the role of sex and age in the architecture of ... · of food trees and lianas) between continuous...

Submitted 4 December 2015Accepted 23 February 2016Published 14 March 2016

Corresponding authorJulieta Benitez-Malvidojbenitezciecounammxjbeniteziiesunammx

Academic editorHannah Buckley

Additional Information andDeclarations can be found onpage 14

DOI 107717peerj1809

Copyright2016 Benitez-Malvido et al

Distributed underCreative Commons CC-BY 40

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The role of sex and age in the architectureof intrapopulation howler monkey-plantnetworks in continuous and fragmentedrain forestsJulieta Benitez-Malvido1 Ana Paola Martiacutenez-Falcoacuten12 Wesley Dattilo3Ana Mariacutea Gonzaacutelez-DiPierro4 Rafael Lombera Estrada4 and Anna Traveset5

1 Instituto de Investigaciones en Ecosistemas y Sustentabilidad Universidad Nacional Autonoma de MexicoMorelia Michoacaacuten Mexico

2Centro de Investigaciones Bioloacutegicas Universidad Autoacutenoma del Estado de Hidalgo Pachuca HidalgoMexico

3Red de Ecoetologiacutea Instituto de Ecologiacutea AC Xalapa Veracruz Mexico4Unidad Acadeacutemica Multidisciplinaria Las Margaritas Universidad Intercultural de Chiapas Las MargaritasChiapas Mexico

5 Ecologiacutea Terrestre Group Biodiversity and Conservation Institut Mediterrani drsquoEstudis Avanccedilats(CSIC-UIB) Esporles Mallorca Spain

ABSTRACTWe evaluated the structure of intrapopulation howler monkey-plant interactions byfocusing on the plant species consumed by different sex and age classes in continuousand fragmented forests in southern Mexico For this we used network analysis toevaluate the impact of fragmentation on howler population traits and on resourceavailability and food choice A total of 37 tree and liana species and seven plant items(bark immature fruits flowers mature fruits immature leaves mature leaves andpetioles) were consumed but their relative consumption varied according to sex andage classes and habitat type Overall adult females consumed the greatest number ofplant species and items while infants and juveniles the lowest For both continuousand fragmented forests we found a nested diet for howler monkey-plant networksdiets of more selective monkeys represent subsets of the diets of other individualsNestedness was likely due to the high selectivity of early life stages in specific foodplants and items which contrasts with the generalized foraging behaviour of adultsInformation on the extent to which different plant species and primate populationsdepend on such interactions in different habitats will help to make accurate predictionsabout the potential impact of disturbances on plant-animal interaction networks

Subjects Animal Behavior Biodiversity Conservation Biology EcologyKeywords Age Alouatta pigra Consumer-resource interactions Habitat fragmentationSex-classes Nestedness Age class Sex class

INTRODUCTIONTrophic interactions among species constitute a central topic in ecology (PetcheyMorin amp Olff 2009) Some studies have evaluated how feeding relationships vary withinpopulations (Bolnick et al 2003) Often within the same population we can find bothmore selective (those that feed off a few plant species) or more opportunistic (those that

How to cite this article Benitez-Malvido et al (2016) The role of sex and age in the architecture of intrapopulation howler monkey-plantnetworks in continuous and fragmented rain forests PeerJ 4e1809 DOI 107717peerj1809

feed off many plant species) individuals (Bolnick et al 2003 Arauacutejo et al 2008 Arauacutejoet al 2010 Pires et al 2011) During the development and growth of an organismfood requirements often change quantitatively and qualitatively principally because ofmetabolic costs sex- and age-related preferences and foraging ability (Stevenson Pinedaamp Samper 2005) In addition consumer growth can be accompanied by shifts in habitatuse which may result in changes in food availability constraining the consumer capacityto exploit different types of resources (Bolnick et al 2003 Petchey Morin amp Olff 2009)According to the lsquolsquoOptimal foraging theoryrsquorsquo individuals consume a subset of potentialresources depending on the resource and individual traits in this sense individuals alwayseat the most valuable resources When preferred resources are scarce individuals can eatunutilized resources (Marshall amp Wrangham 2007 Arauacutejo et al 2008 Arauacutejo et al 2010Arauacutejo Bolnick amp Layman 2011)

An important ecological interaction in the Neotropics occurs between primates andthe plant species they consume and disperse (Rivera amp Calmeacute 2006) Primate species suchas howler monkeys (Alouatta spp) have a flexible diet (eg leaves fruits flowers andbark) that allows them to persist in human-disturbed habitats (Marsh amp Loiselle 2003)Groups of howler monkeys including infants juveniles and adults like some other primatespecies (eg Chiropotes spp and Saguinus spp) are able to cope with changes in resourceavailability within fragmented habitats through behavioural adjustments (eg food choiceand foraging activity) (Jones 2005 Isabirye-Basuta amp Lwanga 2008) Recent studies haveshown that the degree of dietary variation in A pigra is affected by both environmental(ie forest fragment size) and social (ie group size) factors (Dias amp Rangel-Negriacuten 2015)In fact the persistence of primate populations andor species in forest fragments largelydepends on their ability to adjust their diet (Rivera amp Calmeacute 2006)

Several studies have used tools derived from network analysis to describe the dietaryvariation found in populations of animals with a focus on individual-based plant-animalnetworks (Pires et al 2011 Tinker et al 2012 Cantor et al 2013) Recently it has beenshown that intrapopulation primate-resource networks are highly nested diets of specialistindividuals are a subset nested within the diets of generalist individuals (Daacutettilo et al2014) There is no information however about potential factors determining suchnetwork structure In this study we used a network approach to investigate the structureof individual-based howler monkey-plant networks and their underlying mechanismsThe application of network theory allows the recognition of non-random patterns ofinteractions in food webs (Bascompte amp Stouffer 2009) and in our case the identificationof the role of each individual within a food web based on the roles of all individualswithin preserved and disturbed habitat conditions (continuous and fragmented forests)Moreover a network approach in the study of primate diets enables us to assess the level ofselectivity of an individual towards using plant species in a resource-limited environmentsuch as small forest fragments

To answer the question of what is driving diet selectivity and nestedness in howlermonkey populations firstly we assessed differences in resource availability (ie samplingof food trees and lianas) between continuous and fragmented rain forests Secondly wehypothesized that nestedness in howler monkey-plant networks results from the most

Benitez-Malvido et al (2016) PeerJ DOI 107717peerj1809 218

selective age and sex class (male and female infants) feeding on a subset of the broaderdiet of another age and sex class (male and female adults) We used howler monkeysrsquo ageand sex class to analyse consumer-plant interaction because these categories have showndifferences in behaviour and foraging patterns in primates as well as in other mammals(Brent Lehmann amp Ramos-Fernaacutendez 2011 Fuentes-Montemayor et al 2009 StevensonPineda amp Samper 2005) For instance species in theAteline have showndifferences betweenthe sexes in diet Adult females of spider monkeys (Ateles geoffroyi) eat live and decayingwood (eg Licania platypus trees) more often than do adult males possibly to satisfytheir mineral (eg sodium andor calcium) requirements during pregnancy and lactation(Chaves Stoner amp Arroyo-Rodriacuteguez 2012) by contrast adult females of black howlermonkeys (A pigra) are less active and feed mostly on fruits of high energy content whenlactating (Dias amp Rangel-Negriacuten 2015) Moreover fruit selection could differ betweensexes and age classes within primate populations with adult individuals consuming thelargest seedsfruits within a plant species (eg Lagothrix lagothricha in Stevenson Pineda ampSamper 2005) Considering the postulates of the optimal foraging theory in the absence orscarcity of their preferred resources in forest fragments howler monkeys might consume asubset of the plant species consumed in continuous forests which maintains the nestednessin both habitat types From a conservation viewpoint this information is useful if certainhabitat elements such as forest fragments are to be employed effectively in the conservationof primates attention will need to be paid to their diet requirements

METHODSStudy area and habitat typesThe research was conducted at the Lacandon rain forest Chiapas in southeastern Mexico(1607prime58primeprimeN 9056prime36primeprimeW 120m elev) Forest conversion has reduced the original forestedarea (500000 ha) by two-thirds in the last 40 years (De Jong et al 2000) Nevertheless thisregion encompasses the largest remaining portion of tropical rain forest in Mesoamerica(Medelliacuten 1994) The primary vegetation type is lowland tropical rain forest reaching upto 40 m in height in alluvial terraces The temperature averages 239 C and annual rainfallis 28812 mm (Gonzaacutelez-Di Pierro et al 2011) The study was conducted in two areas oflowland tropical rain forest separated by the Lacantuacuten River the Marqueacutes de Comillasregion (MCR eastern side of the river) and the Montes Azules Biosphere Reserve (MABRwestern side) The protected area of theMABR consists of 3310 km2 ofmature undisturbedforest We selected three forest fragments occupied by black howler monkeys within theMCR area (one fragment of 6 ha and two fragments of 3 ha in area) Each fragment hasits own independent howler monkey group Fragments were isolated by 1ndash7 km fromeach other All fragments have been isolated from continuous forest for at least 20 years(Gonzaacutelez-Di Pierro et al 2011) In the continuous forest within the MABR we selectedthree sites used by three different howler monkey groups that were separated by 2 km fromeach other Although howler monkeys have been observed crossing cattle pastures in thestudy area individuals in this study did not move between sites andor habitat types (AMGonzaacutelez-Di Pierro pers obs 2006ndash2008)


Resource availabilityIn the two habitat types we sampled and identified all trees species (ge10 cm diameterat breast height) to determine if resource availability (food availability Tutin et al 1997Doran et al 2002) differed between habitats (fragments and continuous forest) Withineach site (three fragments and three continuous forest sites) we randomly located ten50 times 2 m transects (01 ha) to sample trees (following Gentry 1982) We minimized edgeeffects by locating all transects at least 100 m from the edge We calculated the importancevalue index (IVI) of each species within each habitat (Moore amp Chapman 1986) whichis an overall estimate of the percentage of relative frequency of a plant species in thecommunity Differences in tree community attributes (ie tree species richness treeabundance number of food tree species and IVI) between continuous forest and fragmentswere analysed with t -tests after log(x+1) or an arcsine transformation of the data (thelatter in the case of IVI) To test if differences in tree species similarity (Jaccardrsquos coefficient)were related to geographical distances among transects of each study site we performedMantel tests (Sokal amp Rohlf 1995)

Howler monkeys and dietary compositionThis research complied with protocols approved by CONANP care committee (ComisioacutenNacional de Aacutereas Naturales Protegidas) and DGVS (Direccioacuten General de Vida Silvestrepermission number SGPADGVS07830) The collection of vegetation and feedingbehaviour data did not interfere with primates in any way The black howler monkey(Alouatta pigra) is present in Mexico Guatemala and Belize but most (ca 80) of itsdistribution range is found in Mexico It is one of the largest Mesoamerican primatesThe conservation status of the species is lsquolsquoendangeredrsquorsquo according to the IUCN Red List(httpwwwiucnredlistorgappsredlistsearch) and habitat loss is probably the mostimportant threat affecting the populations Howler population density within the MABRis 013 individualsha but within the study fragments (3ndash6 ha MCR) population densityaveraged 13 individualsha Home-range size of black howlers in continuous forest islt25 ha (Estrada Van Belle amp Garciacutea del Valle 2004)

Dietary composition of howler monkeys was studied during a period of 18 monthsthree months in the dry season from February to April of 2006 2007 and 2008 and threemonths in the rainy season from August to October of the same three years We did notexamine between seasons andor year changes in the food availability for primates becausewe needed a large and complete data set in which all plant species and age and sex classeswere represented to construct the ecological networks Feeding behaviour was documentedduring three consecutive days once every three weeks using five minutes of focal animalsampling (Altmann 1974 Martin amp Bateson 2007) Each individual was recognized bycharacteristically unique marks on their skin and hair Monkeys were systematicallyobserved from 700 am to 1730 pm

At the beginning of the study we categorized the focal individuals by age and sex classinto six groups as follows adult male adult female (adults are full-grown individualswith conspicuous sexual organs males have an enlarged noticeable hyoid bone) juvenilemale and female (juveniles are completely independent from adult females but not yet


full-grown) and male and female infant (infants depend on their mothers for locomotionand feeding in some instance) To construct the ecological networks individuals were keptin their initially designed age and class category despite the fact that infants were moreindependent at the end of the study In continuous forest we recorded 15 individualssix adult females four adult males two juveniles (female and male) two infant femalesand one infant male In forest fragments we recorded a total of 18 individuals five adultfemales four adult males four juvenile females three juvenile males one infant female andonemale Howler monkey population size and structure remained unchanged in fragmentsand continuous forest during the course of the study There was no birth or death in anyof the studied groups

All howler monkey individuals were observed for the same period of time in each habitattype The effect of habitat on feeding time and on the usage of different plant items wasanalysed by comparing the fraction of time spent consuming different plant items (ieflowers petioles young and mature leaves mature and immature fruits and bark) withincontinuous forest and forest fragments with a nested-ANOVA of angular transformed dataData were analysed using the statistical program SigmaStat for Windows 35 Furthermorewe refer to a preferred food (ie an over selected food) as those plant species and itemsselected (usage) disproportionately often relative to their abundance (availability) and to afallback food as those plant species and items that howler monkeys utilized when preferredfoods are scarce (Marshall amp Wrangham 2007) Typically fallback foods are plant speciesand items of low preference but high importance in the diet (eg liana leaves) Plant itemsof high importance are those most frequently consumed regardless of their nutritionalquality whereas preferred items are those of high quality with quality defined as rate ofenergy return to an organism (eg ripe fleshy fruits)

Network metricsWe used the NODF metric (nestedness metric based on overlap and decreasing fillAlmeida-Neto et al 2008) to evaluate whether or not the diets of more selective monkeysrepresent subsets of the diets of monkeys that consumed a broader based diet for eachhabitat Because not all age and sex classes were present in all sites we pooled individualspresent within each habitat type (fragments and continuous forest) to construct thenetworks from an intrapopulation perspective NODF is recommended in ecologicalnetwork analysis because it is less prone to type I errors (Almeida-Neto et al 2008) Wegenerated theoretical matrices to test the significance of the nestedness observed againstnull distributions of these values generated by the Null Model II (Bascompte et al 2003)in ANINHADO software (Guimaratildees amp Guimaratildees 2006) We generated random matricesto test the significance of nestedness according to the Null Model II by using functionswithin the software ANINHADO (n= 1000 randomizations for each network) In thisnull model the probability of occurrence of an interaction is proportional to the numberof interactions of both plant species and monkey individuals (Bascompte et al 2003) Inour intrapopulation networks plant species and monkeys are depicted as nodes andtheir feeding interactions are depicted by links describing the use of plant species byindividuals Our qualitative approach in calculating nestedness decreases the probability of


overestimating the amount of resources (eg leaves vs fruits) ingested by monkeys (Daacutettiloet al 2014) Biologically nestedness describes the organization of niche breadth in whichmore nested networks tend to have the highest niche overlap (Bluumlthgen 2010)

Other network parameters considered in the analysis were as follows (i) mean linkagelevel (mean number of linksinteractions per species) (ii) connectance (the proportionof realized links of the total possible in each network defined as the sum of links dividedby the number of cells in the matrix) (iii) interaction diversity (based on the Shannondiversity index) and (iv) resource selectivity at the network level (H prime2) This selectivityindex ranges from 0 (extreme generalization) to 1 (extreme specialization) and is extremelyrobust with changes in sampling intensity and the number of interacting species (Bluumlthgen2010) Network features were estimated with the Bipartite package (Dormann Gruber ampGroup 2011) Network plots were obtained by using Bipartite in lsquoRrsquo (Dormann Gruber ampGroup 2011 R Development Core Team 2011)

The categorical core vs periphery analysis was used to describe plant species as core(generalist species those with the most interactions) or peripheral (those with fewerinteractions) components of the network Corendashperiphery analyses were performedwith UCINET for Windows 60 (Borgatti Everett amp Freeman 1999) which performs tworoutines for detecting corendashperiphery structures in bipartite graphs (n= 20 runsnetwork)and obtains the percentage of occurrence of corendashperiphery species (see Borgatti et al1999 Diacuteaz-Castelazo et al 2010)

RESULTSOverall we found that important food resources including plant species and items changedwith habitat type and age and sex classes indicating that forest fragmentation affects thefeeding behaviour and level of resource selectivity of howler monkey populations in ourstudy sites

Resource availabilityContinuous forest and fragments presented similar tree species richness and density(diameter at breast height gt 10 cm) similar numbers of tree species consumed by howlermonkeys and a similar IVI of food species (for all cases t lt 2 df = 5 P gt 005 Table 1)Tree speciesrsquo similarity (Jaccardrsquos coefficient) between continuous forest and fragments wasca 70 The Mantel test showed no significant association between tree species similarityand geographical distances within and between habitat types (tprop= 057 P = 060) specieswere as likely to be found in 01 ha blocks close together as in those far apart Fragmentsand continuous forest shared 50 of the 10 tree species with the greatest importance valueindex (IVI) all of which are consumed by howler monkeys (Table 2)

Howler monkey dietary compositionOverall the total time spent making focal observations in fragments was 16730 h but was14666 h in continuous forest because there were more individuals in fragments Howlermonkeys inhabiting forest fragments spent more time foraging (6122 h or 3674 ofthe time) than those monkeys inhabiting continuous forest (3955 h or 2652 of the


Table 1 Tree community (diameter at breast height gt 10 cm) attributes in continuous forest and for-est fragments inhabited by howler monkeys (Alouatta pigra) in the Lacandonian rain forest ChiapasMexico The values are the average (plusmnSD) of ten 50times 2 m transects (01 ha) in each of three forest frag-ments and three continuous forests Tree community attributes did not differ significantly between habitattypes (for all cases t lt 2 df = 5 P gt 005)

Tree attributes Continuous forest Forest fragments

Mean tree species richness (plusmnSD) 337 (47) 333 (21)Mean number (plusmnSD) of primate-dispersed tree species 160 (31) 120 (06)Mean density of trees (dbh gt 10 cm) 141 (164) 137 (55)IVI of food species 67 65

NotesThe importance value index (IVI) was calculated by summing the density the frequency and basal area of each species withineach habitat (Moore amp Chapman 1986)

Table 2 The ten tree species with the highest importance value index (IVI) in continuous forest andforest fragments occupied by howler monkeys (Alouatta pigra) at the Lacandonian rain forest MexicoAll tree species are present in the diet of howler monkeys

Family Species IVI

Continuous forestMoraceae Brosimum alicastrum 052Meliaceae Guarea excelsia 040Moraceae Ficus sp 036Ulmaceae Ampelocera hottlei 022Burseraceae Bursera simaruba 019Anacardiaceae Spondias mombin 015Moraceae Trophis racemosa 015Fabaceae Acacia usumacintensis 013Moraceae Castilla elastica 012Fabaceae Albizia leucocalyx 011

Forest fragmentsFabaceae Dialium guianense 053Moraceae Brosimum alicastrum 046Fabaceae Pterocarpus bayesii 028Ulmaceae Ampelocera hottlei 026Moraceae Ficus sp 021Moraceae Castilla elastica 019Chrysobalanaceae Licania platypus 018Sapotaceae Pouteria campechiana 016Moraceae Trophis racemosa 015Meliaceae Guarea excelsia 011

time) Adults and juveniles in forest fragments spent more time foraging (adults 4614 hjuveniles 1496 h) than adults and juveniles in continuous forest (adults 3130 h juveniles556 h) whereas in continuous forests infants spent more time foraging (280 h) thaninfants present in forest fragments (012 h) We found 30 plant species consumed for all


Figure 1 Diet composition of howler monkeys (Alouatta pigra) in continuous forest and fragments ac-cording to percentage of total feeding time consuming different plant items Significant (P lt 005) dif-ferences in the consumption of plant items is indicated with an asterisk () The items consumed per plantspecies are indicated for each habitat type where items are B bark IF immature fruit FL flower MFmature fruit IL immature leave ML mature leave and P petiole

age and sex classes in forest fragments and 27 in continuous forest (Fig 1 and Table 3) Atotal of 37 plant species and seven plant items (ie bark immature fruits flowers maturefruits immature leaves mature leaves and petioles as in Table 3) were consumed in bothhabitats These included 32 species of trees four species of woody lianas (Abuta panamensisBignonaceae spMacherium sp and Malpighiaceae sp) and one species of a climbing herb(Araceae sp) The time devoted to consuming different plant items was similar for bothhabitats (F113= 053 P = 049) while plant items within habitats were consumed withsignificantly different frequency (nested-ANOVA F613= 1313 P = 0003) Overall theplant items consumed with significantly greatest frequency (number of records per feedingtime) were mature fruits and immature leaves for both habitat types (Fig 1) Feeding timechanged among plant items between habitat types as follows mature fruits in continuousforest 545 vs 376 in forest fragments immature leaves 310 vs 562 immaturefruits 12 vs 50 petioles 62 vs 05 mature leaves 38 vs 08 and finallybark 23 and flowers 073 only in continuous forest Not all items were consumed inall plant species and habitats (Table 3) in continuous forest flowers (ieMachaerium sp)were only consumed by females of all ages whereas bark (ie Licania platypus) was onlyconsumed by adult and infant females and by adult males In continuous forest howlersspent more time eating mature fruits (more than 50) followed by immature leaves (31)regardless of age-class By contrast in fragments adults and juveniles of both sexes spentmore time consuming immature leaves (50) followed by mature fruits (30) whereasinfants spent all of their time eating immature leaves


Table 3 Occurrence of plant species in the core and in the periphery for each network (continuous forest and fragments) The items consumedper plant species are indicated for each habitat type Plant items per species are arranged from left to right with items at the far left being the mostconsumed where items are B bark IF immature fruit FL flower MF mature fruit IL immature leave ML mature leave and P petiole

Plant species Continuous forest Forest fragments

Item core periphery Item core periphery

Brosimum alicastrum IL MF IF 100 0 IL MF IF 100 0Ficus sp ndash 0 0 IL MF IF 100 0Abuta panamensis MF 0 45 MF IL 100 0Acacia usumacintensis IL 0 45 IL 95 5Ampelocera hottlei MF ML 100 0 MF 100 0Araceae sp ndash 0 0 IL 95 5Bignoneaceae sp IL 0 100 IL MF 100 0Cecropia obtusifolia MF IL 85 15 IL MF 95 0Cojoba arborea IL 0 100 IL 100 0Dialium guianense MF 100 0 MF 100 0Licania platypus B IL 45 55 IL 100 0Machaerium sp IL FL MF 0 100 IL 100 0Pourouma bicolor MF 100 0 MF IL 90 10Trophis racemosa ndash 0 0 IL 100 0Brosimum lactescens MF 100 0 MF 25 75Castilla elastic MF 0 100 IL 40 60Combretumsp ndash 0 0 IL 0 80Hirtella Americana ndash 0 0 MF 30 0Liana sp ndash 0 0 IL 20 80Paulinia fibrigera IL 50 50 IL 20 80Pseudolmedia oxyphillaria ndash 0 0 IL 30 70Talauma Mexicana P 0 100 P 0 90Albizia leucocalyx ML IF 100 0 IL 0 100Garcinia intermedia MF 95 5 MF 0 100Inga sp IL 60 0 IL 0 100Platimiscium yucatanum IL 0 45 IL 0 100Sapindaceae sp - 0 0 MF 0 100Schizolobium arboreum P 0 100 P 0 100Spondia mombin - 0 0 MF 0 100Bursera simaruba ndash 0 0 IL 0 100Ficus tecolotensis MF IL 100 0 ndash 0 0Bravaisia sp IL P 55 45 ndash 0 0Maclura tinctoria IL 100 0 ndash 0 0Ficus yoponensis IL 0 100 ndash 0 0Lonchocarpus sp ML 0 100 ndash 0 0Malpigiaceae sp FL 30 70 ndash 0 0Zanthoxylum riedelianum IL 0 100 ndash 0 0


Table 4 Howler monkey-plant network attributes in continuous forest and forest fragments at the La-candon rain forest Mexico see methods for details

Network metrics Continuous forest Forest fragments

No of monkeys 15 18No of plant species 27 30Nestedness (NODF-metric)a 5141 6242Links per species 376 408Connectance (C) 039 036Interaction diversity 506 527Resource selectivity (H prime2) 028 022

NotesaBoth networks were significantly nested (P lt 005)

Howler monkey-plant networksWe found a significant nested pattern in our howler monkey-plant network in bothcontinuous forest (observed matrix NODF = 5141 mean plusmn SD of simulated matricesNODF = 4478plusmn 348 P = 004) and fragment habitats (observed matrixNODF = 6242mean plusmn SD of simulated matrices NODF = 4571 plusmn 289 P = 001) (Fig 2 and Table 4)Network attributes for the two habitats presented similar values of connectance links perspecies interaction diversity and resource selectivity We found however lower links perspecies in continuous forest than in forest fragments which probably generated greaterresource selectivity and specialization in continuous forest (Table 4)

Species turnover as coreperiphery components in fragments and continuous forestsnetworks was very high as plant species fluctuated between habitats as core or peripherycomponents (Table 3) There were however three strict core species (ie A hottlei Balicastrum andD guianense) and one strict peripheral species (ie Schizolobium arboreum)The liana species Bignonaceae sp Macherium sp and the tree Cojoba arborea were corespecies in fragments but periphery in continuous forest whereas the tree species Albizialeucocalyx Brosimum lactescens and Garcinia intermedia were core species in continuousforest but periphery in fragments Not all common food tree species were those preferred byhowler monkeys in the network analysis (Tables 2 and 3) For instance Ficus sp was a corespecies in forest fragments and peripheral in continuous forest whereas Ficus tecolotensiswas core in continuous forest and peripheral in forest fragments Moreover P bicolor wasa core species for continuous forest and fragments however it is not within the 10 treespecies with the highest importance value index (IVI) in either habitat (Table 2)

DISCUSSIONOverall we observed that resource choice in fragments was lower within howler monkeypopulations despite the presence of preferred plant food species for primates in bothhabitat types their relatively low selection may have been driven by habitat attributessuch as the relative scarcity of the most favoured feeding plant species and items of forestfragments (Dias amp Rangel-Negriacuten 2015) Furthermore we found a novel pattern of agesex diet composition variation indicating the presence of a sex age class selectivity in the


Figure 2 Intrapopulation howler monkey-plant networks (Alouatta pigra) for (A) continuous forestand (B) forest fragments Each node represents one monkey (left) or plant species (right) and lines rep-resent monkeyndashplant interactions Codes for A pigra age-classes are the following light box adult femaledark box adult male light triangle juvenile female dark triangle juvenile male light diamond infant fe-male and dark diamond infant male


interaction between howler monkeys and the plant species they consume This study isthe first to show that age and sex classes determine the structure of ecological networksin primate-plant interactions Regardless of habitat type howler monkey populations arecomposed of both more selective and less selective individuals (Fig 2) In this monkey-plant system we have shown that less selective individuals (ie adult males and females)consumed large amounts of resources independent of type and availability thus building acohesive network to which more selective individuals were attached (ie male and femaleinfants) (Bascompte et al 2003) Thus as lsquolsquogeneralistrsquorsquo consumers adults maintain thestability of the network

Habitat food choice and availabilityPlants and animals contributed to the nested pattern in both habitat types The highplant species turnover as corendashperiphery between continuous forest and fragments wasevident in the consumption of Abuta panamensis Bignonaceae sp Brosimum lactescensCojoba arborea and Macherium sp (Table 3) All except B lactescens were core species infragments and peripheral in continuous forest One unidentified species of Ficus sp hada higher importance value index in both habitats however howler monkeys consumed itmore often in fragments than in continuous forest Several fig species are common andvery important in the diet of several Neotropical primate species in different habitat types(Chaves Stoner amp Arroyo-Rodriacuteguez 2012 Daacutettilo et al 2014)

The preferred plant species and items in continuous forests are limited or unavailable infragments Therefore howler monkeys in fragments may rely on resources of relatively lowpreference (fallback foods) to fulfil their nutritional requirements (Marshall amp Wrangham2007) Plant parts or items of liana species as well as immature fruits and leaves of treespecies were more frequently consumed in fragments than in continuous forest (Fig 1) Inpreserved forests howler monkeys are known to select large ripe fruits and immature leavesthat are more easily digested (Leighton 1993 Behie amp Pavelka 2015) Lianas in fragmentsby contrast are typically abundant and important in the diet of howler monkeys butsome of their plant parts or items may provide low rates of energy gain when compared topreferred foods Forest fragmentation is known to increase the mortality of large fruit treesto favour the proliferation of several liana species and to negatively affect tree phenology(reduced fruit set Laurance et al 2001 Chaves Stoner amp Arroyo-Rodriacuteguez 2012)

Fragmentation affects the availability of mature fruits to primates through reductionin the abundance and richness of large food trees as larger trees produce more fruitsthan smaller ones (Chapman et al 1992 Laurance et al 2001 Chaves Stoner amp Arroyo-Rodriacuteguez 2012) The decreased richness and abundance of large trees could negativelyaffect the distribution and abundance of many tropical primates especially in the case ofhighly frugivorous species (Chapman et al 2007) A large proportion of tropical tree speciesproduce fleshy fruits allowing a year-round offer of resources that maintains several speciesof frugivores (Howe amp Smallwood 1982 Fleming amp Kress 2013) However substantialchanges in resource availabilitymdashboth temporally and spatiallymdashwithin fragments mayprevent howler monkeys from searching for and consuming their preferred plant itemsand species (eg ripe fruits)


Network attributes (nestedness connectance mean linkage density interactiondiversity) were similar between habitats The higher consumption of preferred itemsin continuous forest might arise because howler monkeys are not limited and have thepossibility to range freely and feed on the best resources (ie ripe fruits) In fragments bycontrast they have to consume what is available which may represent a restricted set offood choices (resulting in a greater overlap of plant species consumed items) causing themonkeys to spend more time feeding in fragments to fulfil their nutritional needs

Age and sex class plant items and habitatOur findings indicate that individuals do not forage randomly when compared to nullmodels and that the diets of more selective monkeys (infants) represent subsets of plantsand items consumed by other group members (adults) implying that individuals differin their foraging strategies Adult individuals are able to consume a wide range of plantspecies and items and therefore make the strongest contribution to the nested structure ofthe system Infants may become generalists as they learn how to eat a wider range of plantspecies Howler monkey infants tended to be more selective while juveniles consumed amore diverse set of plant species than infants did Adults though consumed the greatestvariety of plant species and items Male and female infants tended to consume moreplant species in continuous forest than in fragments whereas adult females were the mostextreme lsquolsquogeneralistsrsquorsquo in the resulting networks (Daacutettilo et al 2014) This study is the firstone to show that the specialized diets of male and female infants determine the nestedstructure of primate-plant networks in howler monkey populations

Plant species making the greatest contribution to community nestedness (promotingasymmetry) were those species yielding greater fruit supplies and therefore a greaternumber of interactions and greater plant items consumed by howlers (Table 3) Thesestrict core species were not necessarily the most abundant in either habitat Accordingto the optimal foraging theory (Arauacutejo et al 2008 Arauacutejo Bolnick amp Layman 2011) theindividual niche depends on the availability of resources in the habitat we indeed foundthat howler monkeys in fragments consume resources that were not utilized in continuousforest (eg lianas)

CONCLUSIONWe aimed to understand the intrapopulation factors affecting the feeding ecology of howlermonkeys and to link habitat fragmentation and howler monkey-resource interactions byusing a network approach Our findings indicate that both age and sex class drive a nestedpattern in howler monkey-plant interactions in this sense this study is the first to providea mechanism that structures such networks Furthermore we found that within howlergroups adultsmdashparticularly female adultsmdashare likely to be the main seed dispersers forseveral old-growth forest tree species (eg seeds gt 1 cm in length) in the study regionbecause their diet consists primarily of mature fruits (Behie amp Pavelka 2015 Dias ampRangel-Negriacuten 2015) In the Lacandon forest we were able to detect that howler monkeysinhabiting fragments displayed less dietary selection because of the limited availability of


preferred food readily available in continuous forest which may threaten their long-termpersistence in disturbed habitats

ACKNOWLEDGEMENTSWe warmly thank J M Lobato H Ferreira and A Valencia for careful technical support

ADDITIONAL INFORMATION AND DECLARATIONS

FundingThis research was supported by grants from Consejo Nacional de Ciencia y Tecnologiacutea(CONACyT CB2005-C01-51043 CB2007-79121 to JBM) and Universidad NacionalAutoacutenoma de Meacutexico (UNAM IN206111 to JBM) The Posgrado en Ciencias Bioloacutegicas(UNAM) and CONACyT are greatly acknowledged for providing a doctorate scholarshipto AMG and UNAM for a post-doctoral scholarship to APM This manuscript was partiallywritten while JBM was on sabbatical at IMEDEA and supported by DGAPA (UNAM) Thefunders had no role in study design data collection and analysis decision to publish orpreparation of the manuscript

Grant DisclosuresThe following grant information was disclosed by the authorsConsejo Nacional de Ciencia y Tecnologiacutea CB2005-C01-51043 CB2007-79121Universidad Nacional Autoacutenoma de Meacutexico IN206111

Competing InterestsAnna Traveset is an Academic Editor for PeerJ

Author Contributionsbull Julieta Benitez-Malvido conceived and designed the experiments performed theexperiments contributed reagentsmaterialsanalysis tools wrote the paper preparedfigures andor tables reviewed drafts of the paperbull Ana Paola Martiacutenez-Falcoacuten and Wesley Dattilo analyzed the data wrote the paperprepared figures andor tables reviewed drafts of the paperbull Ana Mariacutea Gonzaacutelez-DiPierro conceived and designed the experiments performed theexperiments analyzed the data contributed reagentsmaterialsanalysis tools preparedfigures andor tables collected and processed the data basesbull Rafael Lombera Estrada performed the experiments collected de primate and vegetationdata followed the monkey troopsbull Anna Traveset wrote the paper reviewed drafts of the paper

Animal EthicsThe following information was supplied relating to ethical approvals (ie approving bodyand any reference numbers)

This research compliedwith protocols approvedbyCONANPcare committee (ComisioacutenNacional de Aacutereas Naturales Protegidas permission number SGPADGVS07830) The


collection of vegetation and feeding behaviour data did not interfere with primates in anyway

Field Study PermissionsThe following information was supplied relating to field study approvals (ie approvingbody and any reference numbers)

CONANP (Comisioacuten Nacional de Aacutereas Naturales Protegidas permission numberSGPADGVS07830)

Data AvailabilityThe following information was supplied regarding data availability

The research in this article did not generate any raw data

Supplemental InformationSupplemental information for this article can be found online at httpdxdoiorg107717peerj1809supplemental-information

REFERENCESAlmeida-NetoM Guimaratildees P Guimaratildees Jr PR Loyola D UlrichW 2008 A

consistent metric for nestedness analysis in ecological systems reconciling conceptand measurement Oikos 1171227ndash1239 DOI 101111j0030-1299200816644x

Altmann J 1974 Observational study of behavior sampling methods Behavior49227ndash267 DOI 101163156853974X00534

ArauacutejoMS Bolnick DI Layman CA 2011 The ecological causes of individual specialisa-tion Ecology Letters 14948ndash958 DOI 101111j1461-0248201101662x

ArauacutejoMS Guimaratildees Jr PR Svanbaumlck R Pinheiro A Guimaratildees P Dos Reis SFBolnick DI 2008 Network analysis reveals contrasting effects of intraspecificcompetition on individual versus population diets Ecology 891981ndash1993DOI 10189007-06301

ArauacutejoMS Martins EG Cruz LD Fernandes FR Linhares AX Dos Reis SF GuimaratildeesJr PR 2010 Nested diets a novel pattern of individual-level resource use Oikos11981ndash88 DOI 101111j1600-0706200917624x

Bascompte J Jordano P Melian CJ Olesen J 2003 The nested assembly of plant-animalmutualistic networks Proceedings of Natural Academy of Science of USA 1009383ndash9387 DOI 101073pnas1633576100

Bascompte J Stouffer DF 2009 The assembly and disassembly of ecological net-works Philosophical Transactions of the Royal Society of Biology 3641781ndash1787DOI 101098rstb20080226

Behie AM Pavelka MS 2015 Fruit as a key factor in howler monkey population densityconservation implications In Kowalewski MM Garber PA Cortes-Ortiz L UrbaniB Youlatos D eds Howler monkeys New York Springer 357ndash382

Bluumlthgen N 2010Why network analysis is often disconnected from communityecology a critique and an ecologistrsquos guide Basic and Applied Ecology 11185ndash195DOI 101016jbaae201001001


Bolnick DI Svanback R Fordyce JA Yang LH Davis JM Hulsey CD Forister ML2003 The ecology of individuals incidence and implications of individual special-ization American Naturalist 1611ndash28 DOI 101086343878

Borgatti SP Everett MG Freeman LC 1999UNICET 6 for Windows software forsocial network analysis Harvard Analytic Technologies Available at httpwwwanalytictechcomdownloaduc6htm

Borgatti SP Mehra A Brass DJ Labianca G 1999 Network analysis in the socialsciences Science 323892ndash895

Brent LJN Lehmann J Ramos-Fernaacutendez G 2011 Social network analysis in the studyof nonhuman primates a historical perspective American Journal of Primatology73720ndash730 DOI 101002ajp20949

Cantor M Pires MM Longo GG Guimaratildees PR Setz EZF 2013 Individual variationin resource use by opossums leading to nested fruit consumption Oikos1221085ndash1093 DOI 101111j1600-0706201200070x

Chapman CA Chapman LJ Wangham R Hunt K Gebo D Gardner L 1992 Estimatorsof fruit abundance of tropical trees Biotropica 24527ndash531 DOI 1023072389015

Chapman CA Naughton Treves L Lawes MJWassermanMD Gillespie TR2007 Population declines of Colobus in Western Uganda and conservationvalue of forest fragments International Journal of Primatology 28513ndash528DOI 101007s10764-007-9142-8

Chaves OM Stoner KE Arroyo-Rodriacuteguez V 2012 Differences in diet betweenspider monkey groups living in forest fragments and continuous forest in MexicoBiotropica 44105ndash113 DOI 101111j1744-7429201100766x

DaacutettiloW Serio-Silva JC Chapman C Rico-Gray V 2014Highly nested diets in in-trapopulation tropical monkey-resource food webs American Journal of Primatology76670ndash678 DOI 101002ajp22261

De Jong BHG Ochoa-Ganona S Castillo-SantiagoMA Ramiacuterez-Marcial M CairnsMA 2000 Carbon flux and patterns of land-useland-cover change in the SelvaLacandona Mexico Ambio 29504ndash511 DOI 1015790044-7447-298504

Dias PAD Rangel-Negriacuten A 2015 Diets of howler monkeys In Kowalewski MMGarber PA Cortes-Ortiz L Urbani B Youlatos D eds Howler monkeys New YorkSpringer 21ndash56

Diacuteaz-Castelazo C Guimaratildees Jr PR Jordano P Thompson JN Marquis RJ Rico-Gray V 2010 Changes of a mutualistic network over time reanalysis over a 10-yearperiod Ecology 91793ndash801 DOI 10189008-18831

Doran DMMcNeilage A Greer D Bocian C Mehlman P Shah N 2002Westernlowland gorilla diet and resource availability new evidence cross-site comparisonsand reflections on indirect sampling methods American Journal of Primatology5891ndash116 DOI 101002ajp10053

Dormann CF Gruber B Group R 2011 Package Bipartite visualizing bipartite networksand calculating some ecological indices R Statistical Software Available at httpwwwr-projectorg


Estrada A Van Belle S Garciacutea del Valle Y 2004 Survey of black howler (Alouattapigra)and spider (Ateles geoffroyi) monkeys along the Riacuteo Lacantuacuten Chiapas MexicoNeotropical Primates 1270ndash75

Fleming TH KressWJ 2013 The ornaments of life coevolution and conservation in thetropics (Interespecific Interactions) Chicago University of Chicago Press

Fuentes-Montemayor E Cuaroacuten AD Vaacutezquez-DomIacutenguez E Beniacutetez-Malvido JValenzuela-Galvaacuten D Andresen E 2009 Living on the edge roads and edgeeffects on small mammal populations Journal of Animal Ecology 78857ndash865DOI 101111j1365-2656200901551x

Gentry AH 1982 Patterns of neotropical plant diversity Journal of Evolutionary Biology151ndash84

Gonzaacutelez-Di Pierro AM Beniacutetez-Malvido J Meacutendez-Toribio M Zermentildeo I Arroyo-Rodriacuteguez V Stoner KE Estrada A 2011 Effects of the physical environment andprimate gut passage on the early establishment of Ampelocera hottlei Standley in rainforest fragments Biotropica 4359ndash466 DOI 101111j1744-7429201000734x

Guimaratildees PR Guimaratildees P 2006 Improving the analyses of nestedness forlarge sets of matrices Environmental Modellling amp Software 211512ndash1513DOI 101016jenvsoft200604002

Howe HF Smallwood J 1982 Ecology of seed dispersal Annual Review of Ecology andSystematics 13201ndash228 DOI 101146annureves13110182001221

Isabirye-Basuta GM Lwanga JS 2008 Primate populations and their interactionswith changing habitats International Journal of Primatology 2935ndash48DOI 101007s10764-008-9239-8

Jones CB 2005 Behavioral flexibility in primates causes and consequences New YorkSpringer

LauranceWF Peacuterez-Salicrup D Delamocircnica P Fearnside PM DrsquoAngelo S Jerozolin-ski A Pohl L Lovejoy TE 2001 Rain forest fragmentation and the structure ofAmazonian liana communities Ecology 82105ndash116DOI 1018900012-9658(2001)082[0105RFFATS]20CO2

LeightonM 1993Modeling diet selectivity by Bornean orangutans evidence forintegration of multiple criteria for fruit selection International Journal of Primatology14257ndash313 DOI 101007BF02192635

Marshall AJ Wrangham RW 2007 Evolutionary consequences of fallback foodsInternational Journal of Primatology 281219ndash1235 DOI 101007s10764-007-9218-5

Marsh LK Loiselle BA 2003 Recruitment of black howler fruit trees in fragmentedforests of Northern Belize International Journal of Primatology 2465ndash86DOI 101023A1021446512364

Martin P Bateson P 2007Measuring behaviour an introductory guide CambridgeCambridge University Press

Medelliacuten R 1994Mammal diversity and conservation in the Selva Lacandona ChiapasMexico Conservation Biology 8780ndash799 DOI 101046j1523-1739199408030780x

Moore PD Chapman SB 1986Methods in plant ecology Oxford Blackwell Scientific


Petchey OL Morin PJ Olff H 2009 The topology of ecological interaction networksthe state of the art In Verhoef HA Morin PJ eds Community ecology processesmodels and applications Oxford University Press 7ndash22

Pires MM Guimaratildees Jr PR ArauacutejoMS Giaretta AA Costa JCL Dos Reis SF 2011The nested assembly of individual resource networks Journal of Animal Ecology80896ndash903 DOI 101111j1365-2656201101818x

RDevelopment Core Team 2011 R a language and environment for statistical comput-ing Vienna R Foundation for Statistical Computing Available at httpwwwR-projectorg

Rivera A Calmeacute S 2006 Forest fragmentation and its effects on the feeding ecology ofblack howlers (Alouatta pigra) from the Calakmul area in Mexico In Estrada AGarber PA Pavelka MSM Luecke L eds New perspectives in the study of Mesoamer-ican primates distribution ecology behaviour and conservation New York Kluwer189ndash213

Sokal R Rohlf JH 1995 Biometry New York FreemanStevenson PR PinedaM Samper T 2005 Influence of seed size on dispersal patterns

of woolly monkeys (Lagothrix lagothricha) at Tinigua Park Colombia Oikos110435ndash440 DOI 101111j0030-1299200512898x

Tinker MT Guimaraes PR NovakMMarquitti FMD Bodkin JL Staedler M BentallG Estes JA 2012 Structure and mechanism of diet specialisation testing modelsof individual variation in resource use with sea otters Ecology Letters 15475ndash483DOI 101111j1461-0248201201760x

Tutin CEG Ham RMWhite LJT HarrisonMJS 1997 The primate community ofthe Lope Reserve Gabon diets responses to fruit scarcity and effects on biomassAmerican Journal of Primatology 421ndash24DOI 101002(SICI)1098-2345(1997)421lt1AID-AJP1gt30CO2-0


feed off many plant species) individuals (Bolnick et al 2003 Arauacutejo et al 2008 Arauacutejoet al 2010 Pires et al 2011) During the development and growth of an organismfood requirements often change quantitatively and qualitatively principally because ofmetabolic costs sex- and age-related preferences and foraging ability (Stevenson Pinedaamp Samper 2005) In addition consumer growth can be accompanied by shifts in habitatuse which may result in changes in food availability constraining the consumer capacityto exploit different types of resources (Bolnick et al 2003 Petchey Morin amp Olff 2009)According to the lsquolsquoOptimal foraging theoryrsquorsquo individuals consume a subset of potentialresources depending on the resource and individual traits in this sense individuals alwayseat the most valuable resources When preferred resources are scarce individuals can eatunutilized resources (Marshall amp Wrangham 2007 Arauacutejo et al 2008 Arauacutejo et al 2010Arauacutejo Bolnick amp Layman 2011)

An important ecological interaction in the Neotropics occurs between primates andthe plant species they consume and disperse (Rivera amp Calmeacute 2006) Primate species suchas howler monkeys (Alouatta spp) have a flexible diet (eg leaves fruits flowers andbark) that allows them to persist in human-disturbed habitats (Marsh amp Loiselle 2003)Groups of howler monkeys including infants juveniles and adults like some other primatespecies (eg Chiropotes spp and Saguinus spp) are able to cope with changes in resourceavailability within fragmented habitats through behavioural adjustments (eg food choiceand foraging activity) (Jones 2005 Isabirye-Basuta amp Lwanga 2008) Recent studies haveshown that the degree of dietary variation in A pigra is affected by both environmental(ie forest fragment size) and social (ie group size) factors (Dias amp Rangel-Negriacuten 2015)In fact the persistence of primate populations andor species in forest fragments largelydepends on their ability to adjust their diet (Rivera amp Calmeacute 2006)

Several studies have used tools derived from network analysis to describe the dietaryvariation found in populations of animals with a focus on individual-based plant-animalnetworks (Pires et al 2011 Tinker et al 2012 Cantor et al 2013) Recently it has beenshown that intrapopulation primate-resource networks are highly nested diets of specialistindividuals are a subset nested within the diets of generalist individuals (Daacutettilo et al2014) There is no information however about potential factors determining suchnetwork structure In this study we used a network approach to investigate the structureof individual-based howler monkey-plant networks and their underlying mechanismsThe application of network theory allows the recognition of non-random patterns ofinteractions in food webs (Bascompte amp Stouffer 2009) and in our case the identificationof the role of each individual within a food web based on the roles of all individualswithin preserved and disturbed habitat conditions (continuous and fragmented forests)Moreover a network approach in the study of primate diets enables us to assess the level ofselectivity of an individual towards using plant species in a resource-limited environmentsuch as small forest fragments

To answer the question of what is driving diet selectivity and nestedness in howlermonkey populations firstly we assessed differences in resource availability (ie samplingof food trees and lianas) between continuous and fragmented rain forests Secondly wehypothesized that nestedness in howler monkey-plant networks results from the most


























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Family Species IVI





































































































the role of sex and age in the architecture of ... · of food trees and lianas) between continuous...

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