Post-breeding migrations of newts (Triturus cristatus andT. marmoratus) with contrasting ecological requirements

R. Jehle1 and J. W. Arntzen2

1 Institute of Zoology, University of Vienna, Althanstr. 14, A-1090 Vienna, Austria2 Universidade do Porto, Unidade de GeÂnetica Animal e ConservacËaÄo, Campus AgraÁrio de VairaÄo, Rua de Monte, Crasto, 4480 Vila do Conde,

Portugal

(Accepted 27 July 1999)

Abstract

Two newt species with contrasting ecological requirements (Triturus cristatus and T. marmoratus) were

radio-tracked after leaving a shared breeding pond in western France. Movements of 30 individuals were

recorded over 491 days. During the ®rst terrestrial night, the newts migrated with high directionality up to

137 m to their refuges. Burrows of small mammals were among the favourite refuges. Movements after the

®rst night were mostly underground and over shorter distances (< 6.8 m). Sixty-four per cent of all tracked

newts stayed within 20 m of the pond edge, and migrations were recorded up to 146 m away from the

pond. Both species preferred areas with bushes, hedgerows and trees, and avoided pastures and open areas.

Migration in the direction of a habitat type characterized by trees and underground shelters was favoured

over migrations in other directions. A clear terrestrial niche separation between the two newt species was

not observed. The results are discussed in relation to previous ®ndings that T. cristatus supersedes

T. marmoratus as a consequence of anthropogenic change to the terrestrial environment, affecting the

species differentially.

Key words: amphibians, migration, radio-tracking, Triturus cristatus, Triturus marmoratus

INTRODUCTION

Terrestrial migrations represent an important phase inthe life of many amphibians. Data are frequentlyobtained on their migratory activity, for example withdrift fences when entering or leaving the reproductivesite (Semlitsch, 1985; Pechmann et al., 1991), butdetailed information, such as on the pathways and timebudgets of movements, is rare. Moreover, the relativeease of studying aggregated breeding populationsduring their aquatic phase has not been matched bysimilar research on ecology and behaviour during theirterrestrial phase, although it is becoming increasinglyclear that the terrestrial environment plays an importantrole in the amphibian life cycle (Denton & Beebee, 1994;Dodd & Cade, 1998; Semlitsch, 1998).

In western France, the crested newt Triturus cristatusand the marbled newt T. marmoratus have contrastingecological requirements. Although they breed in pondsconspeci®cally, with no discernible niche differentiationbetween either adults or larvae, they are largely separatedin terms of their occurrence in different landscapes.Triturus marmoratus predominates in forests and hilly

terrain with scrub and hedges, whereas T. cristatuspredominates in ¯at and open areas (ValleÂe, 1959;Schoorl & Zuiderwijk, 1981). This qualitative assess-ment is supported by the observation that T. cristatus iscurrently expanding its range at the expense ofT. marmoratus populations, a process probably trig-gered or accelerated by human-induced changes to thelandscape (Arntzen & Wallis, 1991). In particular, theremoval of hedgerows would lead to a local decline andextinction of T. marmoratus (Schoorl & Zuiderwijk,1981; Arntzen & Wallis, 1991).

The extent and nature of movements are key factorsaffecting the vulnerability of terrestrial vertebrates tolandscape change (Law & Dickman, 1998). Trituruscristatus and T. marmoratus are characterized by theirrelatively large body size, which makes them favourablecandidates for the ®rst telemetric investigations onEuropean newts. The aim of the present study is todocument migration patterns, microhabitats and terres-trial time budgets of T. cristatus and T. marmoratuswhen leaving a shared breeding pond, and to test thehypothesis that T. cristatus and T. marmoratus havedifferent terrestrial niches.

J. Zool., Lond. (2000) 251, 297±306 # 2000 The Zoological Society of London Printed in the United Kingdom

MATERIALS AND METHODS

Study site and newt collecting

The study site, including a 50 m2 breeding pond (pond232 from Arntzen & Wallis, 1991), is located near thevillage of Jublains, `DeÂpartement' Mayenne, westernFrance. In 1997, the pond was inhabited by 242 (se 34.6)large-bodied newts (67% T. marmoratus, 30%T. cristatus, 3% T. cristatus6marmoratus hybrids, asdetermined with Begon's weighted mean method with 10samples (Begon, 1979)).

Large-bodied newts leaving the pond were interceptedwith a drift-fence pitfall-trap system (described inArntzen, Oldham & Latham, 1995), placed 5±8 m westof the shoreline and covering approx. 30% of the pondperimeter. The traps were checked daily at 08:00 from18 June to 30 July 1997, a time which includes part ofthe emigration period of both species (Bouton, 1986).Alternatively, newts were captured in the pond withdip-nets and then kept in enclosures, which were 220 lmetal barrels cut lengthways and covered with nettingto prevent escape, placed at the pond edge. Half of theenclosure was ¯ooded and in constant exchange withpond water, with the second half being on land. Thebottom was covered with soil and dead leaves toprovide shelter. The enclosures were inspected everythird day. Newts that had moved out of the water wereassumed to be leaving the pond, equipped with atransmitter and released. Thirty newts (T. cristatus: 9females, 4 males, and 1 sub-adult, T. marmoratus: 6females, 9 males, 1 T. cristatus6marmoratus hybridfemale) were equipped with radio transmitters (Table 1).Radio-tracked individuals were released at 21:00, eithernear the trap where they were caught but at the oppositeside of the fence (7 T. cristatus, 8 T. marmoratus, 1T. cristatus6marmoratus hybrid), or in the middle ofthe pond (7 T. cristatus and 7 T. marmoratus), de-pending on the way they were obtained.

Transmitter implantation and radio-tracking

Three types of transmitters were used: Holohil SystemsLtd model LB-2 (weight 0.48±0.57 g, battery life 8±21days), BD-2A (weight 0.69±0.78 g, battery life 25±35days), and BD-2G (weight 1.19 g, battery life 9 weeks ormore). The average mass was 7 g for T. cristatus males,7.7 g for T. cristatus females, and 8.9 g for both sexes ofT. marmoratus (Table 1). The weight of the transmittervaried between 7.0% and 19.8% of the body mass(median value 9.2%). If the transmitter exceeded 10% ofthe body mass, the tracking was limited to 7 days.Before implantation, the newts were anaesthetized withMS 222 (Sandoz), until the muscular system was relaxedand the animals stopped moving (10±20 min). The bodycavity was opened at the ventro-lateral side for 10±15 mmwith a scalpel, and the transmitter was inserted usingforceps. Transmitters were cleaned but not sterilized, asevidence from toads suggest that sterilization is not

necessary (U. Sinsch, pers. comm.). The antenna wascut to a length of 3±5 cm, guided out of the body and®xed between 2 stitches when closing the wound(n = 21), or twisted around the transmitter and ®xedwith `superglue' before implantation (n = 9). The woundwas closed with 4±5 sutures using an iris cutting needle(a C-shaped needle 7 mm long) and surgical silk sup-plied for human medicine. Bleeding rarely occurred, andno other complications were observed during the im-plantation procedure. All animals recovered fromanaesthesia after approx. 30 min with no ill effects(Fig. 1). The range of the radio signal varied between 30and 200 m, depending on the antenna length, and

R. Jehle and J. W. Arntzen298

Table 1. Individual data on 30 radio-tracked newts (Trituruscristatus, T. marmoratus and T. cristatus6marmoratushybrid). IN, individual number (corresponds to the three lastdigits of transmitter frequency); BM, body mass (g); TM,transmitter mass; TD, tracking duration (days); NF, numberof ®xes; NM, number of movements; MD, migrated distance(m). Fate: A, recovered alive; B, bare transmitter found; D,recovered dead; L, signal lost; P, predated by snakes; U,unrecoverable transmitter as in inaccessible shelter. *, Newtsfor whom antennas were glued onto transmitters

IN BM TM TD NF NM MD Fate

T. cristatusMales

092 6.2 0.57 19 58 7 13.1 U231 7.5 0.69 22 67 12 24.0 A352 7.3 0.69 11 33 6 18.8 P421 6.9 0.69 21 67 3 6.6 B

Females080 6.9 0.57 19 58 11 22.6 A313 8.0 0.69 14 42 9 24.7 B460 7.5 0.69 20 70 11 12.6 U561 8.5 0.78 24 77 12 23 B672 9.4 0.78 28 85 4 2.1 L770 6.0 1.19 8 26 3 9.7 A*832 7.0 1.19 5 17 5 99.5 P*971 7.0 1.19 8 26 6 78.6 A*993 9.0 1.19 5 17 5 58.7 A*

Sub-adult051 4.8 0.48 5 19 8 4.4 D

T. marmoratusMales

060 7.4 0.57 19 58 1 15.1 A252 9.0 0.69 22 70 4 145.7 D272 ? 0.69 21 67 2 9.6 B512 8.8 0.78 24 70 8 4.6 A581 9.7 0.78 25 71 12 43.6 B591 8.8 0.78 27 81 3 27.8 U621 11.1 0.78 30 91 11 34.2 B912 8.0 1.19 8 26 4 17.6 A*922 8.0 1.19 5 18 4 57.1 A*

Females352/1 9.0 0.69 11 44 12 59.3 A*485 8.1 0.69 21 72 10 11.6 L650 10.1 0.78 31 93 15 15.4 A730 10.6 1.19 3 13 9 20.7 P750 7.0 1.19 4 15 6 10.3 A*860 9.0 1.19 6 17 7 62.2 A*

T. cristatus6marmoratusFemale

550 10.5 0.78 25 76 5 3.6 B

environmental conditions (such as depth of the animalsunder ground). The receiver was a TRX 1000S7 (Wild-life Materials, frequency range 151.000±151.999 Hz)with a hand-held Yagi antenna.

The radio-tracking was conducted between 21 Juneand 30 July 1997. The animals to be tracked werereleased on the day of transmitter implantation, andtheir location was recorded at 8-h intervals (around00:00, 08:00 and 16:00). The location of each ®x wasmarked with a numbered ¯ag. The accuracy of thelocalizations was c. 50 cm. Movements below thisthreshold were ignored. Fourteen individuals (47%)were followed on the ®rst night of release in short butirregular intervals (0.5±6 h) between 21:00 and 08:00 onthe next day. One individual migrated a signi®cantdistance on the second night, when it was detectedvisually by chance at approx. 22:00 and followed for theremainder of the night. Newts were excavated fromtheir refuges 2±3 days before the battery was due toexpire, anaesthetized, and their transmitters wereremoved. The wounds were re-sutured and the studyindividuals were released in the pond, as excavationmostly destroyed their hiding place.

Data collection and analysis

Distance measures were taken with a measuring tape tothe nearest 0.1 m: (1) to the nearest edge of the pond, (2)

for individuals which were released at the fence to thepoint of release, (3) to the last recorded location. Anindex of directionality (di) was obtained as di = d /id, dbeing the total distance moved and id the line-in-sight-distance between the release point and the last localiza-tion (Batschelet, 1981). In determining migration speed,records were included for those newts moving between 2or more consecutive observations in up to 6-h intervals.Directions of migrations were expressed in 36 classes of108 centring on the pond. Eleven spatial reference pointscoinciding with trees and other tall features wereselected on the basis of good visibility (ranging from 3to 10 per localization point) and spacing (White, 1985).A hand-held compass with 18 accuracy was used incombination with triangulation and trigonometric func-tions to plot recordings in a grid system. Localizationpoints and 95% con®dence ellipses were calculated withthe software Locate II, using the Huber estimator oflocation (Lenth, 1981; White & Garrot, 1990). Bearingsto reference points giving a con®dence ellipse > 4p(12.6 m2) were not taken into account, but wheneverpossible 5 or more bearings were included for eachposition ®x. To analyse migratory directions from thepond, standard vectors were calculated and tested fordeviations from random distribution using the Raleightest (Batschelet, 1981). To test for signi®cant differencesin migratory directions between 2 samples, we appliedKuiper's test (Batschelet, 1981). To test whether thecentre point of localizations deviates signi®cantly from

299Migration patterns of newts

Fig. 1. Male T. marmoratus with transmitter implanted laterally on the left side. The antenna is visible behind the left hind leg.

The picture was taken about 30 min after surgery.

the assumed origin (= release site), we applied thebivariate Hotelling 1-sample test (Batschelet, 1981).

A habitat map of the study area was constructedusing aerial photographs taken from an Ultra-LightGlider. Habitat classes were: pasture (grazed by cattleor deer), hedgerow (rows of shrubs, bushes, and trees),avenue (a derelict double hedgerow, with undergrowthremoved), orchard (cultivated trees on pasture), androad (Fig. 2). Minimum convex polygon areas werecalculated with the software HomeRange 2.0.2 to repre-sent activity ranges. G-tests for goodness-of-®t wereused to compare the number of sightings in each classwith the available habitat, de®ned as the proportion ofthe different types within the activity range underconsideration.

RESULTS

General data

Thirty animals were implanted with radio-transmittersand tracked for 491 days, yielding 1544 ®xes and 215observed movements (Table 1). Three implanted newtswere predated by snakes (two grass snakes Natrix natrixand one aesculapian snake Elaphe longissima). Twonewts were recovered dead (one T. marmoratus male,one T. cristatus sub-adult), and two were lost withouttrace. Seven newts who had their transmitters implantedin a way that the antenna was guided out of the bodywhen closing the wound, lost their transmitters. On twooccasions the implanted newt was found alive nearby

(< 20 cm), on ®ve occasions the transmitters were recov-ered without the study individual. Three transmitterscould not be recovered as the refuge of the newt carryingthem was not accessible (Table 1).

Ninety-two percent of all locations could be deter-mined with a 95% con®dence ellipse of < 12.6 m2, and68% of all locations had a con®dence ellipse of < 5 m2.No signi®cant correlations were found between thebody/transmitter mass relationship and the totaldistance moved (Spearman rank correlation: n = 29,rs = 0.07, P > 0.05). The total distances moved did notdiffer between individuals captured at the fence andindividuals captured in the pond (Mann±WhitneyU-test, sexes pooled: T. cristatus Z = 0.65, T. marmor-atus Z = 0.47; P > 0.05 for both). The minimum convexpolygon covered 4803 m2 for T. cristatus (n = 14), and4006 m2 for T. marmoratus (n = 15), and 11 484 m2 forboth species together (Fig. 3).

Temporal and spatial patterns of migrations

In the night following release almost all tracked newtsmigrated over relatively large distances of up to 136.7 mand 98% of the total distance moved in the ®rst 19 h wascovered before 08:00. Movements from day 2 to day 30were over small distances not exceeding 6.8 m (Fig. 4),with the exception of one T. marmoratus female thatmoved 50.3 m on day 2 after covering 7.1 m on day 1.The maximum recorded migration distance of 146 mwas observed for a T. marmoratus male. A skeweddistribution was observed in the maximum distance

R. Jehle and J. W. Arntzen300

N

50 m

Hedgerow

'Avenue'

Orchard

Arable

Paved road

Pasture

Pond

Single trees

Fig. 2. Study area surrounding a breeding pond of the newts T. cristatus and T. marmoratus in Mayenne, France, with a

schematic indication of terrestrial habitat types.

moved away from the pond, with 60% of all observa-tions for T. marmoratus and 68% of all observations forT. cristatus being < 20 m away from the pond (Fig. 5).Signi®cant differences between species or among thesexes within species were not observed (Mann±WhitneyU-test: Z = 0.97, T. cristatus Z = 1.00, T. marmoratusZ = 0.71; P > 0.05 in all cases). The median migrationspeed was 4.6 m/h for both species. The maximumobserved speed was 28.4 m/h for T. cristatus and19.4 m/h for T. marmoratus over 2-h intervals. Nosigni®cant differences were observed between species oramong the sexes within species (Mann±Whitney U-test,Z = 0.54, resp. T. cristatus: Z = 0.65, T. marmoratus:Z = 0; P > 0.05 in all cases). The directionality indexranged from 0.07 to 0.94 for T. cristatus and from 0.13to 0.97 for T. marmoratus. Median di values were 0.52and 0.69, respectively.

After the night following release, for T. cristatus thefrequency of movements was 42% between 00:00 and08:00, 47% between 08:00 and 16:00 and 11% between16:00 and 00:00, for T. marmoratus the correspondingnumbers were 13%, 65% and 22%. The species used thethree daily periods with signi®cant deviation fromrandom expectation (G-test for goodness-of-®t:T. cristatus G = 28.5, d.f. = 2, P < 0.001; T. marmoratusG = 48.5, d.f. = 2, P < 0.001). The temporal activitypattern was signi®cantly different for T. cristatus andT. marmoratus (G-test for independence: G = 24.3,d.f. = 2, P < 0.001).

Among non-moving individuals a preference wasobserved for both species to be situated facing thenorth-western side of the pond (T. cristatus 2908azimuth, standard vector length = 0.39, Raleigh testr = 0.42, P < 0.01; T. marmoratus 310 azimuth, standardvector length = 0.43, r = 0.37, P < 0.001, Fig. 6). Thebivariate Hotelling one-sample test showed a signi®cantdirectional deviation of the localization points from arandom distribution around the pond (T. cristatusT2 = 7.07, P < 0.05; T. marmoratus T2 = 62.40, P < 0.01).Signi®cant differences in migratory directions were ob-served between fence captures and pond captures(Kuiper's test k = 5229, P < 0.01), but not betweenT. cristatus and T. marmoratus (k = 734, P > 0.05).

Shelter and habitat use

Sixty-one (44%) of all localizations were under theleaf-litter without an obvious hiding place, 47 (34%) ofall refuges were burrows of small mammals, 17 (13%)were under the vegetation but above the soil, eight(6%) under logs and seven (5%) were under man-madepiles of stones. No signi®cant difference in refugeselection was observed between the species (G-test forindependence, G = 1.10, d.f. = 4, P > 0.05). Three of therecovered newts (one T. marmoratus male, twoT. cristatus females) were stomach ¯ushed, but no fooditems were recovered.

301Migration patterns of newts

179155

11

50 m

115 3

9110711

N

3

31.5

1.5

11

115

5

32

2

43147

123

5

14711

T. cristatus T. marmoratus

Fig. 3. Minimum convex polygons of registered locations of the newts. The movements of six newts are given as examples (three

T. cristatus, solid symbols; three T. marmoratus, open symbols). Numbers, timespan (h) following release of the individual.

For both species a signi®cant difference between therelative number of locations per habitat and habitatavailability was observed (G-test for goodness-of-®t,T. cristatus G = 89.4, P < 0.001 T. marmoratus G = 86.2,d.f. = 4, P < 0.001). Observations were made in pastureless frequently than might be expected from the spatialavailability of this habitat, whereas observations wereover-represented in hedgerows (Table 2). Trituruscristatus and T. marmoratus differed in their relative useof the terrestrial habitat, with T. cristatus observedmore often in the pasture and hedgerow habitat andT. marmoratus observed more often in the avenuehabitat (G-test for independence: G = 12.0, d.f. = 2,P < 0.01). Signi®cant differences were also obtained inhabitat use between newts obtained at the fence and inthe pond (G-test for independence, G = 80.2, d.f. = 2,P < 0.001, orchard excluded). Treating the two capturemethods separately yields signi®cant differences inhabitat use between the newt species for the fencecaptures only (G-test of independence: fence capturesG = 4.26, d.f. = 1, P < 0.05 (pasture vs avenue, hedgerowand orchard excluded); pond captures: G = 0.07, d.f. = 1,P > 0.05 (hedgerow vs pasture, avenue and orchardexcluded)).

DISCUSSION

Evaluation of the radio-tracking technique

For amphibians, the method of radio-tracking has beenin use for about two decades (van Nuland & Claus,1981; Stephens, Sinsch & Alford, 1994), but the presentstudy is the ®rst on European newts. Mechanical andtelemetric tracking techniques have revealed importantinsights into migration and orientation patterns ofanurans, particularly for toads of the genus Bufo (Smits,1984; van Gelder, Aarts & Staal, 1986; Sinsch, 1987,1988a,b; Kyek, Winding & Palzenberger, 1997).Tracking urodeles with either technique poses problems,because newts and salamanders often have a smallerbody mass than frogs and toads, making transmittertransport more burdensome. Additionally, an elongatedbody shape makes ®tting a tracking device moredif®cult, and migration patterns of urodeles are poorlyknown. In fact, the ®rst telemetric investigations onurodeles have only recently been published for northAmerican salamanders (Madison, 1997, 1998; Madison& Farrand, 1998). Although T. cristatus and T. marmor-atus are big and heavy compared to other newts species,they are at the limit for the implantation of minimum-sized transmitters. Indeed, the recommendation thattransmitters should not exceed 10% of the body mass(White & Garrot, 1990) could not always be adhered to,but for non-¯ying and somewhat inactive animals suchas newts, the strict application of the guideline may beless necessary, as no correlation between weight ratioand magnitude of migration could be found. Further-more, the average transmitter weight±body weight ratiowas < 10%. Newts had the relatively heavy transmittersimplanted for a short period (Table 1), and we includedall radio-tracked newts in our analyses.

R. Jehle and J. W. Arntzen302

6.1%12.5%

82.2%76.5%

4

3.5

3

2.5

2

1.5

1

0.5

00 10 15 20 25 30

Tracked days

Pro

port

ion

of m

oved

dis

tanc

e (%

)

Fig. 4. Temporal distribution of distances moved by the newts T. cristatus (solid bars) and T. marmoratus (open bars) after

release at day 0.

n T.cristatus

T.marmoratus

13

15

0 20 40 60 80 100 120 140 160Distance to pond (m)

Fig. 5. Maximum distances of located newts T. cristatus and

T. marmoratus to the edge of the pond.

The major migratory activity is immediately afterrelease, which might be caused by a ¯ight reaction dueto the treatment. However, there is indirect evidencethat migratory behaviour was not affected by the trans-mitter, as radio-tracked newts were frequently found intheir refuges together with untracked individuals. Acommon problem for radio-tracking studies (White &Garrot, 1990), is that a direct comparison of theinvestigated parameters from tracked and untrackedindividuals is practically impossible, and it is possiblethat implantation affects the behaviour of the newts.The observed loss rates are comparable to thosereported in radio-tracking studies of anurans (e.g.Spieler & Linsenmair, 1998). The death of one sub-adultT. cristatus 2 days after transmitter insertion was prob-ably caused by the surgery. It is, however, notrecommended to keep salamanders in prolongedcaptivity after surgery, as this might cause disorientation(Madison & Farrand, 1998). The other dead newt

observed (number 252) was recorded 22 days aftersurgery, with the last migration record 8 days afterimplantation. Most newts that had their transmitterremoved were released the same day, and no data areavailable on the long-term effects of the surgery.However, ®ve newts were kept for a few days and theirwounds visibly healed, and the released newts werefrequently observed to use similar migration routesagain, and three of the newts were even recaptured inthe same bucket. Frequent recovery of transmitterswithout the study individual was also reported byMadison & Farrand (1998). In our study the only newtswho lost their transmitters were those who had antennaled out of the body during the implantation procedure(Table 1), apparently causing transmitter loss throughthe wound. The two study individuals, who were foundnearby the transmitter, seemed to be in good conditionand their wounds had already closed. The fate of theother individuals is unknown. The antenna is a thin and

303Migration patterns of newts

Table 2. Recorded habitat use in the newts Triturus cristatus and T. marmoratus compared to habitat availability. Habitatavailability is the spatial frequency of habitat types within the activity ranges of the newt species (see text and Fig. 3). Number ofobservations in parentheses

Availability (%) Observed locations (%)

Habitat T. cristatus T. marmoratus T. cristatus T. marmoratus

Pasture 54.0 65.2 27.5 (n = 14) 20.0 (n = 11)Hedgerow 5.8 3.4 54.9 (n = 28) 36.4 (n = 20)Avenue 29.3 28.8 13.7 (n = 7) 43.6 (n = 24)Road 3.7 2.7 0.0 0.0Orchard 7.1 ± 3.9 (n = 2) ±

T. marmoratus

T. cristatus N

50 m

Fig. 6. Directions of location and mean vector for stationary individuals: T. cristatus, solid symbols; T. marmoratus, open

symbols. Half symbols represent one location.

¯exible wire, and no evidence was found for negativein¯uences on burrowing behaviour, or that newts gotcaught on external objects. However, in a follow-upstudy antennas were glued on all transmitters, as thismethod seems better for animal welfare. The reducedtransmitter range (about 30±50 m compared with up to200 m) did not markedly affect the data collectionprocedure. Transmitters with antennas designed forimplantation were not used, as they were not commer-cially available with suf®ciently small weight and size.Technical advancements (e.g. plastic-coated batteries),might enable smaller and lighter transmitters with evenbetter performance to be produced in the near future.

Patterns of migrations

On leaving the pond, the newts migrated above groundfor a few hours, covering relatively large distances.Movements after the night following release mostlyinvolved short distances, were registered throughout theday and were probably performed underground. Migra-tion from the breeding pond takes place in all directionsbut with signi®cant preferences for angles towardscertain habitat types, and is similar to that shown bydrift-fence studies (Dodd & Cade, 1998). Additionally,the present study shows that the preferred angle ofemigration coincides with the preferred terrestrialhabitat at a relatively long distance from the pond. Fora related species, T. dobrogicus, it was shown thatindividuals left the pond in approximately the samedirection year after year, more often than would beexpected by chance from the migration pro®le for thepopulation as a whole, and translocation experimentswith radio-tracked newts indeed suggest ®delity toterrestrial refuge areas. Studies on American salaman-ders have shown that individuals displaced 12 km awaymigrate back to their original sites (Twitty, Grant &Anderson, 1966), and that magneto-reception in combi-nation with the wavelength of the ambient light plays amajor role for orientation mechanisms (e.g. Phillips &Borland, 1994). The initial migrations with high speedand directionality suggests that the studied newts eitherpossess a certain degree of knowledge about their localenvironment, or it requires a geographical positionsense and a compass, the prerequisites for `true naviga-tion' (Phillips, Adler & Borland, 1995). Radio-trackingof the same individuals in > 1 year and experimentalstudies on migratory cues would further elucidate orien-tation mechanisms in European newts (Joly & Miaud,1993).

The importance of terrestrial microhabitat character-istics on the presence/absence of newts has beenrecognized before (van Gelder & Grooten, 1993;Marnell, 1998), but so far quantitative data on refugeuse have been scarce. The frequent use of small mammalburrows was reported for American salamanders(Loredo, van Vuren & Morrison, 1996; Madison, 1997;Madison & Farrand, 1998), and is con®rmed in thisstudy. It may be a common feature for urodeles. The

preferred direction of migration in T. cristatus and T.marmoratus was towards the avenue habitat where baresoil dominated, which suggests that the availability ofunderground refuges might be more important than thehabitat structure above ground.

Single T. cristatus were found to migrate up to1290 m within about 1 year (Kupfer, 1998), and theassumed dispersal distance of 1 km/year (Arntzen &Wallis, 1991; Halley, Oldham & Arntzen, 1996) is about10 times greater than the migration distances obtainedfrom radio-tracking. Triturus cristatus can undertakesigni®cant migrations in autumn (Glandt, 1986), anddispersal events might be rare or take place in springrather than after breeding. Alternatively, the maindispersal might take place at the juvenile stage, as wasshown for some nearctic salamanders and toads (Gill,1978; Breden, 1987). However, in northern France thecolonization of a new pond by T. cristatus was achieved,at least in part, by adults (Arntzen & Teunis, 1993). Inthe present study, there was no evidence of adult newtsdispersing to other sites.

Newts remained underground during hot and dryweather and, unlike radio-tracked toads (Sinsch, 1988b)and salamanders (Madison, 1997), they failed to takeopportunities for surface foraging during rainy nights.The radio-tracked newts did not seem to have fedrecently, a ®nding which is consistent with data fromT. helveticus and T. vulgaris captured at a drift fence(R. Grif®ths, pers. comm.). Three newts (10%) wereeaten by snakes. High predation pressure was alsoobserved in the African frog Hoplobatrachus occipitalis,with 23 (48%) tracked individuals lost to snakes in2.5 months (Spieler & Linsenmair, 1998), and a`surprisingly high predation rate' was reported for thesalamander Ambystoma tigrinum in its subterraneanrefuges (Madison & Farrand, 1998). Predator avoidancemay be among the reasons why T. cristatus andT. marmoratus restrict their surface migrations to shorttime periods.

Habitat preferences

Species-speci®c activity ranges were calculated to deter-mine the area which was frequented by the trackednewts, and to de®ne habitat availability. For T. cris-tatus, the activity range area is highly affected by oneindividual that migrated towards the north-east,however, proportional areas of habitat types are similarfor both species (Table 2), and the species-speci®cdifferences in habitat use should not be biased by theavailability de®nition. As there was a limited number ofindependent ®xes and as individual newts mostly used asingle habitat type, the data were not suitable for, forexample, compositional analysis of habitat use(Aebischer, Robertson & Kenward, 1993).

Triturus marmoratus is thought to be better adaptedto a terrestrial existence than T. cristatus. For example,it has a shorter aquatic breeding period, no aquaticjuveniles and a longer sub-adult terrestrial phase

R. Jehle and J. W. Arntzen304

(Bouton, 1986; Francillon-Vieillot, Arntzen & GeÂraudie,1990). Also, as is typical for terrestrial species,T. marmoratus has a stouter body than the moreaquatic T. cristatus and, unique for the genus Triturus,T. marmoratus has a dorsal rather than a ventral patternof aposematic coloration (Arntzen & Wallis, 1999). Inline with these observations, the species locally show amarked ecological displacement. Triturus cristatus ispredominant in ¯at and open areas and T. marmoratusis predominant in hilly areas with hedgerows and forests(Schoorl & Zuiderwijk, 1981; Arntzen & Wallis, 1991).However, apart from different temporal patterns inshort-distance underground movements, no signi®cantdifferences in migration parameters were observed. Thehypothesis that T. marmoratus is more dependant onthe terrestrial habitat than is T. cristatus could not becon®rmed with the radio-tracking data. The presentstudy revealed that the terrestrial habitat niche segrega-tion between the two newt species is more complex than,for example, the preference of T. marmoratus for adisappearing habitat type which remains unused byT. cristatus. The observation of signi®cant differences inhabitat use for fence captures vs pond captures wasprobably caused by different habitat types present closeto the point of release. The fence might have increasedthe species-speci®c difference as it is facing towards theavenue habitat, for which T. marmoratus has asigni®cant preference.

Many terrestrial vertebrates are not dependant on asingle, but on a suite of habitat types arranged in amosaic (Law & Dickman, 1998). Consequently, themechanisms of competition and specialization betweenspecies may vary enormously across a landscape (e.g.Bowers & Dooley, 1991). For two congeneric toads(Bufo bufo and B. calamita) with much broaderoverlapping geographical ranges than T. cristatus andT. marmoratus, radio-tracking demonstrated clearterrestrial habitat separation (Denton & Beebee, 1994).When forced to breed in syntopic ponds due to succes-sion processes, one species outcompeted the other(Bardsley & Beebee, 1998). Species coexisting in largegeographic areas over long periods may have becomeadapted to different spatial niches. This may not applyto the studied newts as their range overlap is narrowand, perhaps, relatively recent. Additional studies, forexample on terrestrial foraging, could test thehypothesis that T. cristatus is better adapted thanT. marmoratus to exploit the available resources inthose habitats that in the area of range overlap areincreasingly abundant.

Acknowledgements

This study was ®nanced by the Austrian ScienceFoundation (FWF grant P-11852 BIO to W. HoÈdl) andNERC (GR3/10333 to T. Burke, R. Thorpe and J. W.Arntzen), and conducted under permit 97/204 of theMinisteÁre de l'Environment, Paris. We thank A. Ohlerfor help in obtaining the permit, and A. Amezquita,

D. Bachtrog, M. Hable E. Rip¯, I. Strauss andS. Szcepka for help in the ®eld. Thanks also to S. Cornuand J.-P. Deruault for access to their property, and toD. Bardou for friendship and hospitality. RichardGrif®ths, D. Madison and U. Sinsch provided construc-tive comments to an earlier draft of the manuscript.

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