School of Zoology, University of Tasmania, PO Box 252C-05, Tas, 7001, Australia

Functional Ecology 2000 Maternal basking opportunity affects juvenile phenotype Blackwell Science, Ltd in a viviparous lizard E. WAPSTRA School of Zoology, University of Tasmania, PO Box 252C-05, Tas, 7001, Australia Summary 1. The effects of external conditions on embryonic development have been repeatedly examined in oviparous reptile species, but the effect of gestation conditions on offspring traits in viviparous species has rarely been examined. 2. The influence of maternal basking opportunities on gestation length and juvenile phenotype was investigated in a viviparous scincid lizard, Niveoscincus ocellatus. Females were housed under one of two experimental regimes (10 or 4 h access to basking) which reflected the natural variation in temperature, potentially one of the most important proximate sources of life-history variation. 3. Females with longer access to basking gave birth significantly earlier than those with reduced basking opportunities. Maternal access to basking significantly affected the phenotype and growth rate of her offspring. 4. Offspring born after relatively rapid development were longer, heavier and in better condition than offspring born after slower development. 5. In standard laboratory conditions, offspring born after rapid development grew more rapidly than those born after slower development, thus amplifying the difference in body size between these two groups postpartum. 6. These results suggest the existence of a strong selective pressure on female basking behaviour through the effect of the maternal environment on embryo development and offspring phenotype and highlight the role of temperature as a proximate source of variation in both the timing of reproductive events and in key life-history traits of neonates. Key-words: Gestation conditions, growth rate, life-history variation, maternal effects, phenotypic plasticity Functional Ecology (2000) Ecological Society Introduction Offspring phenotype is one of the most important life-history traits and as such the sources of its variation are well documented (Roff 1992). Recent studies have concentrated on the effect of environmental conditions during incubation as a proximate source of variation in offspring phenotype (e.g. Fox & Mousseau 1998; Lacey 1998). Reptiles have assumed a prominent place in these studies. For example, the environment in which a reptile embryo develops profoundly influences its morphology, physiology and behaviour (e.g. Burger 1989; Van Damme et al. 1992; Cagle et al. 1993; Elphick & Shine 1998), and has only recently been recognized as a source of variation in postnatal growth rate (Overall 1994; Rhen & Lang 1995). Most studies that have examined effects on embryonic development in reptiles have concentrated on temperature effects during incubation in oviparous species (e.g. Burger 1989; Rhen & Lang 1995; Roosenburg & Kelley 1996; Shine & Harlow 1996; Qualls & Shine 1998). The relationship between embryonic developmental rate and temperature has been demonstrated in a range of reptilian taxa; generally development proceeds more rapidly at higher temperatures (e.g. Beuchat 1988; Packard & Packard 1988; Shine & Harlow 1996). Furthermore, incubation temperature also affects juvenile phenotype in a variety of oviparous reptile taxa (e.g. Webb, Choquenot & Whitehead 1986; Burger 1989; De Souza & Vogt 1994; Roosenburg & Kelley 1996; Shine & Harlow 1996). There have been fewer studies examining the potential role of the gestation environment in embryonic development of viviparous species (Beuchat 1988; Shine & Harlow 1993; Sorci & Clobert 1997; Shine & Downes 1999). Since viviparity places incubation conditions under the control of the female, there are a priori reasons to expect selection on maternal thermoregulatory behaviour to affect the fitness of their offspring. I therefore chose a viviparous skink species as my model system. 345

346 E. Wapstra In viviparous reptiles, the thermal environment experienced by embryos is a consequence of the thermoregulatory behaviour of the female (Beuchat 1988; Shine & Harlow 1993), which has been studied extensively (e.g. Schwarzkopf & Shine 1991; Daut & Andrews 1993; Tosini & Avery 1996 and references therein). Although the effects of variation in climate on reptile activity has been studied as a source of variation in life history (Adolph & Porter 1993, 1996), the influence of female basking opportunity as a proximate source of variation in juvenile phenotypic characteristics has only recently begun to be investigated (Shine & Harlow 1993; Shine & Downes 1999). Increased basking by gravid lizards, compared with non-gravid ones, has been recorded (e.g. Beuchat 1986, 1988; Schwarzkopf & Shine 1991; Daut & Andrews 1993; Lecomte, Clobert & Massot 1993), presumably as a means by females to reduce gestation length. Although an increase in basking may not be without cost to the female (Schwarzkopf & Shine 1992), there ought to be a selective benefit in terms of reducing the costs of reproduction to the female by reducing gestation length. However, the implications for the developing embryo and ultimately the effect on the phenotype of the juvenile and fitness are less clear. There is evidence that developmental conditions influence variation in offspring size and shape, which are likely to be ecologically important traits (Smith & Fretwell 1974; Brockelman 1975; Roff 1992). For example, size at birth affects neonatal growth, locomotor performance and survivorship of many reptile species (e.g. Jayne & Bennett 1990; Sinervo et al. 1992; Sinervo & Adolph 1994; Elphick & Shine 1998). In addition, larger offspring may grow more rapidly, pass quickly through size classes more vulnerable to predation, better survive poorer environmental conditions, reach reproductive size more rapidly and catch food more effectively than smaller offspring (Ferguson & Fox 1984; Sinervo 1990; Sinervo et al. 1992; Sinervo & Adolph 1994). Niveoscincus ocellatus is an ideal species to test the influence of gestation conditions on juvenile phenotype. This species is a small viviparous scincid lizard (4 10 g) endemic to the island state of Tasmania, Australia, where it occupies a wide geographic and climatic range. Throughout its range it displays significant differences in all life-history characteristics and in the timing of key reproductive events including parturition (Wapstra et al. 1999). Specifically, offspring phenotypic traits differ significantly between populations occupying different climates (Wapstra 1998). The mechanism(s) responsible for these differences is not yet known. However, gravid lizards from climatically different sites may experience very different opportunities for basking. Consequently the developing embryos are likely to experience different thermal environments during development, suggesting that this is a proximate source of variation in juvenile phenotypic traits. In this study, the effect of basking opportunity provided to female lizards is investigated and four specific questions addressed: 1. Does basking opportunity provided to females affect gestation length? 2. Does basking opportunity affect female postparturient condition? 3. Does female basking opportunity affect juvenile phenotype? 4. Do juvenile morphological traits covary with performance and/or growth rate? Materials and methods In order to test the effect of maternal basking behaviour on phenotypic traits of offspring, two experimental regimes were chosen in which gravid lizards were provided with the opportunity to bask for 4 ( short basking regime ) or 10 h ( long basking regime ) per day. This approximately reflects the difference in basking regimes at the climatic extremes of the species distribution during summer (Wapstra et al. 1999). Female N. ocellatus were collected from Orford (42 34 S, 147 52 E) between 7 and 17 October 1996 shortly after ovulation takes place (Jones, Wapstra & Swain 1997; Wapstra et al. 1999). Reproductively active females were identified in the field by the presence of obvious mating marks (Jones et al. 1997; Wapstra et al. 1999). The lizards were taken to the laboratory where they were measured (snout vent length, SVL; ±0 5 mm) and weighed (±0 1 mg). The presence and number of large follicles/eggs was assessed by palpation of the female s abdomen. Ovulation was confirmed in a small subsample (eight lizards) by dissection. The presence of implanted eggs within the oviducts and large corpora lutea confirmed recent ovulation. Thirty-four females were included in the experiment and these were randomly assigned to one of the two treatments on 20 October 1996; 18 and 16 lizards were provided with 4 and 10 h access to a radiant heat source, respectively. All animals were maintained in these conditions until their young were born. Lizards were maintained in the same air-conditioned room under bright fluorescent tube lighting ( 20 000 lux) and UV lighting (14L:10D). Females were housed in pairs (because of space constraints) in plastic terraria (200 300 100 mm 3 ) and supplied with water ad libitum and were offered mealworms and fruit three times weekly. Both females within each terrarium appeared to have ample opportunity to feed. Previous observations had indicated no agonistic interactions between females. Each terrarium was positioned randomly within the experimental regime, and terraria were repositioned daily on the shelves to minimize position effects. Basking heat was supplied by a 25-W spotlight positioned 80 mm above a basking surface. An overturned terracotta pot was used as the basking surface and for shelter. A thermal gradient from 30

347 Female basking affects juvenile phenotype to 35 C at the basking surface to 14 C in the rest of the terrarium allowed lizards to thermoregulate while the basking light was on. Thermoregulatory behaviour was constrained only by the potential basking time, not by available basking space. The ambient temperature fell to 10 C at night. Terraria were checked daily for newborns. Females that had given birth were removed from their terrarium and weighed (±0 1 mg) and palpated to confirm that parturition of all young was completed. Relative clutch mass (RCM) was calculated and is defined as total mass of newborns (mg)/mass of female after birth (mg). Following parturition, a subsample of females was killed and fat bodies removed and weighed to assess maternal condition. Newborns were removed from the maternal terrarium and SVL (±0 05 mm), total length (±0 1 mm) and mass (±0 1 mg) measured. Each lizard was given a unique toe clip for permanent identification. Within 12 h of birth, the performance of the juveniles was assessed using sprint speed as a measure of whole body performance. Toe-clipping does not affect sprint speed in other lizard species (Huey, Overall & Newman 1990; Dodd 1993). The lizards were placed in a sealed container within a water bath held at 27 ± 0 1 C for a minimum of 20 min. This temperature represents the temperature at which sprint speed in this species is maximized (Melville 1998). While it was not possible to measure the body temperature of such small lizards, 20 min was the time required for the body temperature of adult lizards (4 8 g) to equilibrate to the ambient temperature under the same conditions (T body = 26 9 ± 0 14; N = 23). They were then run down a heated (27 ± 0 1 C) racetrack 2 m in length. Lizards were sprinted down the track twice in rapid succession, encouraged by tapping their tails lightly with a paintbrush. The time taken was recorded by three infrared lightbeams held 0 5 m apart and linked to a computer. Each run generated two times (one for each of the 0 5-m sections) and the fastest of the four times obtained was chosen as the maximum sprint speed. Trials in which the neonate refused to run, stopped, or turned and ran the wrong way were excluded from the analysis. Following performance testing and measuring, juvenile lizards were maintained in plastic terraria (200 300 100 mm 3 ) in the same conditions as described for females except that basking opportunities were provided for 12 h each day. Eight to ten lizards were held in each terrarium (i.e. the first ten lizards born were placed in the first terrarium, the second ten in the second terrarium and so on, in order to minimize size and age effects on growth rate within each terrarium). Observations confirmed that there was no competition for basking sources or food. Each terrarium was supplied with water and small mealworms ad libitum, and mashed banana mixed with multivitamins (Wombaroo Herpatavite Reptile Supplement and vitamin D3) was given approximately three times weekly. Terraria were randomly repositioned daily. To minimize disturbance all lizards were measured on the same day which resulted in total growth periods of between 41 and 91 days (because lizard birth dates differed). Growth rates of reptiles are most commonly expressed as rates of change in SVL or mass (Andrews 1982; Sinervo & Adolph 1989, 1994). In this study, SVL was used because changes in mass, especially in small lizards, may reflect recent nutritional history and changes in intestinal contents rather than a change in body size (Dunham 1978). Previous growth rate experiments of N. ocellatus which involved repeated body measurements at regular intervals (2 3 weeks) over the first 130 days of life indicated that growth is linear during this phase (r 2 = 0 83 0 99 for individual lizards; N = 78; E. Wapstra, unpublished data) and a single measurement at the end of the time period provides an accurate estimate of growth. This reduces the confounding effect of measuring and comparing growth rates of individuals of different ages provided that their ages fall within the period of linear growth. Therefore a linear relationship between body size and time of measurement is assumed. All surviving females and their offspring were released at the site of capture at the conclusion of the experiment. Analyses were conducted using SYSTAT 8 0. Differences between groups of females prior to the start of the experiment were analysed by ANOVA and after the completion of the experiment by nested ANOVA (female terraria nested within basking treatment). Differences between groups of young (average of each phenotypic character within each clutch) were analysed by nested ANOVA (using the same nesting as for their mothers; terraria within treatment). Differences in body condition of females and young were analysed using ANCOVA (mass with SVL as covariate). Survival of young in captivity was analysed in two ways; the Cox Proportional Hazard Model to detect differences in survival patterns between groups of young and chisquare analyses of number of deaths within the first 10 days of life (assumed to be a peri-natal effect) and the number of deaths in the following 40 days of life. Results FEMALE CHARACTERISTICS The range in SVL of the 34 females was 57 68 mm (mean 63 6 mm), with the predicted clutch determined by palpation ranging from 1 to 4 (mean 2 5). In all cases the predicted clutch based on palpation was the same as the actual clutch obtained from birth details. There was no significant difference in female characteristics between groups (Table 1) including their initial mass (ANOVA: F 1,32 = 0 085, P > 0 1) or condition (ANCOVA: F 1,31 = 0 410, P > 0 5). The majority of females settled well into captivity. There were no observed agonistic behaviours between pairs of lizards, no bite marks which would indicate aggression were found, and the lizards often shared

348 E. Wapstra Table 1. Characteristics of female Niveoscincus ocellatus. All values are means ± standard error and the sample size is indicated in parentheses Trait 10-h basking 4-h basking F P SVL (mm) 63 1 ± 0 87 (15) 64 1 ± 0 81 (18) F 1,32 = 0 723 >0 1 Initial mass (g) 4 4 ± 0 21 (15) 4 5 ± 0 23 (18) F 1,32 = 0 085 >0 1 Clutch size (number of young) 2 3 ± 0 21 (15) 2 7 ± 0 23 (18) F 1,31 = 1 628 >0 1 Gestation length (days) 104 1 ± 1 93 (13) 142 1 ± 1 62 (18) F 1,16 = 30 133 <0 001 Gestation length (range) 102 121 (13) 121 170 (18) Abdominal fat bodies (mg) 322 5 ± 46 85 (11) 282 6 ± 29 63 (15) F 1,24 = 0 569 >0 1 RCM (clutch mass/female mass) 0 23 ± 0 022 (13) 0 26 ± 0 018 (18) F 1,16 = 2 169 >0 1 Table 2. Juvenile Niveoscincus ocellatus phenotype characteristics. All values represent means (mean phenotypic character per clutch) ± standard errors with sample size indicated in parentheses Juvenile character 10 h 4 h F P SVL (±0 05 mm) 29 7 ± 0 19 (13) 29 1 ± 0 16 (18) F 1,16 = 4 372 <0 05 Total length (±0 1 mm) 68 5 ± 0 64 (13) 66 5 ± 0 54 (18) F 1,16 = 4 080 0 06 Tail length (±0 1 mm) 38 8 ± 0 53 (13) 37 4 ± 0 44 (18) F 1,16 = 4 669 <0.05 Mass (±0 1 mg) 541 2 ± 9 49 (13) 475 4 ± 10 30 (18) F 1,16 = 16 595 <0 001 Growth rate (mm day 1 ) 0 07 ± 0 006 (12) 0 04 ± 0 005 (16) F 1,15 = 6 023 <0 05 Sprint speed at birth (m s 1 ) 0 96 ± 0 073 (21) 0 94 ± 0 056 (36) F 1,55 = 0 031 0 1 Survival (% during first 45 days of life) 93 80 NS the basking surface and shelter. Overt basking typically began when the overhead light was switched on, but lizards spent most of the day heating covertly beneath the basking surface. All of the lizards assigned to the short basking regime gave birth, while in the long basking group one female died early in the experiment, one female aborted premature young (approximately stage 37 38; Dufaure & Hubert 1961) and no birth was recorded from a third female who most likely consumed her young before parturition was monitored. Of the eight lizards that were dissected in early October prior to the beginning of the experiment, six had ovulated and two had large follicles that were judged to be close to ovulation. Thus, for the purposes of comparison, it is assumed that all females had ovulated by 1 October and gestation lengths were therefore calculated from this date. Females from the long basking regime gave birth significantly earlier than the short basking group (range in gestation length 102 121 days and 121 170 days, respectively) (Table 1). Females from the same population gave birth in the wild between 1 January and 3 February 1997 (gestation length 92 126 days; N = 72; Wapstra et al. 1999). There were no differences in the clutch size or RCM between groups. Furthermore, female condition (ANCOVA of postpartum body mass with SVL as covariate: F 1,28 = 2 530, P > 0 1) and mass of abdominal fat bodies did not differ between the groups at the conclusion of the experiment. Thus, females provided with reduced basking opportunities still fed consistently indicating that body temperatures were elevated for long enough for appetite and digestive processes to be maintained. JUVENILE CHARACTERISTICS Almost all juvenile phenotypic characters differed between groups. Juveniles from long basking females were significantly larger at birth (SVL, total length, tail length and mass) (Table 2) and were in better condition than those from short basking females (ANCOVA; F 1,28 = 16 840, P < 0 001). However, juveniles from the two groups did not differ in allometry (ANCOVA of tail length with SVL as covariate; F 1,28 = 1 922, P > 0 1). Growth rates also differed between groups with offspring from the long basking group growing significantly faster than those from the short basking group (Table 2). Survival in the laboratory did not differ between groups (Cox Proportional Hazard Model stratified with female basking length to detect differences in survival patterns; Mantel comparison, χ 2 = 2 220, P > 0 1; with offspring SVL and mass as covariates, Mantel comparison, χ 2 = 1 798, P > 0 1). Survival in the long basking group was higher than in the short basking group (93 and 80%, respectively) although this difference was not significant (Table 2; χ 2 = 2 067; P = 0 15) and there was no difference in the number of juveniles dying in the first 10 days of life or in the subsequent 40 days in captivity (Fishers Exact Test; P > 0 1 in both cases). There was no significant difference in sprint speeds between the two groups (Table 2). Sprint speed at birth was unrelated to survival and mass at birth and postparturient growth rate (P > 0 05 in all cases). Discussion The conditions experienced by an embryo during development have the potential to have dramatic consequences

349 Female basking affects juvenile phenotype for its phenotype at birth and through ontogeny (Lombardi 1996; Fox & Mousseau 1998) and thus its fitness. Although such effects are well documented in a variety of taxa, the effect of conditions during embryonic development on offspring phenotype and ontogeny in ectothermic viviparous taxa/ species are less well documented (Sorci & Clobert 1997; Heath & Blouw 1998), despite the extended period during which the development of the embryo is, at least in part, dependent on the maternal environment. In this investigation, the conditions experienced by developing reptilian embryos were modified through modifying female access to basking sources. Females provided with longer basking opportunity had parturition dates that overlapped with parturition dates for their source population under natural conditions and females provided with shorter basking opportunity gave birth on dates that overlapped with later births at the same site, but generally gave birth on dates that are more typical of cold high-altitude, inland locations (Wapstra et al. 1999). Thus, the conditions in the experiment successfully reflected thermal conditions that females experience in natural situations and the design seems to provide a reasonable test of differences in basking regimes between populations of N. ocellatus living in different climatic regions. It is evident from the present study that gestation length in N. ocellatus depends on the basking behaviour of the mother, which is supported by previous studies (Beuchat 1986; Schwarzkopf & Shine 1991; Shine & Harlow 1993; Shine & Downes 1999). This is not surprising since the rate of development of reptilian embryos is temperature-dependent, and higher temperatures within the normal range accelerate embryonic development (e.g. Beuchat 1988; Packard & Packard 1988; Shine & Harlow 1996). Despite the large difference in mean gestation length between the two treatments, the majority of females in both treatments carried viable embryos to full term. Such an ability to produce viable clutches despite delays and/or repeated interruptions of embryonic development as a result of cool/cold weather patterns may be an important adaptation of species occupying cool/cold climates. More specifically, this ability may be an important adaptation in allowing N. ocellatus to occupy a wide geographic/climatic range in Tasmania, where it is equally common in warm coastal regions and subalpine areas characterized by changeable weather conditions (Wapstra et al. 1999). Throughout its range, N. ocellatus maintains an annual reproductive cycle in which all females reproduce annually (Jones et al. 1997; Wapstra et al. 1999); however, in environments where basking opportunities are constrained even further than in this experiment, annual reproduction is unlikely. Interestingly, species within the genus Niveoscincus that occur at higher altitudes than the limit of the range of N. ocellatus display an unusual biennial reproductive cycle (Hutchinson, Robertson & Rawlinson 1989; Olsson & Shine 1999a,b). Females overwinter with developing embryos and/or full-term young (Olsson & Shine 1999a,b) with spring parturition following an extended hibernation period. This ability to overwinter viable embryos also occurs in cold climate populations of the viviparous gecko Hoplodactylus maculatus (Cree & Guillette 1995). Importantly, access to basking by female N. ocellatus markedly affected the phenotype of their offspring. Thus, geographic and annual variation in basking opportunities available to females (i.e. prevailing climatic conditions) are likely to be a proximate source of variation in offspring phenotype. Females that were given limited basking opportunities gave birth to significantly smaller (length and mass) offspring in poorer condition and with a subsequently lower growth rate than those given more basking time. Many studies have demonstrated that the phenotype of hatchlings may be modified by incubation conditions in a variety of oviparous reptiles (e.g. Webb et al. 1986; Van Damme et al. 1992; Packard, Miller & Packard 1993; Mathies & Andrews 1997), including skinks (Shine, Elphick & Harlow 1997; Elphick & Shine 1998; Qualls & Shine 1998). An increasing body of research indicates that conditions during development also influence offspring phenotype in viviparous species (Sorci & Clobert 1997), especially effects generated by the basking opportunities of pregnant lizards (e.g. Shine & Harlow 1993; Mathies & Andrews 1997; Shine & Downes 1999). Although viviparity provides a mechanism for buffering embryos against deleterious low temperatures (DeMarco 1992), embryos rely on maternal body temperatures for development and thus in populations occupying cold environments, the present study demonstrates that there is strong selection on females to maintain body temperatures at or near conditions optimal for embryonic development. Preovulatory mechanisms influencing the size of reptile offspring at birth are quite well understood. For example, there may be differences in the amount of yolk reserves provided, either through differences in availability of energy or in selection for neonatal size at birth (e.g. Nussbaum 1981; Sinervo 1990; Sinervo & Licht 1991; Sinervo & Doughty 1996; Olsson & Shine 1997a). Postovulatory mechanism(s), however, shaping a neonate s phenotype in viviparous reptiles remains relatively poorly understood. Temperatures during gestation might affect neonatal characteristics either by direct temperature effects on the embryo or indirectly through the mother s response to the amount of basking opportunities (Shine & Harlow 1993). However, an indirect response is unlikely to fully explain differences in neonate phenotype because incubation conditions similarly affect hatchling characteristics of other oviparous species including skinks (Shine & Harlow 1996; Shine et al. 1997; Qualls & Shine 1998). In Niveoscincus metallicus (and presumably N. ocellatus) the yolked egg represents the majority of the female input into the developing embryo (Swain

350 E. Wapstra & Jones 1997). However, in N. metallicus the ability to supplement lecithotrophic nutrition (yolk) with organic matrotrophy after ovulation (Swain & Jones 1997) might provide a means of supplementing yolk reserves. Since all these processes are temperaturedependent, offspring born to females that spend more time at or near their preferred body temperature should be larger and in better condition at birth (the present study and Doughty & Shine 1998). However, results from other studies are less clearcut than in the present one. For example, Shine & Harlow (1993) found that offspring mass was unaffected by female thermoregulatory behaviour. An alternative explanation for the observed differences in body length and mass of N. ocellatus offspring from different basking regimes is that, as gestation length is increased, embryonic energy stores (nutrients and lipid reserves supplied as yolk) are diverted towards embryonic cellular maintenance, rather than somatic growth and fat stores in newborns. The smaller size and poorer body condition of the newborns from the prolonged gestation supports this argument. Clearly, offspring size is likely to result from the interplay of a variety of factors including the amount of yolk provided prior to ovulation, the degree to which the yolk is converted to body tissue (which is temperature-dependent, Shine & Harlow 1993) and the degree of placental transfer of nutrients (Blackburn 1992; Swain & Jones 1997; Doughty & Shine 1998). However, a recent alternative view suggests that variation in offspring phenotype could be an example of adaptive phenotypic plasticity as a result of female selection of the optimum phenotype of her offspring to suit current environmental conditions (Shine & Downes 1999). However, I have no evidence that the offspring phenotypes produced by the two female basking regimes are best suited to these regimes, although the ability of females to modify offspring through behavioural basking choices ( possibly mediated through temperature-dependent placental transfer of nutrients and variation in conversion of yolk to embryonic somatic growth) provides a mechanism whereby females can predictably modify offspring. Recently, Sinervo & Adolph (1994) suggested that growth rates of neonate lizards are probably more sensitive to current thermal environments than to temperatures experienced during development. However, the present study indicates the potential role of embryonic developmental conditions on postnatal growth rate. Offspring born after a shorter gestation grew more rapidly under standard laboratory conditions than offspring born after longer gestation. This is likely to be a reflection of the better condition of these offspring and their ability to devote a greater amount of energy to growth than newborns in poorer condition. Thus, the difference in size at birth actually became amplified throughout the experiment; young born earlier were larger, had a higher growth rate and a longer growing season than those born later. Thus offspring born from good gestation conditions are likely to increase their competitive advantage over rival offspring born from poor conditions through better performance (e.g. Sinervo & Adolph 1989), acquisition of territories (e.g. Fox 1978; Stamps 1988), predator escape (Dunham 1978), survival (Ferguson & Fox 1984; Sinervo et al. 1992; Sinervo & Adolph 1994) and fecundity advantages when sexually mature (Adolph & Porter 1996). Furthermore, growth rate of juveniles has been shown to specifically affect important lifehistory traits of N. ocellatus including size and age at maturity and hence fecundity (Wapstra 1998). Growth rate in N. ocellatus is ultimately set by the interplay of developmental conditions, postparturient environmental conditions (which can result in two-fold difference in growth rate in N. ocellatus; Wapstra 1998; unpublished growth rate data), and perhaps the time of year that offspring are born (Sinervo & Doughty 1996; Olsson & Shine 1997b; Wapstra et al. 1999). In conclusion, geographic and annual variation in the thermal environment (which affects basking opportunity) should not be ignored as a proximate source of variation in neonate phenotype. An important future direction of research is to identify mechanism(s) responsible for variation in offspring phenotype as a result of developmental conditions and their longterm fitness benefits and thus the degree to which they may be considered an adaptive modification vs a simple physiological response. 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