Understanding Behavioral and Physiological Outcomes of Variation in Maternal Care and Glucocorticoids

Abstract

Behavior is one of the most immediate and effective ways to respond to and cope with environmental and social stressors. Our behavioral response to stressors is proposed to be intimately linked to our hormonal stress response in a bidirectional relationship. Understanding the relationship between these two stress responses in adults and their ontogeny in juveniles helps us understand how animals will respond to rapidly changing environmental pressures and provide more context for understanding the role of individual variation in behavior in natural selection. To investigate this question, I collected hormonal and behavioral data from a wild population of North American red squirrels (Tamiasciurus hudsonicus) in Yukon Territory, Canada. Red squirrels in this region have proven to be amenable to large scale experimental hormone manipulations and offspring growth, hormones, and behavior can be tracked for the entire life of hundreds of individuals each year. Using long-term data on behavior and fecal cortisol, I found no relationship between the hormonal and behavioral stress response as was measured. To better understand the ontogeny of this lack of relationship between the stress responses, I examined the role of the maternal early life environment. Maternal behavior and physiology influence the development of phenotypes, many of which are closely related to fitness. However, maternal behavior is often difficult to observe and measure in wild animals, particularly small mammals. To tackle this problem, I measured maternal motivation by recording the time until mothers return to their pups following researchers removing and returning the pups in the nest, or a “simulated predator intrusion”. I found wide variation in the behavioral response of mothers to this nest intrusion. Some mothers were very vocal and aggressively attempt to protect their pups, other moms hung out in a nearby tree and eat quietly waited for us to return their pups. I found a mother’s maternal style, as measured by latency to return to pups following the intrusion, was repeatable within individuals and played a role in increasing the survival and growth rate of offspring. To further explore the impact of the maternal environment on offspring, I tested hypotheses about the impact of maternal glucocorticoid levels on offspring behavior and physiology by conducting a manipulation with mothers across three years to experimentally elevate circulating glucocorticoids in pregnant or lactating mothers. In offspring from these mothers, I found the behavioral traits of activity and aggression in these juveniles were linked to their hormonal stress reactivity, unlike the adults. Through these studies, I expanded on our understanding of the relationship between behavior and physiology, with a particular focus on maternal effects, in a wild small mammal. I added to the growing body of evidence showing a lack of relationship between behavioral and physiological stress responses in wild animals, suggesting the need to develop a more generalizable model of the relationship between the glucocorticoids and animal personality. Furthermore, I leveraged our ability to closely track reproduction in red squirrels to empirically assess the fitness consequences of individual variation in maternal behavior and conduct a unique field experiment to branch across developmental biology, behavioral ecology, and behavioral endocrinology.