UC Riverside

In biparental mammals, becoming a father can significantly alter males’ neural and endocrine systems, but effects of fatherhood on physiology and morphology are less clear. This dissertation tested the hypothesis that fatherhood has significant physiological, morphological and exercise-performance costs, and are modulated by parity and environmental stress. Three studies were performed using the monogamous, biparental California mouse (Peromyscus californicus). In each study, physiology, exercise performance, and morphology were compared among breeding males, non-breeding males (with a tubally ligated female [Chapter One] or an ovariectomized, estrogen/progesterone-treated female [Chapters Two and Three]) and virgin males (with another male [Chapter One] or an ovariectomized, untreated female [Chapters Two and Three]).

Chapter One examined acute effects of fatherhood. New fathers had significantly larger hindlimb muscles than non-breeding males, but virgin males had heavier subcutaneous fat pads than non-breeding or virgin males. No differences were found in physiology (basal metabolic rate [BMR], hematocrit), or locomotor performance (maximum sprint speed, treadmill endurance, maximal oxygen consumption [VO2max]).

Chapter Two characterized long-term effects of fatherhood across multiple reproductive bouts. Breeding males differed from non-breeding and/or virgin males in several physiological (resting metabolic rate [RMR]), exercise performance (sprint speed, predatory aggression), and morphological (lean, fat, and organ masses) measures at specific time points; however, differences among groups were inconsistent, and few differences were found overall. Moreover, the number of differences between breeding males and controls did not increase with parity.

Chapter Three examined effects of cold acclimation and determined how these effects were influenced by males’ reproductive condition. Cold-acclimating virgins had lower body, fat, and lean masses, and higher RMR and VO2max, than virgins at room temperature. Among long-term cold-acclimated animals, breeding males had higher BMR, lower VO2max, longer feet, and larger bacula than non-breeding and virgin males. These results suggest that cold stress may reveal energetic costs of fatherhood.

The results from this dissertation suggest that fatherhood, under the conditions tested, has few pronounced or consistent effects on physiology, exercise performance, and morphology in male California mice. This dissertation has important implications for the understanding of evolution of biparental care and the life-history of this species.