Geographic Variation in Age and Size at Maturity in a Small Australian Viviparous Skink

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Geographic Variation in Age and Size at Maturity in a Small Australian Viviparous Skink

Author(s): Erik Wapstra, Roy Swain, Julianne M. O'ReillySource: Copeia, Vol. 2001, No. 3 (Aug. 6, 2001), pp. 646-655Published by: American Society of Ichthyologists and HerpetologistsStable URL: http://www.jstor.org/stable/1448287

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Copeia,001(3),pp. 646-655

Geographic Variation in Age and Size at Maturity in a Small Australian

Viviparous Skink

ERIK WAPSTRA,RoY SWAIN,ANDJULIANNEM. O'REILLY

Age andsize atmaturity repivotal ife-history raitsthrough heir effects onother

key traits,such as annual and lifetime fecundity.We used skeletochronology o in-

vestigatethe relationshipsamongsize (snout-ventlength), age, andmaturityn two

populationsof a smallviviparousskink,Niveoscincuscellatus,rom Tasmania,Aus-

tralia. The species occupies a wide geographicand climaticrangewithinthe tem-

perate zone, and we chose populations from the climatic extremes of this range.Growth n N. ocellatuss rapid earlyin life but slows considerablyaftermaturityn

both sexes. Withinsites, we found no difference in growthpatterns or length at

maturitybetween the sexes. However,there were large differences between sites.

At our "warm" ite, lizardswere mature at three years of age at a relativelysmall

size. Lizardsfrom the "cold"site typicallydelayed maturityuntil their fourthyear

(althoughsome males were matureat the end of their thirdyear);as a result,theywere significantly argerat maturityand thereafterremainedlargerfor any age than

did warm-site izards. These patternsare consistent withpredictionsfrom models

of the proximate influence of the thermalenvironmenton growthand maturity

patterns n squamatereptiles.Lizards romthe cold site areborn laterin the season

and have a shorteractivityseason prior to obligatorywinterhibernation,and con-

ditions for growthare less favorable n any particularmonth thanat the warm site.

Because delaying maturitys costlyto currentfecundity,we suggestthatin N. ocel-

latus ifetime fecundityis enhanced at the cold site by additionalgrowthand gainsin futurefecunditythroughthe relationshipbetweenbody lengthandreproductive

output.

LONGEVITY and age of attainment of sexual

maturity vary widely among species of

squamates (Tinkle et al., 1970; Dunham et al.,

1988) or between populations of the same spe-cies (e.g., Ballinger, 1979; Grant and Dunham,

1990; Tinkle et al., 1993). Age at maturity is a

pivotal trait because fitness is often more sensi-

tive to changes in this trait than to any other

(Stearns, 1992; Bernardo, 1993; Ferguson and

Talent, 1993). Any variation in size and age atmaturity has important implications for the life-

history pattern of a particular population be-

cause after maturity energy is diverted awayfrom growth, maintenance, and storage and di-

rected toward reproduction (Andrews, 1982;

Adolph and Porter, 1996; Rohr, 1997). There

are two general patterns of maturation in liz-

ards: "early" versus "late" reproduction (Tinkleet al., 1970), with natural selection favoring dif-

ferent genotypes according to environmental

conditions(James, 1991).Selection pressures to mature early must be

balanced by trade-offs with other fitness com-

ponents to explain delayed maturity (Stearns,

1992). In species in which fecundity is closelyrelated to female body size, a delay in repro-duction, although costly to current reproduc-tion, may result in attainment of larger body

size and greater gains in future fecundity(Stearns and Crandall, 1981; Bruce and Hair-

ston, 1990; Stearns, 1992). Species or popula-tions of species that delay maturity tend to be

large-bodied and long-lived, whereas early ma-

turity tends to be associated with small body size

and low survival rates (Tinkle et al., 1970; Shine

and Charnov, 1992; Schwarzkopf, 1994).Differences between populations may result

from several factors, including proximate fac-tors acting on the population. Many studies ad-

dress the role of temperature as a proximatesource of variation in life histories of ecto-

therms (e.g., Conover and Present, 1990; Ber-

nardo, 1994), including those of lizards (e.g.,

Adolph and Porter 1993; Ferguson and Talent,

1993; Sinervo and Adolph, 1994). The thermal

environment determines the growing season for

many reptiles. Reptiles at low latitudes or low

elevations may be active much of the year,whereas the

growingseason is curtailed at

highelevations or high latitudes (Adolph and Porter,

1996). Furthermore, differences in daily ther-

mal conditions during the active season influ-

ence activity and are likely to influence growthrate (e.g., Sinervo and Adolph, 1989; Sinervo,

1990; Niewiarowski and Roosenburg, 1993). Re-

cently, Adolph and Porter (1996) developed a

? 2001bythe AmericanSocietyof IchthyologistsndHerpetologists



WAPSTRA ET AL.-AGE AND SIZE AT MATURITYIN A SKINK 647

model in which age and size at first reproduc-tion may be predicted for lizards living in dif-

ferent thermal environments. In their model,lizards living in colder climates are predicted to

delay maturitybecause of reduced

opportuni-ties for growth and, as a result, mature at a larg-er size at an older age. Empirical data generally

support this model (e.g., Tinkle et al., 1970;Grant and Dunham, 1990; Rohr, 1997), but tests

for a wide variety of taxa are required. Other

factors may need to be considered to fully ex-

plain a delay in maturity (size or age). For ex-

ample, Rohr (1997) compared two populationsof the skink, Eulamprus tympanum,separated byaltitude, and found that the alpine population

delayed maturity longer than was predicted by

the differences in growth rate and the mini-mum size at which maturity could occur.

The spotted snow skink, Niveoscincusocellatus,is a small to medium-sized viviparous skink (3-12 g) endemic to the island state of Tasmania,Australia. Within Tasmania, it is one of the most

widespread species, occupying a wide climatic

zone from the cool temperate coastal region to

the cold temperate subalpine and alpine zones.

We have previously described geographic and

annual variation of reproductive cycles of this

species (Jones et al., 1997; Wapstra et al., 1999;Wapstra and Swain, 2001a); although the spe-cies is constrained by the environment at cold

alpine and subalpine sites, reproduction is an-

nual, and all mature females reproduce each

year in all populations studied. However,

throughout its range, it displays extensive geo-

graphic variation in key life-history traits (Wap-stra and O'Reilly, 2001; Wapstra and Swain,

2001b); mature N. ocellatus rom cold subalpine

populations are significantly larger at maturity

and reacha

largermaximum size than those

from warmer, lowland populations. The differ-

ence in adult size is so marked that the averagesnout-vent length (SVL) of both sexes in a sub-

alpine population consistently exceeded the re-

spective maximum sizes recorded in a lowland

population (Wapstra and Swain, 2001b). Be-

cause fecundity in this species is strongly size

related (Jones et al., 1997; Wapstra and

O'Reilly, 2001; Wapstra and Swain, 2001b),these large differences in size are largely re-

sponsible for the variation in other key life-his-

tory traits between the populations, particularlythe higher reproductive output (litter mass and

number of young) of females from the colder

subalpine populations. In this paper, we inves-

tigate the relationships between length and sex-

ual maturity in two populations that representthe climatic extremes of the species' distribu-

tion. Because body size cannot be used as an

25-

East Coast20 (warm ite)

CentralPlateau

1- (coldsite)

15-

a3 0-0-

E

5- winternactivitywarm itecold ite

<0 111 1z

month

Fig.1. Mean

monthlymaximumair

temperature(C) for 1992-1997 at the cold (CentralPlateau) andthe warm(EastCoast)studysites (courtesyof the Tas-manian Bureauof Meteorology)and typicalperiodof

Niveoscincuscellatus interinactivity.

accurate and reliable indicator of age in specieswith indeterminate growth and large variance in

body size within any age class, we used skeleto-

chronology to establish age. We sought to an-

swer four specific questions: (1) Are the differ-

ences in adult body size (SVL) between the two

populations a reflection of different growthrates that may be attributed to proximate envi-

ronmental factors? (2) Is the larger adult bodysize at the subalpine site a result of a delay in

maturity, or does it reflect enhanced longevity?(3) Are there intersexual differences in growth

patterns in either population? (4) Do growthrates slow following sexual maturity?

MATERIALS AND METHODS

Location and descriptionof studysites.--Studysites

were located on the East Coast (42o34'S,

147'52'E; hereafter "warm site") and on the

Central Plateau (41'51'S, 146'34'E; hereafter

"cold site") of Tasmania. Temperatures differ

greatly at these sites, and as a result the lengthof the activity season differs between sites (Fig.

1). The warm site is typical of much of the coast-

al cool temperate region of Tasmania. It lies 50-

75 m above sea level and forms part of a dol-

erite scree slope, providing an abundance of

shelter and basking sites. The cold site is 1200

m above sea level and lies within the South-WestWorld Heritage Area. It is characteristic of sub-

alpine/alpine areas within Tasmania; snow is

common through winter months but can occur

in any month, and hibernation is obligatory.

Lizard capture and skeletochronological ge assess-

ment.-As part of a larger analysis of reproduc-





WAPSTRA ET AL.-AGE AND SIZE AT MATURITY IN A SKINK 649

LAGs) in older animals. In all cases, several sec-

tions were compared to establish that the num-

ber of LAGs was consistent (see Castanet and

Smirina, 1990). LAGs formed early in life are

generally

stained lighter (S. Hudson,pers.comm.), and this was also the case in this study.

All bones were examined initially over a three-

day period and then again in a blind retrial

three weeks later to confirm the reliability of

earlier estimations.

The age of individuals (in months) was cal-

culated by establishing the number of winters

the individual had lived (the actual number of

LAGs counted plus the number judged to have

been lost through endosteal resorption) plusthe number of months of growth since the last

winter and the capture date, plus the numberof months lived prior to the first winter. All in-

dividuals from the warm site population were

assumed to have been born in January and

those from the cold site in February (Jones et

al., 1997; Wapstra et al., 1999; Wapstra, 2000).

Construction and analyses of growth curves.-We

used the relationship between SVL and age to

construct growth curves. Two asymptotic growthmodels based on linear length measurements

are most often used in reptile studies: the vonBertalanffy and logistic-by-length models (e.g.,Schoener and Schoener, 1978; Hemelaar, 1988;Sexton et al., 1992). The two models make dif-

ferent predictions about the shapes of the

growth curves. The von Bertalanffy curve pre-dicts that growth (in length) is maximum in

newborn lizards, whereas the logistic-by-lengthmodel predicts maximal growth (in length) lat-

er in life (Andrews, 1982). Niveoscincusocellatus

grows most rapidly early in life (Fig. 3; EW un-

publ. growth rate experiments); consequently,the von Bertalanffy growth model was fitted tothe data. This model is also considered partic-

ularly appropriate in species that are long-lived(Andrews, 1982). The general form of the von

Bertalanffyequation used is SVL, = a (1 -be-k)

where SVL is the body size at time t, a is the as-

ymptotic body size, b is a parameter related to

initial body size, and k is the characteristic

growth rate. The parameters a, b, and k were

estimated using nonlinear regression tech-

niques (Andrews, 1982; James, 1991) using SYS-

TAT'. The von Bertalanffy model provided anexcellent fit to the data (adjusted r2 values

0.902-0.952). Growth trajectories were consid-

ered to be significantly different if the 95% con-

fidence intervals did not overlap (Dunham,1978; Schoener and Schoener, 1978; Niewia-

rowski et al., 1997). This is considered to be a

conservative test for differences in growth rates

A. 90-

80-

70- 0

1-60 0

E ,La'oe

>40

C/)50

SJuvenile40

* Male

o Female

20

0 12 24 36 48 60 72 84 96108120132144

B. 90-

80-0oo

oo ?

700-OP

>C50 -s

40-

301

20

0 12 24 36 48 60 72 84 96 108120132144

age (months)

Fig. 3. Relationshipbetween age and size in Ni-veoscincus cellatusn the warm site (A) and the coldsite (B). Age (in months) is estimatedfrom the num-ber of LAGs,the capturedate, and the assumptionthat all individualswere born inJanuaryat the warmsite and in Februaryat the cold site.

(Schoener and Schoener, 1978; Henle, 1989;

James, 1991). Initially, we compared male and

female growth trajectories at each site. Data

from juveniles were included in data for both

males and females to establish the best regres-sion permitted for the correct evaluation of the

age of animals in their early years (see Ryser,1988). Juvenile male and female N. ocellatusdo

not show substantially different growth patternsin field enclosures or under standard conditions

in the laboratory (Wapstra, 1998), and males

and females mature at similar sizes (Jones et al.,

1997; Wapstra and Swain, 2001b) and remain

similar sized throughout life (see below).

RESULTS

Lizards did not differ in SVL between yearsat either site (ANOVA: females: warm site F2,284= 2.526; P > 0.05; cold site F2,1,,

= 2.803; P >

0.05; males: warm site; F2,72= 2.135; P > 0.1;cold site; F2,104= 1.692 P > 0.1), and male and

female SVLs did not differ within sites (ANOVA:



650 COPEIA, 2001, NO. 3

warm site; F1,360= 0.003; P > 0.1; cold site F1,307

= 2.417; P > 0.1). There was a significant dif-

ference in adult SVL between sites (F1,669=

814.427; P < 0.001), with lizards from the cold

sitebeing larger

at sexualmaturity

andreachinga greater maximum length (Fig 2). The typical

minimum SVL of mature females at the warm

site was 55-57 mm, whereas at the cold site a

SVL of 65 mm was typical. Niveoscincusocellatus

has an asynchronous reproductive cycle (Jonesat el., 1997; Wapstra et al., 1999; Wapstra and

Swain, 2001a) in which male gonadal develop-ment peaks in late summer prior to mating,whereas in females gonadal development (vitel-

logenesis) is maximal in spring. Thus, males

have the potential to mature at the end of the

summer season, whereas females will not show

signs of maturity until after the winter. As a re-

sult, the minimum SVL at maturity for males

was similar, but slightly smaller, than that of fe-

males at both sites: 54-55 mm at the warm site

and 62-64 mm at the cold site. Adult male and

female lizards could not be divided into ageclasses based on their SVL although it is likelythat the smallest lizards represent the youngestcohort and progressively larger lizards older co-

horts.

Growth is rapid early in life, and there is astrong relationship between SVL and age. In the

population from the warm site (Fig. 3A), all liz-

ards three years or older are clearly mature,whereas those in their second and third seasons

(one and two years old) are immature. After

maturity, growth slows considerably, and the re-

lationship between age and SVL is not as strong.The oldest females were estimated to be eight

years old, and the oldest male was estimated at

six years. From the individuals aged, females ap-

pear to havethe

greatest longevity, althoughthis may be a result of sampling bias. The largestdifference in the relationship between SVL and

age at the cold and warm sites is the delay in

maturity exhibited by individuals from the cold

site (Fig. 3B). From the sample aged, the youn-

gest males that were mature were approximately65-68 mm SVL and were 36 months old. It ap-

pears that females mature one year later at

much the same SVL (Fig. 3B); however, in part,this is a reflection of the different reproductive

cycles of the sexes and the sampling regimes. In

males, reproductive maturity wasjudged by the

presence of enlarged gonads, which occurs at

the end of the activity season (i.e., prior to win-

ter and the production of a further LAG),whereas in females sexual maturity is indicated

by the presence of enlarged follicles/develop-

ing embryos in spring and summer after a fur-

ther LAG is produced in winter (see Jones et

MalesMales / maleandfemale lines

.................Females superimposed in A.

A. 90 95 %Confidencelimits ormales

---------95 %Confidencelimits orfemales80-

70- . ........

E

E 60-Jo

U) 50

40

30

20

0 12 24 36 48 60 72 84 96 108120132144

B. 90-

80-

70-E

60--J

U) 50

40

30

200 1224364860728496 08120132144

age (months)

Fig. 4. von Bertalanffygrowth curves based ondata from male and female Niveoscincuscellatusromthe warmsite (A) and the cold site (B).Juveniledataare included in the construction of curves for bothsexes. Parameters or these curves are presented inTable 1.

al., 1997; Wapstra et al., 1999). Consequently,the

youngestfemales that

produceda litter were

four years old (in their fifth season), and some

smaller immature females probably matured a

year later than this (Fig. 3B). As a result of the

delayed maturity by both males and females at

the cold site, individuals mature at a larger SVL

than those from the warm site. As at the warm

site, growth slows markedly after maturity. Lon-

gevity at the cold site was greater than at the

warm site; the oldest females here were 11 yearsold, and the oldest males were 12 years old.

We constructed von Bertalanffy growth curves

for the warm site (Fig. 4A) and the cold site

(Fig. 4B) populations (Table 1). Within each

site, there is considerable overlap of the growthcurves of males and females, indicating that the

sexes do not show markedly different growth

patterns. Sexes from each site were consequent-

ly pooled to compare von Bertalanffy growthcurves between sites (Fig. 5). These curves are



WAPSTRAET AL.-AGE AND SIZE AT MATURITY IN A SKINK 651

TABLE1. VONBERTALANFFYROWTH ODEL PARAMETERSSVL, = (1 - be-kt)], WHERE IS THEASYMPTOTIC

BODYSIZE,k IS THECHARACTERISTICSROWTH ATE,ANDb IS THE COEFFICIENTERIVEDROM NITIAL ODY

SIZE,GENERATEDROM ONLINEAREGRESSIONSF SIZE SVL) ANDAGEFORNiveoscincuscellutusATTHEWARM

SITE ANDTHECOLD SITE. Juvenilesare initially ncluded in both male and female data.Growthmodels with

the sexes combinedweregeneratedbecause male and femalegrowthcurvesdid not differ at either site.Upperand lower valuesare the 95%confidence intervals or each value.

Parametersrom von Bertalanffymodel

a b k

Group lower mean upper lower mean upper lower mean upper 72

warm males 62.65 67.01 71.38 0.51 0.55 0.59 0.029 0.042 0.054 0.95

warm females 64.57 66.99 69.41 0.52 0.55 0.58 0.034 0.043 0.052 0.96

warm combined 64.49 66.66 68.82 0.52 0.55 0.58 0.036 0.044 0.053 0.95

cold males 76.51 79.55 82.59 0.58 0.63 0.67 0.022 0.027 0.032 0.91

cold females 81.72 85.33 88.94 0.62 0.65 0.69 0.018 0.022 0.026 0.94

cold combined 78.81 81.08 83.35 0.60 0.63 0.67 0.023 0.026 0.030 0.90

an excellent fit for the data (Table 1). The

growth patterns exhibited at the two sites are

quite different. There are few differences in

growth rate early in life, but the growth curves

begin to diverge at around three years of age.This is consistent with the age at which individ-

uals from the warm site mature, whereas indi-

viduals from the cold site continue to grow rap-

idly priorto

maturity1-2

yearslater. In both

populations, growth slows after sexual maturityis reached. The growth asymptotes of the two

populations differ markedly: that for the warm

site is between 64 and 69 mm, whereas for the

cold site, it lies between 79 and 83 mm. Both of

warmsite

cold site

95 % Confidence imits orwarm site

- 95 %Confidence imits or cold site90

80o ....

70" -

EE 60--J

> 5soc/)

40-

30

20 0 12 24 36 48 60 72 84 96 108120132144156

age (months)

Fig. 5. Combined male, female von Bertalanffy

growth curves for Niveoscincus ocellatus rom the warm

site and the cold site. Juvenile data are included in

the constructionof curves for both populations.Pa-rameters or these curves arepresentedin Table 1.

these estimates are in close agreement with the

largest individuals caught from the populations

during the course of this study (Fig. 2). The low-

er value of k (growth coefficient) at the cold site

does not necessarily imply an absolute lower

growth rate than at the warm site (although

growth rate experiments reveal a strong proxi-mate influence of temperature on growth; EW

unpubl.data) but rather reflects the

longertime taken to reach the growth asymptote (andto reach sexual maturity).

DIscussIoN

Skeletochronology provides a suitable and re-

liable method by which to establish the age of

N. ocellatusand presumably other small skinks.

In both populations studied, distinct dark bands

were formed during the period of winter inac-

tivity. The only difficulties encountered were

the loss of early LAGs resulting from expansionof the medullary cavity and faint staining of ear-

ly lines, especially in the cold site individuals.

However, these problems were overcome

through the measurement of LAGs and bone

diameters in juvenile lizards. Although there is

a strong relationship between SVL and age in

both populations, as in many other squamate

reptiles, considerable variation in size within

age classes precludes accurately using other

methods to age individuals. Growth rates vary

in this species as a consequence of many factorsincluding genetic and phenotypic responses to

environmental conditions and variation in size

at birth (Wapstra, 1998; Wapstra, 2000). There

is also likely to be annual variation in growthrates.

There were no differences between the sexes

in the relationship between SVL and age or in



652 COPEIA, 2001, NO. 3

the growth curves at either site. This is consis-

tent with the observation that growth rates of

juvenile N. ocellatusdo not differ between sexes

and that there are no intersexual differences in

the minimum SVLatmaturity

or maximum SVL

at either site (Jones et al., 1997; Wapstra and

Swain, 2001b). Growth in both populations was

rapid prior to maturation but slowed markedlyafter this point. This is the typical pattern of

growth in reptiles; the slowing of growth after

maturation is associated with the energy de-

mands of gonadal maturation and other costs

associated with reproduction (Andrews, 1982;Shine and Schwarzkopf, 1992; Bernardo, 1993).The observation that males and females mature

at a similar size in both populations and growthslows equally in both sexes after maturation im-

plies that sexual maturity is equally energetically

costly to males and females. This is consistent

with the findings of Anderson and Vitt (1990),

although it should be noted that, in N. ocellatus,

energetic costs associated with reproduction are

incurred at different times of the year (Wapstraand Swain, 2001a).

Although age at maturity is an important life-

history parameter (Roff, 1992; Stearns, 1992;

Bernardo, 1993), relatively little attention has

been paid to variation in age and size at matu-rity among populations of reptiles experiencingdifferent thermal environments (Adolph and

Porter, 1996). Our results are in agreement with

the model of Adolph and Porter (1996), in

which individuals from populations from cool/cold environments are predicted to mature at a

later age but at a larger size, than individuals of

the same species from populations in warmer

areas because of the proximate influence of

temperature on their activity patterns and

growthrates.

Lizards from our cold site ma-tured at a larger size and at a greater age than

those from our warm site. The delay in maturitywas largely responsible for differences in bodysize between populations, although differences

in maximum ages further increased size differ-

ences between the populations. Niveoscincusocel-

latus from our cold site have reduced growth

potential because they are born approximatelyone month later than those from the warm site

(Wapstra et al., 1999); the thermal characteris-

tics at the site are less favorable for growth in

any particular month, and the activity season is

typically 6-8 weeks shorter (Fig. 1). Althoughthese populations show some genetic diver-

gence (Melville and Swain, 2000), common gar-den and reciprocal transplant experiments of

growth rate using newborns from the two pop-ulations showed no genetic basis for differences

in growth rates but revealed the strong proxi-

mate influence of temperature and basking op-

portunity on growth rate (Wapstra, 1998).

Delayed maturity, with a consequent increase

in size at maturity, is common in lizards inhab-

itingcolder climates

(e.g.Tinkle et

al., 1970;Grant and Dunham, 1990; Rohr, 1997) and is

thought to be associated with an inability of liz-

ards occupying these habitats to reach a size

suitable for reproduction at the same age as

those from warmer climates (Andrews, 1982;

Jones et al., 1987; Niewiarowski, 1995). An al-

ternative strategy by lizards from populations

occupying cold habitats is to mature at the same

age as their counterparts from milder habitats

but at smaller sizes (e.g., Parker and Pianka,

1975; Forsman and Shine, 1995; Mathies and

Andrews, 1995). For example, reproductively ac-tive female Sceloporus calaris at high elevations

are smaller than females from low elevations

(Mathies and Andrews, 1995), and Lampropholisdelicata from higher latitudes are smaller than

those from lower (and warmer) latitudes (Fors-man and Shine, 1995).

In stable populations, age at maturity should

evolve to maximize lifetime reproductive suc-

cess (Shine and Charnov, 1992; Bernardo,

1993); the strategies adopted by N. ocellatus at

the climatic extremes of its distribution are con-sistent with this expectation. In this species, an-

nual fecundity in females correlates stronglywith body length (see Jones et al., 1997; Wapstraand O'Reilly, 2001; Wapstra and Swain, 2001b).At the warm site, conditions for growth are com-

paratively benign, and maturity is reached rela-

tively early with a relatively short life span (typ-

ically 3-6 years), and thus lifetime fecundity is

maximized by early maturation. Conversely, at

the cold site where conditions for growth are

relatively poor, maturity is reached later, animalsare longer-lived (typically 5-10 years), and life-

time fecundity is maximized by delaying matu-

rity and devoting available energy to growth.Because delaying maturity involves costs to in-

dividual fitness, strong trade-offs can be expect-ed between fitness losses resulting from delayed

reproduction and gains resulting from the delay(Stearns and Crandall, 1981; Shine and

Schwarzkopf, 1992; Stearns, 1992). At our cold

site, delaying reproduction by one year results

in a small potential fecundity cost of one, po-

tentially two, young based on published fecun-

dity data (Wapstra and O'Reilly, 2001; Wapstraand Swain, 2001b). However, this is compensat-ed for by a higher reproductive output over the

subsequent years because of their larger size

(Wapstra and O'Reilly, 2001; Wapstra and

Swain, 2001b).The enhanced longevity of lizards from the



WAPSTRA ET AL.-AGE AND SIZE AT MATURITY IN A SKINK 653

cold site (maximum of 12 years) compared to

the warm site (maximum eight years) agreeswith previous models (see particularly Adolphand Porter, 1993) in which individuals from

colderpopulations

havehigher

survival rates

than individuals from warmer populations. If

survival rates are largely dependent on preda-tion rates and individuals from colder popula-tions are active for shorter periods of time, both

daily and annually, then their potential expo-sure to predators is reduced (see particularly

Adolph and Porter, 1993). We have no direct

data on predation rates between the two popu-lations but do note that the same predators are

present at both sites (Wapstra, 1998), and activ-

ity time is reduced at the cold site. In addition,

reproducing female N. ocellatushave a relatively

higher chance of predation than nonreprod-

ucing females (Wapstraand O'Reilly, 2001). Be-

cause all adult females reproduce each year

(Wapstra et al., 1999), early maturity at the

warm site introduces risks that are not experi-enced by animals from the cold site until much

later.

Differences in growth rate are often used to

explain differences in life-history traits, includ-

ing size or age at maturity (e.g., Halliday and

Verrell, 1988; Smith et al., 1994; Adolph andPorter, 1996). Our study supports the view that

age and size at maturity are phenotypically plas-tic traits (Stearns and Koella, 1986) modified bythe thermal environment (Ferguson and Tal-

ent, 1993; Adolph and Porter, 1996; Rohr,

1997). The different growth patterns exhibited

by N. ocellatus at our two study sites result in

differences in size and age at maturity,which in

turn are associated with differences in other key

life-history differences between populations,

particularly maximum adult body size and re-productive output.

ACKNOWLEDGMENTS

Numerous field assistants aided in the collec-

tion of lizards. S. Hudson provided early advice

and important references on the technique of

skeletochronology. M. Hindell assisted in the

construction and analyses of growth curves

based on the von Bertalanffy model. Lizards

were collected from the Central Plateau World

Heritage Area with the permission of the Tas-

manian Parks and Wildlife Service (permits95109, 95363, and 96190). The work was con-ducted under Animal Ethics Permit 94070 from

the University of Tasmania. Our research was

assisted by grants from the Peter Rankin Trust

Fund and the World Heritage Fauna and FloraGrants Scheme, Tasmanian Parks and Wildlife

Service. The W. V. Scott Bequest Fund specifi-

cally provided funding for the skeletochronol-

ogical analyses.

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Geographic Variation in Age and Size at Maturity in a Small Australian Viviparous Skink