amphibians live longer at higher altitudes but not at higher latitudes

Amphibians live longer at higher altitudes but not athigher latitudes

LIXIA ZHANG and XIN LU*

Department of Zoology, College of Life Sciences, Wuhan University, Wuhan 430072, China

Received 14 November 2011; revised 15 January 2012; accepted for publication 16 January 2012bij_1876 1..10

Why and how organisms differ in life-history strategies across their range is a long-standing topic of interest toevolutionary ecologists. Although many studies have addressed this issue for several life-history traits, such asbody size and clutch size, very few have been made for some others traits, including longevity. In the present study,we performed a comparative study aiming to develop general patterns of geographical variation in longevity ofurodele and anuran amphibians using published information on demographic age derived from skeletochronology.We conducted within-species meta-analyses using datasets of two (ten urodele and 12 anuran species) and multiple(two urodele and nine anuran species) spatially-separated populations and found that maturation, mean, andmaximum age all increased with altitude but not with latitude in each sex of both amphibian groups. Thisgeographical pattern held true across 33 urodele and 86 anuran species at common body sizes, independent ofphylogeny. It is likely that metabolic rate, reproductive investment, and mortality risk, which are the key factorsthat affect longevity as suggested by ageing theory, vary systemically along altitudinal gradients but not alonglatitudinal gradients. The evolutionary causes behind these puzzling patterns deserve further investigation.© 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, ••, ••–••.

ADDITIONAL KEYWORDS: anuran – geographical gradient – lifespan – phylogenetic comparison – urodele.

INTRODUCTION

Longevity as a critical life-history trait of organismsevolves in response to local abiotic and biotic factors(Ricklefs, 2008). Therefore, populations and speciesare expected to differ in this trait across the latitu-dinal and altitudinal range over which environmentalconditions vary. Clarifying the spatial patterns oflongevity may provide useful cues for understandingthe evolution of life history, including senescence, aphenomenon that increasingly interests evolutionaryecologists (Ricklefs, 2008). However, in contrast toother extensively-studied life-history traits, such asbody size (Gaston, Chown & Evans, 2008) and clutchsize (Jetz, Sekercioglu & Böhning-Gaese, 2008), onlya few attempts have been made to analyse longevitychanges at geographical scales, with the majorityof these studies focusing on intraspecific compari-sons (Bauer, 1992; Tatar, Gray & Carey, 1997; Heibo,Magnhagen & Vøllestad, 2005; Norry et al., 2006;

Karl & Fischer, 2009; Munch & Salinas, 2009; Duycket al., 2010) and the minority on interspecific compari-sons (Møller, 2007; Williams et al., 2010). A majorreason for this gap may be that, although it is unsuit-able to explore this question using lifespan records ofanimals kept in captivity or in laboratory settings(Carey & Judge, 2000; de Magalhães & Costa, 2009),longevity data are difficult to collect in naturalpopulations.

Occupying a transitional state in the evolutionof tetrapod vertebrates, amphibians as ectothemicorganisms most likely differ in their ageing mecha-nisms and longevity adaptations compared to othervertebrate classes (Goss, 1994), especially birds andmammals with which several influential evolutionarytheories of senescence have been frequently tested(de Magalhães, Costa & Church, 2007, Møller, 2007;Ricklefs, 2010; Shattuck & Williams, 2010). However,amphibians remain enigmatic with respect to theevolution of longevity, and almost no systematicalwithin- or across-species investigations have beenundertaken with respect to geographical clines in*Corresponding author. E-mail: [email protected]

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longevity for this ectothermic lineage. Skeletochronol-ogy, a method of counting annual growth rings of thebone tissue, makes it possible to determine the actualage of free-living amphibians that exhibit indetermi-nate growth (Halliday & Verrel, 1988). These ringsform as a result of the ensuing slow growth whenanimals experience hibernation in temperate regions(Cogalniceanu & Miaud, 2003; Ma & Lu, 2009) or anannual food shortage derived from dry climates intropical regions (Guarino, Andreone & Angelini, 1998;Khonsue, Matsui & Misawa, 2000). Subsequent to the1970s, data on the age of amphibian species andpopulations, including attempts to analyse intraspe-cific geographical variation in longevity for a fewindividual species (Morrison, Hero & Browning, 2004;Liao & Lu, 2010, 2012; Chen, Yu & Lu, 2011), haveaccumulated. This offers a good opportunity for us toconduct an integrative evaluation of geographical lon-gevity patterns of these animals within and acrossspecies.

In the present study, we test the prediction thatamphibians living at higher latitudes or altitudeshave longer lifespans than their southern andlowland counterparts or congeners. This prediction ismainly based on three critical theories of ageing.

First, the rate-of-living hypothesis predicts short lon-gevity for animals exhibiting high metabolic levels,presumably because the total amount of energyexpended by each gram of tissue is approximatelyconstant (Speakman, 2005) and, from a biochemicalperspective, elevated metabolism might increase theproduction of oxygen free radicals deleterious to cells(Hulbert et al., 2007). Low temperatures almostinvariably reduce the metabolism of ectothermicanimals, including amphibians, as has been demon-strated in laboratory studies (Gillooly et al., 2001;Speakman, 2005). Because ambient temperaturessystemically decrease with increased latitude or alti-tude, we may expect that populations or speciesinhabiting colder regions will live longer. Manystudies of invertebrates have relied on this hypothesisto explain their findings (Karl & Fischer, 2009; Duycket al., 2010). Furthermore, amphibians living inhigher latitudes or altitudes go through a longerperiod hibernation during which the metabolism ofanimals is shut down (Irwin & Lee, 2003; Lu et al.,2008), and thereby would expend less energy per yearand allow for an extension of lifespan.

Second, the resource allocation hypothesis arguesthat short longevity may evolve if animals increaseinvestment in reproduction to maximize lifetimefitness because the resources available for animalsare finite and, in such cases, they have to reducetheir investment in somatic maintenance (Kirkwood,2002; Roff, 2002). At an intraspecific level, amphibian

populations at higher latitudes or altitudes tend toproduce fewer eggs and lighter clutches and thushave a lower total investment in reproduction(Morrison & Hero, 2003; Wells, 2007). This life-history strategy, as a response to adverse climate andforaging conditions (Lessard et al., 2010), would allowhigher latitude or higher altitude animals to put moreenergy into self-maintenance and thus to live longer.

Third, the extrinsic mortality hypothesis proposesthat animals experiencing high mortality risks shouldbegin reproduction at a young age and have a shortlongevity because, in such cases, animals may poten-tially gain reproductive chances before dying of pre-dation or disease (Kirkwood & Rose, 1991; Williamset al., 2006). Evidence for this hypothesis hasbeen obtained from intraspecific (Loison et al., 1999;Reznick et al., 2001) and interspecific (Blanco &Sherman, 2005; de Magalhães, Costa & Church, 2007;Shattuck & Williams, 2010; Ridgway, Richardson &Austad, 2011) longevity comparisons in both ecto-therms and endotherms. Mortality hazards frombiotic interactions, characterized by diversity anddensity of predators and parasites, should becomeweak towards high latitudes or altitudes (Beck et al.,2008). Furthermore, prolonged periods of inactivity ofthese animals during winter dormancy may eliminateextrinsic mortality risks, as observed in hibernatingmammals (Turbill, Bieber & Ruf, 2011).

In the present study, we employ multivariate statis-tical procedures to clarify the independent effect oflatitude and altitude on longevity of amphibians aftercontrolling for body mass, an important phenotypethat is strongly correlated with longevity presumablyowing to lower mass-related lifetime energy expendi-ture of larger body sizes (Speakman, 2005). Our com-parative analyses are conducted at two taxonomiclevels: (1) Between populations of a single species. Thisapproach may minimize the potential effect of geneticdifferences on longevity evolution if species-specificrates of ageing exists. (2) Across different species. Thisapproach may avoid the potential effect of low geneticvariability and between-population gene flow on localadaptation, allowing us to determine whether theobserved geographical longevity patterns are a resultof evolutionary convergence or ecological plasticity.

MATERIAL AND METHODSDATA COLLECTION

Data on the mean and maximum skeletochronologicalage of adult animals (as two longevity measures) wereextracted from published literature worldwide. Wealso took records of age at sexual maturity, mean bodysize, and latitude and altitude of sampling sites fromthese sources. Most studies only provided information

2 L. ZHANG and X. LU

© 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, ••, ••–••

on body length not mass. We converted body lengthinto body mass using an allometric relationshipbetween the two parameters sensu Ricklefs (2010).The latitude and altitude at which a population wasstudied instead of the midpoint geographical range ofa species was used as the measure of geographicallocation. No distinction was made between the north-ern and southern hemispheres assuming a similarvariation in climate regime across the two gradients(Sunday, Bates & Dulvy, 2010). If data were providedfor more than one study year of the same population,or for multiple populations of a single species, weaveraged demographic parameters over years or overpopulations. When calculating these mean values,sample sizes were weighted. A few studies presenteddata on graphs. We saved the graphs in jpeg formatand imported them into IMAGEJ, version 1.42q(NIMH) for data extraction.

Our dataset included 33 urodele and 86 anuranspecies (see Supporting information, Table S1), whichwere distributed into four (40%) of the ten currentlyrecognized families in the order Urodela and 11 (29%)of the 38 families in the order Anura (AmphibiaWeb,2011). These species exhibited great differences inage, body size, and geographical range, providing thepotential for exploring longevity evolution (see Sup-porting information, Table S2). Because urodeles andanurans differ in morphology and lifestyle, we ana-lyzed their data separately. Patterns of resource allo-cation and life-history strategy in amphibians aresex-specific, and so we analyzed the data accordingto sex.

INTRASPECIFIC COMPARISONS

We conducted meta-analyses using COMPREHEN-SIVE META-ANALYSIS software, version 2.0(Biostat, Inc.), aiming to investigate whether therewas a general pattern of within-species geographicalvariation in longevity. First, we attempted to estab-lish data subsets via extracting mean age, SDs, andsample sizes from published reports that comparedthe demographic differences between two populations,which were at: (1) the same altitude but separated byat least 0.5°N in latitude or (2) the same locality (withalmost no difference in latitude) but separated atleast 50 m in altitude. We set these criteria to distin-guish between two populations differing in latitudeor altitude because some demographical traits ofamphibians have been demonstrated to differ signifi-cantly over these ranges (Lu, Li & Liang, 2006).However, only two species in our dataset metthe former, whereas samples meeting the latterwere relatively common (see Supporting informa-tion, Table S3). Therefore, our meta-analysis wasrestricted to the longevity–altitude relationship to

give a standardized mean difference in mean agebetween higher and lower altitudes.

Second, we selected species for which data fromat least five geographical populations that wereseparated by at least 0.5° in latitude and 50 m inaltitude were available. For each species included,a Pearson correlation coefficient was calculatedto assess the relationship between longevity andlatitude and altitude, respectively (see Supportinginformation, Table S4). Then, an overall effect size(grand mean correlation coefficient) was producedbased on meta-analyses.

Small sample sizes may reduce the precision ofestimates of between-study variances. In such a case,the fixed effects model for a meta-analysis is suitable(Borenstein, Hedges & Rothstein, 2007). Publicationbias against nonsignificant or negative results is apotential problem that can affect the overall conclu-sion. For our dataset, however, this question was lessserious because: (1) most studies were not designed totest a priori hypothesis and (2) data for a few specieswere a result of pooled single-site studies.

INTERSPECIFIC COMPARISONS

We performed multivariate regression analyses toinvestigate whether mean and maximum age can beindependently predicted by latitude and altitude afteraccounting for body mass. Life-history theory predictsthat delayed maturation may lead to greater longevi-ties (Roff, 2002). Thus we also analyzed age at matu-rity in relation to the three parameters usingmultivariate approaches.

In across-species comparisons, trait values cannotbe considered independent from a statistical point ofview because closely-related species may presentsimilar trait values as a result of shared ancestry(Felsenstein, 1985; Freckleton, Harvey & Pagel,2002). Ignoring phylogenetic relationships in statisti-cal analyses of multi-species data can lead to inflationof the number of degrees of freedom and increasedType I error rates (Rohlf, 2006). When a phylogeneticeffect is absent, results from conventional and plylo-genetic procedures will converge (Revell, 2010). Forthis reason, we detected a phylogenetic signal byphylogenetic generalized least-squares models forthe relationship between longevity and its potentialpredictor variables. We downloaded cytochrome bsequences for urodeles and 16S rRNA sequencesfor anurans from GenBank. These sequences werealigned using CLUSTALX, version 1.83 to construct amaximum-likelihood phylogenetic tree by PHYML(Guindon & Gascuel, 2003) (see Supporting informa-tion, Fig. S1). The relationships between species inour phylogenetic trees were generally similar to pre-viously published phylogenies (Pyron & Wiens, 2011).

GEOGRAPHICAL VARIATION IN AMPHIBIAN LIFESPAN 3


Then we detected a phylogenetic signal using a phy-logenetic generalized least-squares model with theAPE package implemented in R (Martins & Hansen,1997). This method, by estimating the maximum like-lihood value of the parameter l to provide a properadjustment to the variance–covariance matrix, mayreflect the phylogenetic signal of the residuals of thelinear model (Freckleton et al., 2002; Revell, 2010).The values of l vary between 0 and 1, with traitevolution being completely unrelated with phylogenyif l = 0 but strongly dependent on phylogeny if l = 1.We forced l to 0 and 1 to produce two null models.The difference between the estimated value of l andthat of the null models was tested using the likelihoodratio (a chi-squared statistic with one degree offreedom). We considered a phylogenetic signal to bepresent if l differed significantly from 0 even thoughit was statistically different from 1 (Revell, 2010).

We set maturation, mean, and maximum age as thedependent variables, respectively, with latitude, alti-tude, and body mass as covariates. Before the analy-ses, raw data were transferred to their naturallogarithm to correct for heterocedasticity and allom-etric effects. Observed age may be affected by sam-pling effort (measured as sample size). Only a fewstudies may have been compromised by small samplesizes. Separate multivariate analyses using the datasubsets with sample size as an additional exploratoryterm showed that, in all cases, sample size failed toenter the models as a significant predictor of longev-ity, and so we did not took this factor into account.

Statistical analyses were conducted using R, version2.13.0 (R Development Core Team, 2011).

RESULTS

Meta-analyses for intraspecific comparisons based onthe fixed effect model showed a significant overalleffect size for males and females in both amphibianclasses, and the positive effects indicated that, amonganuran species, individuals in high-altitude popula-tions lived longer compared to those in low-altitudepopulations (Table 1). For the species with age datafrom at least five widely separated sites, the meta-analyses revealed that, in all cases, the overall grandmean correlation coefficient was not significantbetween longevity and latitude, although significantlypositive between longevity and altitude (Table 1).

Phylogenetic generalized least-squares models forage in relation to other predictor variables produced lvalues that were significantly or nearly significantlydifferent from zero but significantly smaller than 1(see Supporting information, Table S5). Therefore, wepresent the results of both phylogenetic and nonphy-logenetic analyses for across-species comparisons.Both analyses showed that, controlling for bodymass, mean and maximum age of both males andfemales across urodele and anuran species were inde-pendent of latitude, although significantly increasedtowards higher altitudes; age at maturity followed asimilar tendency (Fig. 1, Table 2; see also Supportinginformation Tables S6, S7). Pearson’s correlation

Table 1. Results of meta-analyses with the fixed effects model for the relationship between mean age and geographicalgradient in amphibians

Pooledeffect size 95% CI Z P

Species(N)

High versus low altitudeUrodele male 0.18 0.03–0.34 2.33 0.02 10Urodele female 0.33 0.15–0.50 3.64 < 0.01 10Anuran male 0.63 0.51–0.76 9.78 < 0.01 12Anuran female 0.79 0.61–0.97 8.78 < 0.01 12

Across latitudesUrodele male -0.23 -0.64–0.32 0.81 0.42 2Urodele female -0.32 -0.72–0.23 0.12 0.25 2Anuran male 0.06 -0.27–0.38 0.35 0.73 9Anuran female 0.03 -0.30–0.36 0.19 0.85 9

Across altitudesUrodele male 0.86 0.63–0.95 4.51 < 0.01 2Urodele female 0.93 0.80–0.98 5.78 < 0.01 2Anuran male 0.84 0.71–0.92 7.00 < 0.01 9Anuran female 0.85 0.73–0.92 7.24 < 0.01 9

CI, confidence interval.



analyses revealed a significant positive correlation ofmaturation age with either mean age (33 urodelespecies: male r = 0.59, female r = 0.61; 86 anuranspecies: male r = 0.70, female r = 0.76; all P < 0.01) ormaximum age (32 urodele species: male r = 0.48,female r = 0.53; 81 anuran species: male r = 0.44,female r = 0.57; all P < 0.001).

DISCUSSION

Several longevity-related environmental factors andlife-history strategies of amphibians appear to changein a consistent way along both geographical gradi-ents. Cold climates characterize both high altitudesand latitudes; northern amphibians, such as highlandones, tend to have an overall low investment in repro-duction (Morrison & Hero, 2003; Lu, 2004; Wells,2007); risks from predation and parasitism are con-

sidered to be lower in temperate habitats than thetropics, as well as at higher compared to lower alti-tudes (Schemske et al., 2009). When applying therate-of-living, resource allocation and extrinsic mor-tality theories of ageing to amphibians, we mayexpect to observe increased longevity in amphibianstowards both higher altitudes and latitudes; however,we found an increase in maturation, mean, andmaximum age only along altitudinal gradients. Thispattern was consistently detected in within- andacross-species comparisons for each sex of bothamphibian lineages when body mass was statisticallycontrolled for. The low phylogenetic effect on thecorrelates of across-species longevity implies a highecological plasticity of longevity prevalent acrosstaxa, as suggested by several studies that show aweak phylogenetic signal for ecological traits (Losos,2008).

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 1 2 3 4 5 6 7 8 9

r = 0.73r = 0.66

n = 33

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 1 2 3 4 5 6 7 8 9

r = 0.40r = 0.43

n = 86

Ln m

ean

age

(yr)

0.0

0.5

1.0

1.5

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2.5

3.0

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0 1 2 3 4 5 6 7 8 9

r = 0.33r = 0.33

n = 81

0.0

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2.0

2.5

3.0

3.5

0 1 2 3 4 5 6 7 8 9

r = 0.65r = 0.65

n = 33

Ln m

axim

um a

ge (

yr)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

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0 1 2 3 4 5 6 7 8 9

r = 0.89r = 0.72

n = 33

0.0

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1.0

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3.5

0 1 2 3 4 5 6 7 8 9

r = 0.52r = 0.53

n = 86

Ln m

atur

atio

n ag

e (y

r)

Ln altitude (m)

AnuranUrodele

Figure 1. Mean, maximum, and maturation age of amphibians as a function of altitude. Log-transformed raw data withpredicted regression lines of general linear models (latitude and body mass were set as their mean, P < 0.003 for allPearson correlation coefficients). Males are indicated by closed circles and thick lines, females are indicated by open circlesand thin lines.



Our finding is intriguing. Munch & Salinas (2009)demonstrated an increased lifespan with latitudebased on 67 free-living ectothermic species. Neverthe-less, their study did not take altitude into accountand their dataset only included 12 amphibian species,of which only five followed the prediction aboutlifespan in relation to ambient temperatures. Ifthe observed altitudinal trend in longevity canadequately be explained by the three hypotheses ofageing, as suggested by some studies (Karl & Fischer,2009; Duyck et al., 2010), why is it not mirrored tolatitudinal gradients? A possible cause is that factorspotentially related to longevity adaptation mightcovary with altitude but not with latitude. Ambienttemperature lapse rates are steeper along altitudinalthan latitudinal gradients during the summer whenamphibians are active (Dillon, Frazier & Dudley,2006). As a result, metabolism of amphibians, whichis highly sensitive to ambient temperature (Wells,2007), would fluctuate less extensively across lati-tudes than altitudes. Several studies reported that,across species, tropical anurans had lower metabolicrates than temperate ones (Hutchison, Whitford &Kohl, 1968; Whitford, 1973; Duellman & Trueb,1994). If this pattern is generally true, despite thefact that the underlying mechanism remains unclear(Wells, 2007), it might lead to a longer lifespan ofsouthern populations and species. Animals exposed tohypoxia have low metabolic levels (Hou & Huang,1999; Owerkowicz, Elsey & Hicks, 2009; Harrison &

Haddad, 2011), and oxygen availability is consideredto be responsible for the longer lifespans of animalsinhabiting oxygen-poor environments (Ma, Lu &Merilä, 2009; Buttemer, Adele & Costantini, 2010).Typically, systematic changes in atmospheric oxygenpartial pressure only occur across altitudinalgradients.

Phylogenetic studies of ecological traits are oftenscale-dependent (Krasnov, Poulin & Mouillot, 2011).Amphibians exhibit a stronger inter-populationgenetic divergence compared to other vertebrate lin-eages as a result of the patchy distribution of breed-ing habitats and poor dispersal abilities (Palo et al.,2004). This is particularly true for latitudinal gra-dients that occur over considerably larger spatialdistances than altitudinal scales. As a result, differ-ent phylogenetic histories might lead to a diversepattern of life-history traits, including lifespanamong populations and species across this geo-graphical dimension (Miaud & Merilä, 2001). Thispotentially makes it impossible to find a latitudinallongevity cline. Another possibility for the absence oflongevity–latitude association is that microclimaticconditions do not vary systematically with latitudebut with altitude. For example, Central and SouthAmerica have extensive dry forests, whereas Africadoes not. The influence of different biotic and abioticfactors on lifespan responses to the microcli-matic differences across latitudes could lead to ourfailure to detect a systematical pattern along this

Table 2. Results of phylogenetic generalized least-squares models to determine whether mean, maximum, and matura-tion age of amphibians (28 urodele and 67 anuran species) depended on latitude and altitude after controlling for bodymass

Mean age Maximum age Maturation age

b ± SE t P b ± SE t P b ± SE t P

Urodele maleLatitude 0.05 ± 0.36 0.14 0.89 -0.13 ± 0.11 1.24 0.22 0.38 ± 0.32 1.18 0.25Altitude 0.10 ± 0.04 2.79 0.01 0.09 ± 0.02 3.49 < 0.001 0.10 ± 0.03 3.28 < 0.001Body mass 0.12 ± 0.05 2.35 0.03 0.20 ± 0.04 5.03 < 0.001 0.06 ± 0.04 1.31 0.20

Urodele femaleLatitude -0.19 ± 0.35 0.54 0.60 -0.09 ± 0.09 0.94 0.35 0.39 ± 0.38 1.03 0.31Altitude 0.09 ± 0.03 2.72 0.01 0.10 ± 0.02 4.40 < 0.001 0.08 ± 0.04 2.18 0.04Body mass 0.14 ± 0.05 2.89 0.01 0.18 ± 0.03 5.79 < 0.001 0.09 ± 0.05 1.83 0.08

Anuran maleLatitude 0.06 ± 0.41 0.15 0.88 -0.09 ± 0.13 0.71 0.48 -0.01 ± 0.12 0.10 0.92Altitude 0.08 ± 0.04 1.95 0.06 0.09 ± 0.03 2.91 < 0.001 0.06 ± 0.03 2.20 0.03Body mass 0.10 ± 0.06 1.74 0.10 0.21 ± 0.05 4.39 < 0.001 0.11 ± 0.04 2.48 0.02

Anuran femaleLatitude -0.11 ± 0.40 0.27 0.79 -0.13 ± 0.12 1.07 0.29 -0.03 ± 0.11 0.30 0.77Altitude 0.09 ± 0.04 2.23 0.04 0.11 ± 0.03 3.57 < 0.001 0.08 ± 0.03 3.10 < 0.001Body mass 0.12 ± 0.06 2.08 0.05 0.21 ± 0.04 5.28 < 0.001 0.12 ± 0.03 3.66 < 0.001



geographical dimension when including species fromdifferent continents in a single analysis.

Geographical variation in lifespan of amphibianscan be caused by multiple sources of selection.Despite the general tendency for animals includingamphibians to invest less in reproduction (Morrison &Hero, 2003; Lu, 2004; Wells, 2007) and experiencereduced mortality risks (Schemske et al., 2009)towards both higher latitudes and altitudes, it islikely that there are some subtle differences in thesefactors between the two geographical dimensions.Moreover, the relative strength of the mechanismssuggested by the three hypotheses of ageing mightdiffer according to different gradients. Therefore,straightforward speculations with respect to latitudi-nal and altitudinal longevity clines from the hypoth-eses of ageing should be made with caution. We willonly be able to clarify the mechanisms behind theinconsistent variation in amphibian lifespan betweenthe two geographical gradients when detailed data onmultiple lifespan-related variables become availablefor more species.

Altitudinal clines in other life-history traits do notmatch with latitudinal clines has been reported in avariety of organisms. For example, the observationthat insects have smaller body sizes at high-altitudesites clearly contradicts the latitudinal trend (Dillonet al., 2006). It is a general rule that birds breeding innorthern regions lay larger clutches than those insouthern regions (Jetz et al., 2008), although avianclutch sizes typically decrease towards higher alti-tudes (Badyaev, 1997; Badyaev & Ghalambor, 2001;Lu, 2005, 2008). Moreover, tropical birds have a slowlife history characterized by low metabolism, highsurvival, and great longevity compared to their tem-perate congeners (Williams et al., 2010); however, theslow pace of life has evolved in high-altitude birds(Bears, Martin & White, 2009). Therefore, our findingcould be of general interest to ecologists who work onthe evolution of life history.

Additionally, regardless of the underlying mecha-nisms, our findings have implication for species con-servation. In the past three decades, amphibians haveexperienced an intense population decline worldwideand this is particularly the case for taxa living at highaltitudes (Stuart et al., 2004). Species with delayedmaturation and extended longevity are highly vulner-able to extinction risks because they have low popu-lation turnover rates and are less able to compensatefor increased mortality (Purvis et al., 2000). Thisposes a greater challenge for conserving high altitudeamphibians. On the other hand, global climatewarming may cause an alteration in the lifespan ofspecies and related life-history traits. This, as arguedby Munch & Salinas (2009), will affect structure andfunction of ecosystems.

ACKNOWLEDGEMENTS

We are grateful to Y. H. Li and G. Y. Zhang for theirassistance in collecting data from literature, as wellas Drs Dale Roberts and Daniele Seglie, and threeanonymous referees for their comments on an earlierdraft of the manuscript. Financial support was pro-vided by the National Sciences Foundation of China(no. 30425036).

REFERENCES

AmphibiaWeb. 2011. Amphibian species lists. Availableat: http://amphibiaweb.org/lists/index.shtml Accessed 27December 2011.

Badyaev AV. 1997. Avian life history variation alongaltitudinal gradients: an example with cardueline finches.Oecologia 111: 365–374.

Badyaev AV, Ghalambor CK. 2001. Evolution of life histo-ries along elevational gradients: trade-off between parentalcare and fecundity. Ecology 82: 2948–2960.

Bauer G. 1992. Variation in life span and size of the fresh-water pearl mussel. Journal of Animal Ecology 61: 425–436.

Bears H, Martin K, White GC. 2009. Breeding inhigh-elevation habitat results in shift to slower life-historystrategy within a single species. Journal of Animal Ecology78: 365–375.

Beck E, Kottke I, Bendix J, Makeschin F, Mosandl R.2008. Gradients in a tropical mountain ecosystem: a syn-thesis. In: Beck E, Bendix J, Kottke I, Makeschin F,Mosandl R, eds. Gradients in a tropical mountain ecosystemof Ecuador. Ecological Studies 198: 451–463.

Blanco MA, Sherman PW. 2005. Maximum longevities ofchemically protected and non-protected fishes, reptiles, andamphibians support evolutionary hypotheses of aging.Mechanisms of Ageing and Development 126: 794–803.

Borenstein M, Hedges L, Rothstein H. 2007. Meta-analysis: fixed effect vs. random effects. Englewood Cliffs,NJ: Biostat.

Buttemer WA, Adele D, Costantini D. 2010. From bivalvesto birds: oxidative stress and longevity. Functional Ecology24: 971–983.

Carey JR, Judge DS. 2000. Longevity records. Odense:Odense University Press.

Chen W, Yu TL, Lu X. 2011. Age and body size of Ranakukunoris, a high-elevation frog native to the Tibetanplateau. Herpetological Journal 21: 149–151.

Cogalniceanu D, Miaud C. 2003. Population age structureand growth in four syntopic amphibian species inhabiting alarge river floodplain. Canadian Journal of Zoology 81:1096–1106.

Dillon ME, Frazier MR, Dudley R. 2006. Into thin air:physiology and evolution of alpine insects. Integrative andComparative Biology 46: 49–61.

Duellman WE, Trueb L. 1994. Biology of Amphibians.Baltimore, MD: John Hopkins University Press.

Duyck PF, Kouloussis NA, Papadopoulos NT, Quilici S,Wang JL, Jiang CR, Müller HG, Carey JR. 2010.



Lifespan of a Ceratitis fruit fly increases with higher alti-tude. Biological Journal of the Linnean Society 101: 345–350.

Felsenstein J. 1985. Phylogenies and the comparativemethod. American Naturalist 125: 1–15.

Freckleton RP, Harvey PH, Pagel M. 2002. Phylogeneticanalysis and comparative data: a test and review of evi-dence. American Naturalist 160: 712–726.

Gaston KJ, Chown SL, Evans KL. 2008. Ecogeographicalrules: elements of a synthesis. Journal of Biogeography 35:483–500.

Gillooly JF, Brown JH, West GB, Savage VM, CharnovEL. 2001. Effects of size and temperature on metabolic rate.Science 293: 2248–2251.

Goss RJ. 1994. Why study ageing in cold-blooded verte-brates? Gerontology 40: 65–69.

Guarino FM, Andreone F, Angelini F. 1998. Growth andlongevity by skeletochronological analysis in Mantidactylusmicrotympanum, a rain-forest anuran from southern Mada-gascar. Copeia 1998: 194–198.

Guindon S, Gascuel O. 2003. A simple, fast and accuratealgorithm to estimate large phylogenies by maximum like-lihood. Systematic Biology 52: 696–704.

Halliday TR, Verrell PA. 1988. Body size and age inamphibians and reptiles. Journal of Herpetology 22: 253–265.

Harrison JF, Haddad GG. 2011. Effects of oxygen on growthand size: synthesis of molecular, organismal, and evolution-ary studies with Drosophila melanogaster. Annual Review ofPhysiology 73: 95–113.

Heibo E, Magnhagen C, Vøllestad LA. 2005. Latitudinalvariation in life-history traits in Eurasian perch. Ecology86: 3377–3386.

Hou PC, Huang SP. 1999. Metabolic and ventilatoryresponses to hypoxia in two altitudinal populations of thetoad, Bufo bankorensis. Comparative Biochemistry andPhysiology A 124: 413–421.

Hulbert AJ, Pamplona R, Buffenstein R, Buttemer WA.2007. Life and death: metabolic rate, membrane composi-tion, and life span of animals. Physiological Reviews 87:1175–1213.

Hutchison VH, Whitford WG, Kohl M. 1968. Relation ofbody size and surface area to gas exchange in anurans.Physiological Zoology 41: 65–85.

Irwin JT, Lee JRE. 2003. Geographic variation in energystorage and physiological responses to freezing in the graytreefrogs Hyla versicolor and H. chrysoscelis. Journal ofExperimental Biology 206: 2859–2867.

Jetz W, Sekercioglu CH, Böhning-Gaese K. 2008. Theworldwide variation in avian clutch size across species andspace. PLoS Biology 6: e303.

Karl I, Fischer K. 2009. Altitudinal and environmentalvariation in lifespan in the copper butterfly Lycaena tityrus.Functional Ecology 23: 1132–1138.

Khonsue W, Matsui M, Misawa Y. 2000. Age determina-tion by skeletochronology of Rana nigrovittata, a frogfrom tropical forest of Thailand. Zoological Science 17: 253–257.

Kirkwood TBL. 2002. Evolution of ageing. Mechanisms ofAgeing and Development 123: 737–745.

Kirkwood TBL, Rose MR. 1991. Evolution of senescence:late survival sacrificed for reproduction. PhilosophicalTransactions of the Royal Society of London Series B, Bio-logical Sciences 332: 15–24.

Krasnov BR, Poulin R, Mouillot D. 2011. Scale-dependenceof phylogenetic signal in ecological traits of ectoparasites.Ecography 34: 114–122.

Lessard JP, Sackett TE, Reynolds WN, Fowler DA,Sanders NJ. 2010. Determinants of the detrital arthropodcommunity structure: the effects of temperature, resources,and environmental gradients. Oikos 120: 333–343.

Liao WB, Lu X. 2010. A skeletochronological estimation ofage and body size by the Sichuan torrent frog (Amolopsmantzorum) between two populations at different altitudes.Animal Biology 60: 479–489.

Liao WB, Lu X. 2012. Adult body size = f (initial size + growthrate ¥ age): explaining the proximate cause of Bergman’scline in a toad along altitudinal gradients. EvolutionaryEcology DOI: 10.1007/s10682-011-9501-y.

Loison A, Festa-Bianchet M, Gaillard JM, Jorgenson JT,Jullien JM. 1999. Age-specific survival in five populationsof ungulates: evidence of senescence. Ecology 80: 2539–2554.

Losos JB. 2008. Phylogenetic niche conservatism, phyloge-netic signal and the relationship between phylogeneticrelatedness and ecological similarity among species. EcologyLetters 11: 995–1007.

Lu X. 2004. Annual cycle of nutritional organ mass in atemperate-zone anuran, Rana chensinensis, from northernChina. Herpetological Journal 14: 9–12.

Lu X. 2005. Reproductive ecology of blackbirds (Turdusmerula maximus) in a high-altitude location, Tibet. Journalof Ornithology 146: 72–78.

Lu X. 2008. Breeding ecology of an Old World high-altitudewarbler, Phylloscopus affinis. Journal of Ornithology 149:41–47.

Lu X, Li B, Li Y, Ma XY, Fellers GM. 2008. Pre-hibernationenergy reserves in a temperate anuran, Rana chensinensis,along a relatively fine elevational gradient. HerpetologicalJournal 18: 97–102.

Lu X, Li B, Liang JJ. 2006. Comparative demography of atemperate anuran, Rana chensinensis, along a relativelyfine elevational gradient. Canadian Journal of Zoology 84:1789–1795.

Ma XY, Lu X. 2009. Sexual size dimorphism in relation to ageand growth based on skeletochronological analysis in aTibetan frog. Amphibia-Reptilia 30: 351–359.

Ma XY, Lu X, Merilä J. 2009. Altitudinal decline of body sizein a Tibetan frog Nanorana parkeri. Journal of Zoology,London 279: 364–371.

de Magalhães JP, Costa J. 2009. A database of vertebratelongevity records and their relation to other life-historytraits. Journal of Evolutionary Biology 22: 1770–1774.

de Magalhães JP, Costa J, Church GM. 2007. An analysisof the relationship between metabolism, developmentalschedules, and longevity using phylogenetic independent



contrasts. Journal of Gerontology A: Biological Sciences andMedical Sciences 62: 149–160.

Martins EP, Hansen TF. 1997. Phylogenies and the com-parative method: a general approach to incorporating phy-logenetic information into the analysis of interspecific data.American Naturalist 149: 646–667.

Miaud C, Merilä J. 2001. Local adaptations or environmen-tal induction? causes of populations differentiation in alpineamphibians. Biota 2/1: 31–50.

Møller AP. 2007. Senescence in relation to latitude andmigration in birds. Journal of Evolutionary Biology 20:750–757.

Morrison C, Hero JM. 2003. Geographic variation in lifehistory characteristics of amphibians: a review. Journal ofAnimal Ecology 72: 270–279.

Morrison C, Hero JM, Browning J. 2004. Altitudinalvariation in the age at maturity, longevity, and reproductivelifespan of anurans in subtropical Queensland. Herpeto-logica 60: 34–44.

Munch SB, Salinas S. 2009. Latitudinal variation in lifespanwithin species is explained by the metabolic theory ofecology. Proceedings of the National Academy of Sciences ofthe United States of America 106: 13860–13864.

Norry FM, Sambucetti P, Scannapieco AC, Loeschcke V.2006. Altitudinal patterns for longevity, fecundity andsenescence in Drosophila buzzatii. Genetica 128: 81–93.

Owerkowicz T, Elsey RM, Hicks JW. 2009. Atmosphericoxygen level affects growth trajectory, cardiopulmonaryallometry and metabolic rate in the American alligator(Alligator mississippiensis). Journal of ExperimentalBiology 212: 1237–1247.

Palo JU, Schmeller DS, Laurila A, Primmer CR, KuzminSL, Merilä J. 2004. High degree of population subdivisionin a widespread amphibian. Molecular Ecology 13: 2631–2644.

Purvis A, Gittleman JL, Cowlishaw G, Mace GM. 2000.Predicting extinction risk in declining species. Proceedingsof the Royal Society of London Series B, Biological Sciences267: 1947–1952.

Pyron RA, Wiens JJ. 2011. A large-scale phylogeny ofAmphibia with over 2,800 species, and a revised classifica-tion of extant frogs, salamanders, and caecilians. MolecularPhylogenetics and Evolution 61: 543–583.

R Development Core Team. 2011. R: a language and envi-ronment for statistical computing. Vienna: R Foundation forStatistical Computing.

Revell LJ. 2010. Phylogenetic signal and linear regressionon species data. Methods in Ecology and Evolution 1: 319–329.

Reznick D, Buckwalter G, Groff J, Elder D. 2001. Theevolution of senescence in natural populations of guppies(Poecilia reticulata): a comparative approach. ExperimentalGerontology 36: 791–812.

Ricklefs RE. 2008. The evolutionary ecology of senescence:the evolution of senescence from a comparative perspective.Functional Ecology 22: 379–392.

Ricklefs RE. 2010. Life-history connections to rates of agingin terrestrial vertebrates. Proceedings of the NationalAcademy of Sciences of the United States of America 107:10314–10319.

Ridgway ID, Richardson CA, Austad SN. 2011. Maxi-mum shell size, growth rate, and maturation age correlatewith longevity in bivalve mollusks. Journal of GerontologyA: Biological Sciences and Medicine Sciences 66: 183–190.

Roff DA. 2002. Life history evolution. Sunderland, MA:Sinauer Associates.

Rohlf FJ. 2006. A comment on the phylogenetic correction.Evolution 60: 1509–1515.

Schemske DW, Mittelbach GG, Cornell HV, Sobel JM,Roy K. 2009. Is there a latitudinal gradient in theimportance of biotic interactions? Annual Review of Ecology,Evolution, and Systematics 40: 245–269.

Shattuck MR, Williams SA. 2010. Arboreality has allowedfor the evolution of increased longevity in mammals.Proceedings of the National Academy of Sciences of theUnited States of America 107: 4635–4639.

Speakman JR. 2005. Body size, energy metabolism andlifespan. Journal of Experimental Biology 208: 1717–1730.

Stuart SN, Chanson JS, Cox NA, Young BE, RodriguesASL, Fischman DL, Waller W. 2004. Status and trends ofamphibian declines and extinctions worldwide. Science 302:1783–1786.

Sunday JM, Bates AE, Dulvy NK. 2010. Global analysis ofthermal tolerance and latitude in ectotherms. Proceedings ofthe Royal Society of London Series B, Biological Sciences278: 1823–1830.

Tatar M, Gray DW, Carey JR. 1997. Altitudinal variationfor senescence in Melanoplus grasshoppers. Oecologia 111:357–364.

Turbill C, Bieber C, Ruf T. 2011. Hibernation is associatedwith increased survival and the evolution of slow life his-tories among mammals. Proceedings of the Royal Society ofLondon Series B, Biological Sciences 278: 3355–3363.

Wells KD. 2007. The ecology and behavior of Amphibians.Chicago, IL: University of Chicago Press.

Whitford WG. 1973. The effects of temperature onrespiration in the Amphibia. American Zoologist 13: 505–512.

Williams JB, Miller RA, Harper JM, Wiersma P. 2010.Functional linkages for the pace of life, life-history, andenvironment in birds. Integrative and Comparative Biology50: 855–868.

Williams PD, Day T, Fletcher Q, Rowe L. 2006. Theshaping of senescence in the wild. Trends in Ecology andEvolution 21: 458–463.



SUPPORTING INFORMATION

Additional Supporting Information may be found in the online version of this article:

Figure S1. Maximum likelihood phylogenetic trees for 28 urodele (a) and 67 anuran species (b).Table S1. List of species and data sources used in the present study.Table S2. A summary of dataset used in the present study.Table S3. Results of meta-analyses with the fixed effects model for the relationship between mean age andaltitude in amphibians where the data from two geographically-separated populations were available.Table S4. Results of meta-analyses with the fixed effects model for the relationships between mean age andlatitude or altitude in amphibians where the data from at least five geographically-separated populations wereavailable.Table S5. Significance of phylogenetic signal l estimated by phylogenetic generalized least-squares models forlongevity in relation to body mass, latitude, and altitude in amphibians.Table S6. Results of general linear mixed model analyses to determine whether mean, maximum, andmaturation age of amphibians depended on latitude and altitude after controlling for body mass.Table S7. Results of general linear model analyses to determine whether mean, maximum and maturation ageof amphibians depended on latitude and altitude after controlling for body mass.

Please note: Wiley-Blackwell are not responsible for the content or functionality of any supporting materialssupplied by the authors. Any queries (other than missing material) should be directed to the correspondingauthor for the article.



amphibians live longer at higher altitudes but not at higher latitudes

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