A behavioral ecology approach to the causes and consequences of sibling growth rate variation in the eastern fence lizard, Sceloporus undulatus
Open Access
- Author:
- Rosier, Renee Louise
- Graduate Program:
- Biology
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 14, 2012
- Committee Members:
- Tracy Lee Langkilde, Dissertation Advisor/Co-Advisor
Victoria Anne Braithwaite Read, Committee Chair/Co-Chair
James Harold Marden, Committee Member
Sonia Angele Cavigelli, Special Member - Keywords:
- corticosterone
activity rates
boldness
feeding competition
reproductive tradeoff
body size
reproductive tradeoff
maturity - Abstract:
- This dissertation is composed of seven chapters: an introduction, five data chapters, and a conclusion, with an overall goal of increasing knowledge of how behavior can impact growth rates and body size. In the Introduction, I give an overview of how siblings can prove a useful tool for understanding causes and consequences of variation in growth rates, how behavior can play an important role in growth rate variation, and introduce the eastern fence lizard (Sceloporus undulatus) as a study system for behavioral ecology research. The first data chapter (Chapter 2) builds on research to understand factors affecting hind limb length in eastern fence lizards. Lizards that coexist with invasive, red imported fire ants (Solenopsis invicta) have relatively longer hindlimbs than those lizards that live without these ants that serve as both predator and toxic prey. This greater length of hindlimbs could be an evolved trait in response to fire ant invasion, as longer limbs aid in fire ant defense, or it could be a plastic response to higher movement rates as lizards flee ants and forage more for non-toxic prey. In this first data chapter, I tested whether fence lizards exhibit limb length plasticity by experimentally forcing lizards to climb more or less – an activity which affects limb length in other taxa. The results of this study reveal that climbing did not increase relative hind limb length of fence lizards. This suggests that observed differences in limb length between populations of lizards that are exposed, or naïve to, invasive fire ants are likely caused by evolved changes in response to altered selective pressures imposed by fire ants, rather than due to plastic, within-lifetime changes. In Chapter 3, I used my study on how climbing behavior could impact growth rates of lizards to inform considerations of ethical laboratory conditions for reptiles. Although many studies now use reptiles for research, little is known about captive housing requirements for these species. Many institutions require researchers to provide environmental enrichment for test animals, but the efficacy of these enrichment practices has only been validated in mammals and birds. I aimed to determine if the provision of climbing enrichment, providing the opportunity for these partially arboreal lizards to exhibit natural behavior, improved the quality of life for these captive animals by measuring: 1) body growth and condition, 2) baseline levels of hormonal stress (as plasma corticosterone, CORT), 3) behavior (time spent active, basking, or hiding), and 4) survival. My results showed that lizards that were provided the opportunity to climb were no larger, were similarly stressed, showed no difference in time spent active, basking, or hiding, and lived just as long as lizards that had no climbing opportunities. These results suggest that, although climbing may be a common escape and basking behavior for these lizards in the wild, the provision of climbing opportunity is unlikely to be an effective form of environmental enrichment for this species. These findings emphasize the need to quantify the impacts of environmental enrichment on common reptilian research species before determining their potential benefit, rather than relying on human intuition as a guide. Behaviors, such as activity rates and boldness, have been tied to body size for this and other species. This implies that behavior affects growth rates, driving differences in final body size. Researchers typically assume that this link is driven by these behaviors allowing animals greater access to food resources, such as through increased competitive ability; alternatively, behavior could be the consequence, rather than cause, of body size variation. I tested the relationship between behavior and body size, and the role of competition in mediating this relationship, in the third data chapter (Chapter 4). I measured the activity rates, boldness during open field trials, feeding boldness (i.e., willingness to eat in a new environment), and baseline stress (CORT, an indicator of social status) of sibling lizards. I housed sibling lizards either alone or in social groups and determined if these behaviors or body size had a greater effect on outcomes of high- and low- intensity feeding competition. This study showed that larger lizards are more active, but no bolder than, smaller, same-aged conspecifics, and had similar levels of baseline stress. I found that individuals that scored high for activity, boldness, feeding boldness, and had lower CORT values, were no more likely to eat crickets under high or low competition. Larger lizards did eat more crickets under low feeding competition, but were no more likely to eat under high competition. These results imply that the behaviors measured here do not increase access to resources by making lizards better competitors for food. Indeed, body size had the greatest impact on competitive success. My findings suggest that these behaviors, although frequently correlated with body size, are likely not driving body size variation through impacts on competitive ability but may instead be the result of differences in body size. I took this question a step further and determined if activity rates, which positively correlate with body size, could be driving growth rate and body size variation among siblings by quantifying behavior and body size of individual lizards over time (Chapter 5). Eastern fence lizards hatch at approximately the same size, but my previous research reveals that they show considerable variation in body size as sub-adults, reflecting differences in growth rates. If these lizards differ in activity rates at hatching, individuals with higher activity levels may grow to be the larger lizards through greater food intake. Such a relationship between early activity rates and later body size would help explain why activity rates are correlated with body size later in development, and allow us to predict future growth patterns and adult body sizes from behavior expressed very early in life. To test this hypothesis, I measured activity rates of lizards when they were 8 days old, and again after siblings had significantly diverged in body size, at 8 weeks old. The results of this study demonstrated that, although the most active hatchlings at 8 days old are also the most active at 8 weeks of age, and that these more active lizards are also larger at 8 weeks of age, an individual’s activity rates at 8 days of age did not predict body size at 8 weeks of age. This provides additional evidence to support the conclusions of Chapter 4, that higher activity rates may be a consequence of intrinsic growth rates, as individuals that are growing faster are forced to be more active to obtain the resources necessary to support growth, rather than a cause of growth rate variation. Fast growth rates, and associated larger body size, can provide important fitness benefits. However, energy limitations can mean that growth is prioritized over other traits such as reproductive investment. As a result, individuals that grow faster may do so at the expense of sexual maturity. For example, larger body size often increases reproductive output, but for animals that reproduce seasonally, there may not be enough time to acquire the energy necessary for large body size and reproductive maturity. However, such tradeoffs may not be present in individuals or populations that are not energetically constrained, or in cases where the selective advantages of large size and reproductive maturity are both strong. Eastern fence lizards breed only during the summer and have few opportunities to reproduce in a lifetime. Furthermore, offspring produced earlier in the breeding season have more time for growth and have higher rates of survival. As a result, selection may favor individuals that grow to larger size and mature later with higher reproductive output later (at cost to current reproduction), or individuals that mature earlier and reproduce at the beginning of the following season, thereby breeding earlier but producing fewer offspring. I tested for a tradeoff between growth rate and sexual maturity in male and female fence lizards that were approximately the same age and reared in the laboratory. I used testes mass and secondary sexual characteristics (signaling badge size and color) to quantify male reproductive status. For females, I counted the number of egg follicles, weighed them, and determined the number of these that were vitellogenic (yolked). My results show no evidence of a tradeoff between growth and reproductive maturity in this species. Larger males had larger testes but did not differ in badge size or color. Larger females had heavier follicles, but did not have more follicles overall, and were more likely to have vitellogenic follicles. For females that did have these yolked follicles, there was no relationship between body size and number of vitellogenic follicles. This study shows that there may not be a tradeoff between growth and sexual maturity in this species; instead, some individuals are able to allocate more energy into both body growth and reproductive maturity than others. This dissertation provides insight into the role of behavior in affecting limb morphology, competitive success, growth rates, and the potential for reproductive costs of faster growth, while controlling for genetic factors and age. Together, this body of work reveals that growth rates (of limbs and body size) 1) are likely under strong selective pressure, 2) exhibit relatively little plasticity, and 3) that behavior is primarily a consequence, rather than a cause, of intrinsic variation in growth rate and resulting body size. Inter-individual variation in body size is not the result of differential tradeoffs with reproductive maturity; instead, some individuals are able to allocate more energy to both of these functions than can others, likely with important fitness benefits. The research presented here provides an important foundation for future scientific inquiries regarding the impacts of behavior on morphology, reproduction, and survival in wild populations.