Stress Response and Coloration as Mediators of Behavioral and Physiological Variation

Differences in behavior and physiology can arise and change because of variation in life history strategies or differences in discrete polymorphisms. These differences can result from the indirect effects of predation, or responses to erratic, repeated exposures to predators in which the prey individual is not consumed (non-consumptive predation). Behavioral responses in the presence of predators, like changes in mating or antipredator behavior may differ because of life history variation or discrete morphs within populations. There are also physiological responses that occur with behavioral responses. These physiological responses occur through the animals’ stress response. A stress response is an accumulation of endocrine and neuroendocrine responses that return the body back to its normal metabolic and hormonal levels. The least killifish, Heterandria formosa is an excellent system to understand how life history variation influences responses to predation, while, melanic and silver morphs in the Eastern mosquitofish, gambusia holbrooki, are ideal for understanding how variation within populations influence responses to predation. The overarching goals of this dissertation were to evaluate variation in behavior and physiology from predation risk as a result of differences between populations of the least killifish and within a population of the Eastern mosquitofish as well as to review pleiotropic effects that stem from the central melanocortin system and their potential influence on behavior and stress within and between populations. To answer our first questions in the least killifish we created control and predator treatments and measured both populations’ differences in male mating behavior when exposed to a predator. We also analyzed female cortisol levels between the two populations to see if their stress responses varied. We exposed male and female least killifish from both populations to predator and control treatments over 30 days to measure the effect of treatment and population on reproductive output. Results showed that males from the high-predation population were more active overall than males from the low predation population in the control treatment but that males from both populations responded to the presence of predators. Females from the low predator population had higher cortisol levels in predator treatments however cortisol levels from females in high predator population were similar in control and predator treatments. Our results reflect conclusions of another study in which predator presence did not influence reproductive output in another population of H.formosa. In the Eastern mosquitofish we exposed silver and melanic males to predator and control treatments and followed behavior assays with cortisol analyses. We found that melanic males were more active than silver males in their mating behavior and this result is consistent with other studies. All males decreased their mating behaviors in the presence of predators and increased antipredator freezing behavior in predator treatments. Melanic males had higher cortisol levels on average than silver males in control treatments; melanic male cortisol levels were only half as high in the predator treatments as in the control treatments. Differences in morph behaviors and physiology may be mediated by the pleiotropic effects correlated with black coloration and also may help rare eastern mosquitofish morphs persist. Finally, we reviewed the melanocortin system and its pleiotropic effects determined the frequencies of melanic morphs within natural populations and reflected on other pathways that influence black coloration. We found that melanic morphs are rare in polymorphic populations. We also found that populations with discrete polymorphisms mostly consisted of melanic morphs that occurred 33% or less of the time in the population with melanic morphs frequencies higher than 75%. In conclusion, this work is the first to review the frequencies of melanic morphs in populations and assess behavior and stress response in the least killifish and Eastern mosquitofish. We highlight the importance of assessing short-term behavioral responses and also measuring long-term responses through predation exposure in H.formosa. We also provide evidence for the use of short and long-term measures and not solely relying on short-term responses as a means to predict fitness. We also demonstrate how variation within populations, specifically melanic coloration may influence differences in morph behaviors, the propensity to be approached or attacked by predators, and additional behavioral and physiological responses that may arise because of pleiotropy in the melanocortin system.