Reproductive and environmental influences on energy regulation

Grey squirrels are polygynous breeders that reproduce in midwinter and early summer. Bimodal changes in reproductive condition accompany these seasonal breeding cycles. I tested the hypothesis that males exhibit suites of responses that anticipate seasonal variation in reproductive requirements and energetic conditions. Field observations indicated that body mass, overall activity, caloric intake, and food caching rates increased prior to each reproductive season, although these changes were greater in the fall. Body mass, overall activity and feeding rates declined post-reproduction, and no caching occurred. Reproductive condition was positively correlated with changes in all parameters except fall foraging rates. These results suggest that squirrels increase energy stored as fat and cached food in anticipation of increased reproductive demands, and decrease energy acquisition when reproductive demands cease. Energy regulation patterns were also sensitive to year-long patterns of food abundance and increased costs of homeothermy. Laboratory experiments were conducted to control for variation in resource availability and temperature. Under ad libitum food access, body mass exhibited a unimodal cycle which peaked in the fall. Masses did not increase prior to the summer reproductive season. Within a season, mass changes positively correlated with reproductive condition, suggesting that body mass set-point shifts throughout the year. Caching rates did not differ among seasons; thus, food storage is not a programmed energy regulation strategy. Food intake increased during the spring and fall, although diet composition differed between these seasons, and declined during the winter and summer. Activity rates were greatest in the spring and lowest during the winter. When food deprived, foraging and overall activity rates increased as mass decreased; these behaviors were exacerbated by increased reproductive condition. Following deprivation, caching rates were positively correlated with mass loss and reproductive condition. These results suggest that squirrels are more sensitive to energetic stress when they are reproductive, and defend body mass prior to each reproductive season with equal tenacity, despite lower mass during the spring. Taken together, these results illustrate the hierarchical nature of energy regulation; squirrels exhibited short-termed reactive responses to energy deprivation. These reactive responses are nested within long-term anticipatory responses to two sets of selective pressures: bimodal reproductive cycles and unimodal cycles in energetic conditions