Life History Trade-offs

LH Trade-offs

• Trade-offs have a central role in life history theory

• Trade-offs have been experimentally manipulated in both the lab and field, measured as phenotypic correlations, or genetic correlations

Life-history Trade-offs

• Trade-offs are the linkages between traits that constrain the simultaneous evolution of two or more traits

• Some of the most commonly studied trade-offs include: – survival vs. reproduction– Current vs. future reproduction– Reproduction and growth– Reproduction and condition– Quality and quantity of offspring

LH Trade-offs

• One of the reliable methods for quantifying trade-offs include measuring a particular trait and measure the correlated responses in the other trait

• Another approach is to manipulate the phenotype and study the consequences in the same individuals (e.g. clutch sizes)

LH Trade-offs

• Examples: red deer• Adult mortality is higher in females that are

nursing offspring• There is both a physiological and ecological

mechanism

LH Trade-offs

• What is the mechanism? • Lactating females do not have large fat

reserves, thus causing higher over-winter mortality

• This is also age-dependent

LH Trade-offs

• E.g. Beech Trees• Beech trees will have ‘mast’ years• During those years, it is not surprising the

growth ring may only be ½ as large as in ‘normal’ years

LH Trade-offs

• Beech tree trade-off

LH Trade-offs• Grasshoppers will

trade ‘quantity’ relative to ‘quality’…possibly based upon environmental conditions

LH Trade-offs

• E.g. Neotropical frogs• Female frogs are not the only ones that hear

calling males!• The capture rate is positively correlated with

calling rate • They can distinguish between species and size

and make ‘informed’ decisions

Bats

Life History Trade-offsLife History Traits Mangrove Warbler Yellow Warbler

Territoriality Year-round Seasonal

Breeding season length 3.5 months 2.5 months

Bigamous male percent 10 5 or less

Average clutch size 3 eggs 4.5 eggs

Average incubation time 13 days 11 days

Average brooding time 11 days 8.5 days

Depredation percent 65 30

Nesting success percent 26 55

Nesting attempts 2 ?

Females double brooding percent 5 1 or less

Cowbird parasitism percent 8 40

Parental care percent 44 57

Adult survivorship 65 50

LH Trade-offs

• There may be several types of trade-offs• Physiological: allocation decisions between

two or more processes that compete directly with one another for limited resources within a single individual

• E.g. red deer, beech tree, grasshoppers

LH Trade-offs

• Microevolutionary: broader than physiological trade-offs; include trade-offs in which one trait increases fitness while linked to a second trait that directly decreases fitness

• Microevolutionary trade-offs are defined by the response of populations whereas physiological trade-offs may exist without any microevolutionary trade-off

LH Trade-offs

• Consider the grasshopper in which there is a reaction norm for number and size of offspring, but with no genetic variation for the reaction norm

• When conditions are poor, they produce fewer, larger eggs

• This is really a case of individual plasticity (without a genetic component)

LH Trade-offs

• It is important to remember that physiological trade-offs that are not genetically variable may have been previously, but have become fixed because they were the optimal allocation

LH Trade-offs

• Macroevolutionary trade-offs are defined by comparative analysis of variation in traits among independent phylogenetic events

• Consider two traits that are not plastic and for which there is no genetic variation (fixed)

• Within phylogenetic groups, the two traits are negatively correlated

• Also, the traits are apparently adaptively associated with habitats

LH Trade-offs

• Such patterns could only exist because physiological and microevolutionary trade-offs that existed in the past have left their traces in an entire lineage even though we cannot now measure them within species

LH Trade-offs

• By identifying the comparative pattern within which the intraspecific trade-offs occur, we identify conditions common to who lineages

• This gives greater generality to evolutionary patterns and potentially mechamisms

Physiological Trade-offs

• Physiological ecology demonstrates the lineage-specific effects that constrain microevolutionary optimization – condition thresholds for breeding, growth rates as a function of body size, limits on maximum performance, and the amount of energy that it takes to produce a gram of offspring

Physiological Trade-offs

• While these traits are relatively constant (conservative) within species, but vary among lineages

Physiological Trade-offs

• Physiology is the basis of phenotypic correlations and is the filter through which genetic conditions are expressed

Physiological Trade-offs

• Genome consists of a part carrying lineage-specific effects characteristic of a species and a variable part carrying the differences among individuals

Physiological Trade-offs

• Physiological tradeoffs constrain adaptation: with limited resources, an increase in energy allocation must result in a proportional decrease in materials and energy allocated to another (the Principle of Allocation)

• What is left is after standard metabolic use is sometimes referred to as a surplus

• It can be allocated to growth (u) and reproduction (1-u)

Physiological Trade-offs

• What value of u will maximize fitness?

• Given values of fitness for every pair of values of growth and reproduction, one can plot the fitness values on the growth-reproduction plane and draw contours through points of equal value

Physiological Trade-offs

• There may be a single combination of growth and reproduction that produces the highest fitness, there would be a peak, with declining fitness around it

• The trade-off should represent a straight line (although the slope does not have to be 1)

• Where the trade-off intersects with the highest value on the fitness contour, fitness is maximized

Physiological Trade-offs

Physiological Trade-offs

• There are many caveats: see handout

Physiological Trade-offs

• The physiological models focuses on how materials and energy are acquired, processed and utilized

• It is based upon rates (e.g. feeding, metabolic, growth…)

Physiological Trade-offs

• Consider the fate of ingested material for a carnivorous fish swimming and foraging optimally

Physiological Trade-offs

• Feeding constraints and efficiencies connect physiological ecology, behavioral ecology and life history evolution

• Male Kestrals feed females and young

Physiological Trade-offs

• Males with broods from 4 to 7 chicks all spent an average of 4.75 hours per day in flight independent of brood size (382kJ/day foraging)

• Males with larger broods hunted more efficiently and provision equally well (63 g/day)

• When nestlings were manipulated (number or quantity of food), males increased delivery rates by almost 3x

Physiological Trade-offs

• The energy spent was extremely high and sustained (up to 11 days)

• However, they still only foraged during half of the daylight hours…what does that mean?

Physiological Trade-offs

• Foraging and reproductive success in geese: geese pair bond before arriving on the breeding grounds

• The quality of forage and efficiency by which females can graze depend upon male status

• Consequently, dominant females return in the fall with more young and females with subordinate males get divorced more often

Physiological Trade-offs

• Reproductive effort is a key concept in LHE, but costs are poorly understood

• Individuals of two species could devote the same quantity of energy to reproduction at the equivalent body sizes, but differ greatly in the absolute amount of energy gathered or in the time during which it was gathered

• However, the ratio would be equal, but true investment is not

Physiological Trade-offs

• Second, even if energy budgets were identical for two species, a comparison of clutch weight/body wt ratios might not provide comparable measure of effort if the species differed in the number of clutches produced in a single season

Physiological Trade-offs

• Individuals may also differ in their ability to detect predators, thus determining the investment in predator detection is not equal either

• Investments are difficult to follow!

Physiological Trade-offs

• Do we really need to study or measure reproductive effort?

• What we really need is the quantity of reproduction and the cost of reproduction (changes in B or D), but not reproductive effort (physiological allocations)

NO

Microevolutionary Trade-offs

Microevolutionary Trade-offs

Microevolutionary Trade-offs

Microevolutionary Trade-offs

Physiological Trade-offsLife History Traits Mangrove Warbler Yellow Warbler

Territoriality Year-round Seasonal

Breeding season length 3.5 months 2.5 months

Bigamous male percent 10 5 or less

Average clutch size 3 eggs 4.5 eggs

Average incubation time 13 days 11 days

Average brooding time 11 days 8.5 days

Depredation percent 65 30

Nesting success percent 26 55

Nesting attempts 2 ?

Females double brooding percent 5 1 or less

Cowbird parasitism percent 8 40

Parental care percent 44 57

Adult survivorship 65 50