Honors Theses Biology Fall 2011 Fitness benefits from climate change in a temperate lizard Donald Nathaniel Clarke Penrose Library, Whitman College Permanent URL: http://hdl.handle.net/10349/1002 This thesis has been deposited to Arminda @ Whitman College by the author(s) as part of their degree program. All rights are retained by the author(s) and they are responsible for the content.
FITNESS BENEFITS FROM CLIMATE CHANGE IN A TEMPERATE LIZARD by Donald Nathaniel Clarke A thesis submitted in partial fulfillment of the requirements for graduation with Honors in Biology. Whitman College 2011
Certificate of Approval This is to certify that the accompanying thesis by Donald Nathaniel Clarke has been accepted in partial fulfillment of the requirements for graduation with Honors in Biology. Peter A. Zani Whitman College May 11, 2011 ii
Table of Contents Preface...1 Title Page...2 Abstract...3 Introduction...3 Materials and Methods...7 Results...12 Discussion...18 Acknowledgements...24 References...25 iii
List of Figures and Tables Table 1. Second-clutch characteristics...31 Table 2. Embryo characteristics...32 Table 3. Characteristics of second clutch hatchlings...33 Table 4. Hatchling sex ratios...34 Figure 1. Second clutch success...35 Figure 2. Average interclutch interval...36 Figure 3A. Residual hatchling mass...37 Figure 3B. Residual hatchling SVL...37 Figure 4. Average hatchling embryonic period...38 iv
Preface This thesis was prepared in appropriate style and format to be submitted for consideration for publication the Journal of Experimental Biology.
FITNESS BENEFITS FROM CLIMATE CHANGE IN A TEMPERATE LIZARD Nat Clarke Whitman College, Department of Biology, 280 Boyer Avenue, Walla Walla, WA 99362 2
Abstract Temperate ectotherms have been predicted to benefit from climate change, but few data yet exist to verify these predictions, and most studies utilize a model of uniform annual temperature increase. In this study, I examine the effects of asymmetric climate warming in temperate ectotherms by subjecting female Uta stansburiana to differing nighttime temperature treatments during their reproductive cycle. I found that higher temperatures during the ovarian cycle increased the probability of reproductive success and decreased the duration of the cycle, but did not affect clutch size, egg mass, or relative clutch mass. Higher incubation temperatures increased hatchling size, and decreased the incubation period, but had no effect on incubation success. These findings indicate that higher temperatures during the breeding season could increase reproductive output and subsequent fitness in temperate ectotherms. Introduction Anthropogenic climate change has the potential to affect many organisms as global temperatures rise. Global annual average temperatures have increased steadily in recent decades (0.2 C per decade), and estimates of average warming over the next century range from 1.1 C to 6.4 C (IPCC, 2007). Climate warming has already produced observable effects on the distribution, abundance, and evolution of some species (Bradshaw et al., 2006; Parmesan, 2006). In light of these effects, accurately predicting the future impacts of climate change on organisms is a major concern to biologists. Most 3
studies analyzing effects of global warming have focused on changes in mean temperature (e.g. Buckley, 2010), but examining average trends such as these overlooks the regional effects of climate change (IPCC, 2007) as well as the underlying asymmetry inherent in climate warming (Easterling et al., 1997). Temperature increases are asymmetrically distributed over annual (Schwartz et al., 2006) and diurnal timeframes (DeGaetano and Allen, 2002; Karl et al., 1995), with the trend that both yearly and daily minima are increasing at a faster rate than maxima. For example mean nighttime temperatures are increasing at twice the rate of corresponding daytime averages (Easterling et al., 2000). While there is great value in broadly applicable models of climate effects, the biological relevance of asymmetric warming trends cannot be ignored when testing predictions. Recent research has demonstrated that both short- and long-term temperature variation can have significant effects on biotic interactions. For example, the degree of daily temperature fluctuation affects the infection rate of malaria (Paaijmans et al., 2010) and dengue virus (Lambrechts et al., 2011), and climatic variability associated with natural dynamic modes, such as the El Niño-Southern Oscillation, has been implicated in the decline of amphibian biodiversity to due chytrid fungus (Rohr and Raffel, 2010). Similarly, decreases in diurnal temperature variation associated with rising nocturnal temperatures affects growth and developmental rates in plants (Constable and Retzlaff, 2000) as well as in insects (Whitney-Johnson et al., 2005). In this study, I sought to extend our understanding of the impacts of asymmetric warming by testing for direct and latent effects on the reproductive ecophysiology of a lizard. 4
Terrestrial ectotherms are among the most diverse organisms on the planet (Wilson, 1992), but are increasingly threatened by recent and predicted declines associated with climate change (e.g., Deustch et al., 2010; Pounds, 2001; Sinervo et al., 2010). Environmental temperatures have been shown to affect fundamental life processes, such as growth and reproduction, in marine and terrestrial arthropods (Lee et al., 2003; Stillwell and Fox, 2005), fish (Vondracek et al., 1988), and amphibians (Griffiths and Dewijer, 1994). Specific to reptiles, incubation temperatures can affect hatching success and hatchling phenotypes (Hare et al., 2002; Shine et al., 1997), as well as the subsequent growth (Nelson et al., 2004), survival (Hare et al., 2004), and reproductive success (Warner et al., 2010) of hatchlings. In studying relations between temperature and incubation, it is clear that asymmetric climate warming cannot be discounted since daily temperature fluctuations during incubation can affect the timing of hatching (Du and Shine, 2010) as well as hatchling phenotype (Ashmore and Janzen, 2003) beyond the extent observed for constant temperatures. The effects of temperature variability on other aspects of reproduction are less clear. In fishes, temperature is partly responsible for seasonal gonad activity and maturation of oocytes (e.g., Koya and Kamiya, 2000). Thermal effects during the ovarian cycle are poorly studied in reptiles, but there is some indication that temperature can act as an exogenous stimulus to alter the largely endogenously-controlled rhythm of the ovarian cycle in terms of follicular growth and reproductive timing (Medonça, 1987; Tinkle and Irwin, 1965). Both growth rates and phenotypes in the hatchlings of some viviparous reptiles have been demonstrated to vary with thermal conditions (Hare and Cree, 2010). This implies that the maternal control over embryonic developmental 5
conditions ( maternal manipulation hypothesis : Shine, 1995) is not complete, and thus brings the effects of ambient temperature variability on the ovarian cycle of oviparous lizards into question as well. While I know of no reports of temperature during the ovarian cycle affecting hatchling traits, the effects on reproductive timing suggest that diurnal temperature variation could affect this period as well. The temperature effects on reproductive timing and hatchling traits indicate that ovarian development and incubation are likely to result in life-history changes due to climate warming, but it remains to be seen whether these impacts will be positive or negative. The severity of climate warming is predicted to increase with latitude (IPCC, 2007), with organisms living at higher latitudes being the most affected (Parmesan, 2007; Root et al., 2003). However, in temperate ectotherms this exaggerated effect is predicted to increase fitness because current environmental temperatures are below their physiological optimum temperatures (Deutsch et al., 2008; Huey et al., 2009). I hypothesize that asymmetric warming during the breeding season will increase the fitness of temperate ectotherms by advancing the timing of reproductive events and altering hatchling traits. Uta stansburina (Baird and Girard) is an ideal study organism to test the predictions that as temperatures increase asymmetrically: (1) female parental Uta (dams) will have a higher reproductive output; (2) the timing of oviposition will be advanced; (3) hatchlings will be larger; and (4) the total developmental period will decrease. In order to test these predictions, I manipulated nighttime temperatures to simulate different degrees of diurnal temperature variation. 6
Materials and Methods Study Organism and Study Site The side-blotched lizard, Uta stansburina, is a small (40-60 mm adult snout-vent length [SVL]) diurnal lizard widely distributed across arid habitats in western North America, ranging from Baja California, Mexico to the Columbia River Basin in Washington State. I collected Uta for this study at Wrights Point, OR (1,318 m elevation, 43 26' 12 N, 118 55' 40 W), a large rock outcrop 20 km south of Burns, OR at the northern edge of the Great Basin Desert. In more southern populations, Uta are active year round, and females regularly produce multiple clutches of eggs (Cowles, 1941; Sinervo and Doughty, 1996; Tinkle, 1967). The elevation and latitude of the study population, limit Uta activity to a short growing season from April through October, and females are only able to lay one or two clutches depending on their size and age (Nussbaum et al., 1983; Zani, 2005). While two-year-old females regularly lay two clutches, yearlings frequently lay only one unless environmental conditions are favorable (Zani, 2005; Zani and Rollyson, 2011). Ovarian Cycle In order to study the effects of diurnal temperature variation for the complete duration of the ovarian cycle, it was necessary to collect female Uta gravid with their first clutch, so that upon oviposition temperature could be controlled experimentally for the 7
entirety of the second cycle. I collected 59 females from Wrights Point in June and July 2010, marking each lizard with an identification number by removing a unique combination of the distal two phalanges of three to six toes (never the long [fourth] toe; no more than two toes per foot). Upon oviposition of a female s first clutch in the lab (see Zani, 2005), dam SVL (to 0.5 mm using a linear rule) and mass (to 0.01 g using an electronic balance) were measured. Lizards were placed into a common-garden environment of twelve identical 0.8 m 2 cages similar to those described in Zani and Rollyson (2011), modified for the outdoors with mesh covers and a 40 cm 2 piece of commercial garden shade cloth. Uta were assigned to cages at an equal density (3 female, 1 male). Males were added to cages for insemination, and rotated among cages every 7-10 d to allow females a choice of partners. Lizards were fed crickets, vestigial wing fruit flies, and mealworms ad libitum. Each female was systematically assigned based on timing of ovipostition to one of three nighttime temperature treatments in incubators set to simulate the possible climatic changes of asymmetric temperature (Warm: 28.6 ± 0.11 C [hereafter: mean ± SEM]; Intermediate: 17.4 ± 0.28 C; Cold: 10.6 ± 0.32 C). Each evening around sunset females were removed from cages and placed in cloth bags inside the incubators for 11.7 ± 0.06 h each night, and returned to outdoor cages each morning ~2 h after sunrise. This procedure was repeated daily until females oviposited their second clutch, though for logistical reasons several nighttime treatments were missed. On those nights, females remained in the outdoor cages, but no more than four nonconsecutive nights were excluded per lizard. For comparison, temperatures in the incubators, as well as in the common garden cages and natural study area were recorded at 1 h intervals with data loggers (Watchdog 100-Temp 2K; Spectrum Technologies). 8
Control Group In addition to the second clutches collected from the experimental population, I collected clutches from females in the natural population known or suspected to be gravid with their second clutch for use as a control. Females in this group remained in the wild until they were determined (by abdominal palpation) to be gravid with shelled eggs, at which point they were brought to the lab to oviposit, which occurred no more than three days after collection. Determination of first versus second clutch was based on age (estimated by size) and date of encounter because there is a strong correlation between age and lay date (Zani, 2008; Zani and Rollyson, 2011). Incubation To determine the effects of diurnal temperature variation during incubation, I collected the eggs from second clutches produced by common-garden females. Of the 59 experimental dams, 34 successfully laid second clutches (57. 6%), yielding 105 fertile eggs. Upon oviposition, eggs were weighed and one fertile egg from each clutch was preserved in 10% formalin and staged following the procedure described by Andrews and Greene (in press) in order to determine the developmental stage of embryos at the time of oviposition. The remaining eggs were then weighed and half-buried in 60 ml of substrate (9:1 vermiculite to water by volume; replaced every 7-10 d) in a 75 ml beaker and systematically assigned to one of the three nighttime temperature incubation treatments. Relative clutch mass (RCM) was determined by dividing the total clutch mass by the dam 9
mass to estimate the reproductive effort of the dams. Eggs were assigned to incubation treatments based on order of hatching, as opposed to the dam s ovarian temperature treatment, to allow for comparison of ovarian and incubation temperature effects. By reassigning the eggs, I created nine combined ovarian and incubation treatments, respectively (Warm-Warm, Warm-Intermediate, Warm-Cold, etc.). Each beaker was then placed in 6.7-L plastic boxes capable of holding 33 eggs, which were stacked inside the incubators. Boxes were rotated daily to minimize incubator effects. Eggs were incubated on a cycle of 12 h at constant warm conditions (28.6 ± 0.11 C) identical to incubation conditions elsewhere (e.g., Zani, 2008) and 12 h in the treatment incubators (see above). Once hatching began eggs were checked twice daily for hatchlings. Hatching and Release For all hatchling lizards, within 12 h after hatching, I measured the size (SVL and mass) and sex (by presence/absence of enlarged post-anal scales). All hatchlings were marked (as above) with unique identification numbers and housed for up to 12 d before being shipped overnight to Burns, OR to be released into eight semi-natural field enclosures at a site 24 km north of the source population (described in detail in Zani, 2005). Briefly, each enclosure contains 100 m 2 (20 x 5 m) of desert habitat similar to the source population, including a portion of 2-4 m high south-facing cliff, and is bordered by 0.25-m-high walls made of aluminum flashing. The enclosures were designed to allow food (insects) to enter while containing the lizards within, as well as to incorporate overwintering sites in deep cracks in the cliff face. Enclosure walls were not high enough 10
to prevent most predators (mainly snakes and birds) from entering the site (P. A. Zani, unpublished). Hatchlings were released into the enclosures over 56 d (20 Aug. 15 Oct., 2010) in randomized (using Microsoft Excel) cohorts containing a mix of treatments at a relative density of 100/ha, which has no significant impact on growth or survival (Zani, 2008). Statistical Analyses In order to test hypotheses relating to continuous dependent variables I conducted regression and one- or two-factor analysis of variance (ANOVA) tests where appropriate. Since the purpose of these comparisons was to determine variation among treatment groups, for factorial ANOVAs ovarian and/or incubation treatments, but not the controls, were included as the main effects. Where this test indicated significant variation among the treatment groups, I conducted a posteriori comparisons between groups using leastsquares linear contrasts. However, individual groups were compared to the control group using a least-squares-means comparison between the experimental and natural populations to determine if each combination of ovarian and incubation temperature differed from the control. However, for interclutch interval, insufficient control sample size prevented me from making this comparison. Furthermore, hatchling mass and SVL were positively correlated to egg mass (see Hatchling Traits in results). Thus, I calculated statistical residuals of hatchling size with initial egg mass as the independent variable for comparison to the control. For categorical dependent variables (e.g., oviposition success, hatching success) I conducted logistic regression and used 11
subsequent likelihood-ratio tests to determine significance of individual effects. All statistical were performed using JMP v. 7.0.1 for Macintosh (SAS, 2007). Results Comparison of Experimental and Natural Thermal Environments The average daytime (shaded surface) temperature for the common-garden environment was 26.1 ± 0.75 C. The average daytime (shallow crevice) temperature for the natural population was 28.9 ± 0.71 C. A polynomial regression of experimental vs. natural temperatures between these two environments revealed a significant correlation (F 2,30 = 98.28; P < 0.001; R 2 = 0.868), with both linear (F 1,30 = 141.34; P < 0.001) and non-linear (quadratic: F 1,30 = 24.62; P < 0.001) components to temperature variation. These suggested that the daytime experimental environment was not appreciably different from the natural environment. The average nighttime (shallow crevice) temperature from the natural population was 21.6 ± 0.42 C. Analysis of variance comparing the natural nighttime temperatures to each experimental treatment (see methods) indicated that the warm treatment had a significantly higher (+6.0 C) average nighttime temperature than natural (F 1,192 = 224.25; P < 0.001), and both the intermediate and cold treatments had significantly lower (-4.4 and -11.3 C, respectively) average nighttime temperatures than natural (F 1,209 = 85.19; P < 0.001 and F 1,209 = 574.37; P < 0.001, respectively). Thus, the natural variation 12
in nighttime temperature is between the warm and intermediate experimental nighttime environments. Comparison of Experimental and Natural Second-Clutch Characteristics When I compared the mean lay date of second clutches of the natural population to each of the experimental groups I found no difference in reproductive timing of the warm group (F 1,35 = 0.53; P = 0.471; Table 1), but significantly delayed timing of the intermediate (F 1,31 = 8.00; P = 0.008) and cold (F 1,28 = 11.47; P = 0.002) treatment groups. The mean size of second clutches of the natural population was larger than that of the cold group only (F 1,28 = 7.00; P = 0.013; Table 1); neither warm nor intermediate second clutch sizes differed from the natural population (F 1,35 = 3.99; P = 0.054 and F 1,31 = 1.16; P = 0.291, respectively). For all three experimental conditions, average egg mass of second clutches was greater than that of the natural population (warm: F 1,35 = 21.05; P < 0.001; intermediate: F 1,31 = 15.07; P < 0.001; cold: F 1,28 = 4.51; P = 0.043; Table 1). However, the RCM of females was no different between control and experimental groups (warm: F 1,35 = 1.00; P = 0.323; intermediate: F 1,31 = 0.26; P = 0.611; cold: F 1,28 = 0.51; P = 0.482; Table 1). Oviposition Probability, Interclutch Interval, and Second-Clutch Characteristics Since successful completion of a second clutch in nature is related to body size, I included both SVL and ovarian temperature treatment (and their interaction) as factors in 13
the logistic regression to determine the probability of laying a second clutch in experimental dams. The full model indicated significant variation could be explained by these factors (d.f. = 5; Χ 2 = 46.3; P < 0.001), while the probability of laying a second clutch was positively related both to body size (effect likelihood ratio test, d.f. = 1; Χ 2 = 36.16; P < 0.001) and ovarian cycle temperature (d.f. = 2; Χ 2 = 9.42; P = 0.009), but not their interaction (d.f. = 2; Χ 2 = 4.16; P = 0.125; Fig. 1). To determine the effects of temperature on the length of the ovarian cycle, I compared interclutch intervals between treatment groups. For interclutch interval (i.e. ovarian-cycle length) I included only ovarian-cycle temperature as a factor in the ANOVA. There was a significant association between ovarian temperatures and interclutch interval (F 2,31 = 12.82; P < 0.001; Fig. 2; Table 1) such that warmer nighttime temperatures during the ovarian cycle reduced the interclutch interval. A posteriori leastsquares mean contrasts comparing the interclutch intervals indicated significant differences between the warm and both intermediate (F 1,31 = 13.27; P < 0.001) and cold (F 1,31 = 21.58; P < 0.001) groups, but not between intermediate and cold treatments (F 1,31 = 1.60; P = 0.215). When I tested (using one-factor ANOVA) for the effects of ovarian-temperature treatment on second-clutch characteristics (Table 1) I found that the clutch size (F 2,31 = 0.72; P = 0.49), average egg mass (F 2,31 = 0.46; P = 0.635), and RCM (F 2,31 = 0.01; P = 0.906) were not related to temperature. 14
Embryonic Stage at Oviposition In order to compare developmental rates between the natural and experimental populations, I tested for difference in embryonic stage using a one-factor ANOVA with location as the predictor. The full model indicated a significant difference in stage between natural and experimental populations (F 1,49 = 4.37; P = 0.042) such that experimental embryos were oviposited at an earlier developmental stage than control embryos (Table 2). When I compared the stage of second clutch embryos of the natural population to each of the experimental treatments using least-squares mean contrasts I found a significant difference in the intermediate group (F 1,47 = 4.60; P = 0.037), but not in the warm (F 1,47 = 2.32; P = 0.133) or cold (F 1,47 = 0.67; P = 0.417) treatment groups. A similar one-factor ANOVA with location as a predictor of embryo length revealed no significant difference between populations (F 1,49 = 0.36; P = 0.550). To compare embryos between experimental treatments, I performed two onefactor ANOVAs with ovarian treatment as a predictor of embryonic stage and embryo length. These tests indicated that there was no significant difference in embryo stage (F 2,29 = 0.46; P = 0.636) or embryo length (F 1,49 = 0.26; P = 0.770) between the experimental treatments. Incubation Period, Hatching Success, and Hatchling Traits To test immediate and latent effects of temperature on incubation period, I included both ovarian and incubation temperatures and their interaction in a two-factor 15
ANOVA to determine temperature effects on the length of the incubation period. The full model indicated significant variation could be explained by these factors (F 8,58 = 328.47; P < 0.001; R 2 = 0.978), and that both ovarian (F 2,58 = 6.78; P = 0.002) and incubation temperatures (F 2,58 = 1057.42; P < 0.001; Table 3) affected incubation period length, but that their interaction was not significant (F 4,58 = 0.74; P = 0.569). Of 105 eggs in the experimental groups, 67 (63.8 %) hatched. Of the 84 eggs in the control group, 45 (53.6%) hatched. A logistic regression between all experimental groups and the control revealed no difference in hatching success (d.f. = 1; Χ 2 = 2.02; P = 0.155). However, since incubation temperature is related to hatching success in other species, I included both ovarian and incubation treatments as factors in a two-factor logistic regression to determine the probability of hatching within the experimental treatment groups. The full model indicated no effect of nighttime temperature on an egg s probability of hatching (d.f. = 8; Χ 2 = 6.43; P = 0.598). A two-by-two contingency table indicated that the sex ratio of hatchlings in the control group was not different from all experimental hatchlings combined (Χ 2 = 2.28; P = 0.131; Table 4). Since the control and warm incubation groups differed only in the preincubation conditions experienced by embryos, as a further test of the effects of experimental conditions I compared the sex ratio of the control group to the warm incubation treatment. A two-by-two contingency table indicated no difference in sex ratio of hatchlings (Χ 2 = 0.37; P = 0.542; Table 4). Following this I compared just the sex ratios within the experimental groups. Contingency table analysis revealed that neither ovarian temperature treatment (Χ 2 = 0.325; P = 0.850) nor incubation temperature 16
treatment (Χ 2 = 5.77; P = 0.056) was significantly related to the sex ratio of resulting hatchlings (Table 4). I found no difference in hatchling mass based on sex (F 1,65 = 0.55; P = 0.460), therefore I did not include sex in my subsequent analysis of hatchling mass. The initial egg mass was positively related to both hatchling mass (F 1,65 = 68.82; P < 0.001) and hatchling SVL (F 1,65 = 35.07; P < 0.001). A full-factorial ANOVA with ovarian and incubation temperature treatments could explain significant variation in residual hatchling mass (F 8,58 = 2.88; P = 0.009). Effects test revealed a positive relationship between residual hatchling mass and incubation treatment (F 2,58 = 7.46; P = 0.001; Fig. 3A), but no relationship with either ovarian treatment (F 2,58 = 0.13; P = 0.883) or the ovarian by incubation interaction (F 4,58 = 1.42; P = 0.239). A similar full-factorial ANOVA with the same factors as predictors could explain significant variation in residual hatchling SVL (F 8,58 = 4.03; P < 0.001). Effects tests revealed a positive relationship between residual hatchling SVL and incubation temperature (F 2,58 = 9.57, P < 0.001; Fig. 3B), but no relationship with either ovarian treatment (F 2,58 = 1.70; P = 0.191) or the interaction (F 4,58 = 1.34; P = 0.265). Total Embryonic Period To examine the effects of temperature during the entire developmental period (i.e. interclutch interval and incubation period combined), I performed a full-factorial ANOVA with ovarian and incubation temperature treatments as predictors of total embryonic period length. The full model indicated significant variation was explained by 17
these factors (F 8,58 = 109.86; P < 0.001; R 2 = 0.938), and that the length of the embryonic period was affected by both ovarian temperatures (F 2,58 = 27.01; P < 0.001; Fig. 4) and incubation temperatures (F 2,58 = 320.92; P < 0.001), but not the interaction between the two (F 4,58 = 0.28; P = 0.890). Discussion It is clear from the results of this study that in addition to dam body size, ovarian temperature affects the probability of laying a second clutch (Fig. 1) such that higher temperatures may cause an increase in annual reproductive output even in the absence of a longer growing season. Although other studies have reported decreases in reproductive output above optimal temperatures due to physiologic stresses (Luo et al., 2010), climate change in the form of asymmetric warming (e.g., Easterling et al., 1997) could increase mean environmental temperatures without surpassing a species physiologic optima as minimum temperatures disproportionately rise, allowing for an overall increase in fecundity. Two major factors comprising fitness of dams are reproduction and survival, and clearly a doubling of reproductive output by successful second clutches may likely increase reproductive potential. My results indicate that the length of ovarian cycle is negatively related to treatment temperature (Fig. 2), which could minimize risk of mortality. For example, the locomotor capacity of female Uta and other lizards are reduced while gravid, which could reduce their ability to escape predators (e.g., Zani et al., 2008). Shortening this vulnerable period in the female life cycle could increase 18
survivorship in females by reducing predation. Thus, these results indicate that climate change is likely to produce fitness benefits for temperate ectotherm dams. Not only does it appear that climate warming will benefit dams, there are also potential fitness benefits for the hatchlings of temperate ectotherms. I found that hatchling length and mass are both positively correlated with incubation temperature, but not significantly affected by ovarian temperature (Fig. 3). That is, there are direct thermal effects from the incubation environment, but not latent effects from the temperature exposure of the dam. This lack of a latent thermal effect is similar to that reported for other ectotherm species. For example, Zani et al. (2005) showed that immediate survival is affected by temperature, but subsequent fecundity or fertility are not. However, these direct effects were enough to alter fitness, measured as per capita reproduction (Zani et al., 2005). I observed differences in clutch size between clutches from the experimental and control populations, such that the clutch size of experimental treatment groups tended to be smaller (0.37-1.1 fewer eggs per clutch) than second clutches from the natural population (Table 1). In addition, average egg mass was greater (0.04 0.05 g heavier per egg) in experimental treatments when compared to the natural population. While this might indicate a difference in maternal investment between temperature treatments and the control group, since there was no difference in RCM between experimental and natural second clutches (Table 1), I interpret these differences as changes in reproductive strategy (egg size vs. number) as opposed to reproductive effort. Previous research in lizards has shown that relative food availability reduces both clutch size and clutch number (Ballinger, 1977). Furthermore, Sinervo and colleagues demonstrated that a high 19
population density in Uta reduced clutch sizes through stress-induced hormonal pathways (Sinervo et al., 2000). In addition to density, manipulations such as handling or housing in an unfamiliar enclosure can increase plasma corticosterone in lizards (Langkilde and Shine, 2006). Since animals were housed in an enclosure for the duration of their second clutch, handled daily when exposed to temperature treatments, and placed in isolation for oviposition, it is probable that the differences in clutch size and egg mass I observed were due to experimental conditions as opposed to temperature. Which specific experimental factor caused this life-history shift is not clear. However, since animals experienced a common-garden environment except for nighttime temperature treatments, the primary statistical comparison this may confound is between experimental and control populations. One known direct effect of temperature on reptile hatchling phenotype deals with body size (Michaud and Echternacht, 1995). In turn, large hatchling size increases hatchling fitness in Uta (Ferguson and Fox 1984), as well as in other species (Telemeco et al., 2010). Incubation temperature also affects hatchling thermoregulatory behavior (Goodman and Walguarnery, 2007) and locomotor performance (Elphick and Shine, 1998; Hare et al., 2008), which could impact fitness. However, a complicating factor for determining thermal effects on fitness is that any benefit from increased incubation temperature in nature is dependent on dam nest-site choice, which has been shown to affect nest temperatures and characters in resulting hatchlings (Weisrock and Janzen, 1999). Still, all indications are that warming incubation environments due to climate change could benefit temperate ectotherms by producing larger, more robust hatchlings. 20
Even if hatchling traits are unaffected by climate change, there may be substantial fitness benefits from the advancement of hatching date. I found that the total embryonic period (from laying of the first clutch to hatching of the second) is negatively correlated to temperature during both the ovarian cycle and incubation (Fig. 4), but incubation temperature appears to have a more pronounced effect as evidenced by the difference in F-values (see results; Fig. 4). A shorter embryonic period would result in an advanced phenology (earlier time of hatching), which in turn would allow hatchlings more time to grow and store energy for winter, and as the results of this study demonstrate, altered nighttime temperatures can cause significant variation in embryonic period (difference of up to 50 d; Table 2). Developing embryos do not appear to have the capacity to compensate physiologically for cold temperatures during incubation (Booth, 1998) or to hatch prematurely in response to decreasing temperatures, such as those experience naturally in autumn (Shine, 2002). Advancing reproductive phenology is one of the most likely means of maximizing fitness in seasonal environments, and is a common pattern in many lizards (Sinervo and Doughty, 1996; Warner and Shine, 2007) and other ectotherms, such as fishes (Schultz, 1993) and insects (Landa, 1992), as well as in endotherms (Verhulst, 1998). Previous studies in a skink (Bassiana duperryia) have shown that embryonic heart rate increases exponentially with temperature during incubation (Du and Shine, 2010). This could explain the non-linear temperature dependence of development during both the ovarian (Fig. 2) and incubation (Fig. 4) environments observed in this study. Embryonic stage data support the idea that the rate of development in Uta can vary with temperature since all embryos in this study were oviposited within one stage interval 21
(stage 27 28; Table 3) of each other. This stage of development is within the distribution of stages previously observed at oviposition in this species (stage 25 30; Andrews and Greene, in press), despite the high variability in the length of ovarian-cycle length in different treatments (up to 9 d; Table 1). It appears that temperature can cause variation in growth rates between developmental checkpoints such as oviposition and hatching, possibly by altering embryonic heart rate. Future asymmetric warming is likely increase reproductive output of females and subsequent fitness of their offspring, which could subsequently increase abundance and dispersal in certain temperate ectotherms. However, predicting increases in population sizes and geographic ranges is complex. An increase in fecundity and hatchling fitness may not correlate to an in increase in population size if overwinter survival decreases as temperatures rise as has been seen with both insects (Irwin and Lee, 2000) and lizards (Zani 2008). Since Uta have lower survival rates when hibernating at warmer temperatures (Zani, 2008), an increased hatchling population might not translate into increased recruitment of yearlings in the spring. Similarly the diverse affects of changing climatic factors other than temperature can have unforeseen consequences. For example, Hare and Cree (2010) demonstrated that an increase in cloud cover due to climate change could potentially reduce pregnancy success in temperate ectotherms by reducing basking potential. Even excluding complicating environmental factors, it is still unclear that increased reproductive output in temperate ectotherms will increase population sizes. For example, Tinkle (1969) predicted that as fecundity increases in ectotherms, mean life expectancy should decrease to compensate for population growth. Life-history theory is complex, but past studies indicate that predation (Reznick and Endler, 1985) and survival 22
costs of reproduction (Shine, 1980) are important predictors of ectotherm life history. Further study is necessary to quantify these variables in a natural setting before accurate predictions of population growth can be made. If warming temperatures do increase the abundance of ectotherm species, dispersal and range expansion seem probable (Parmesan, 2006). However, recent studies indicate that range shifts due to climate are affected by behavioral and physiological traits, and cannot be simply estimated based on thermal optima (e.g., Buckley, 2010). Furthermore, models predicting species range shifts have a large degree of inherent uncertainty, resulting in predictions ranging from insignificant dispersal to near-complete species turnover in temperate regions (Thuiller, 2004). It is beyond the scope of this study to predict the impacts of these fitness benefits. However, the present study is one of the first to verify climate predictions made for temperate ectotherms (e.g., Huey et al. 2009), and to test experimentally the effects of asymmetric diurnal temperature change (Easterling et al., 2000). Temperatures are rising more rapidly at mid to high latitudes in the Northern hemisphere (IPCC, 2007), and organisms living at higher latitudes appear to be affected disproportionately by climate change (Parmesan, 2007; Root et al., 2003). Among ectotherms, however, tropical species have been predicted to be the most detrimentally affected (Deutsch et al., 2008) and extinctions have already been reported at low latitudes (Sinervo et al., 2010). For this reason, increasing our ability to understand and predict biological responses to future climate change is of critical importance. 23
Acknowledgements I would like to thank Peter Zani for advising this thesis research, Lauren Flynn for assistance with fieldwork, Kathy Flanery for housing and lab assistance, Tree-Tops Island Ranch for access to Wrights Point, James Cooke and family for use of their property for the field enclosures, Robin Andrews for staging embryos, Dave Ganskopp for assistance releasing hatchlings, the Whitman Biology Department for financial support, Delbert Hutchison, Thomas Knight, and Daniel Vernon for helpful comments on a previous draft of this thesis, and the Gonzaga University IACUC for approval of animal care protocol. 24
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