Reproductive Biology of House Mice

Reproduction is at the core of every animal's existence, and virtually all aspects of physiology and behavior have, in one way or another, evolved in support of reproductive tasks. My research involves integrating multiple levels of organization-anatomy, physiology, behavior, and ecology-as it relates to mammalian reproduction. Supervised by postdoctoral fellow Michael Boechler and me, my students are engaged in a wide range of interrelated experiments concerning reproduction in house mice (Mus domesticus, formerly incorrectly classified as M. musculus). One aspect of my research involves energetics and sex differences.

Males and females have fundamentally different goals, so the sexes have evolved different physiological and behavioral strategies to maximize their reproductive success. Obtaining enough food, meeting reproductive demands, partitioning food energy among competing tissue and organ needs, and simply having enough time to complete these tasks are problems shared by all mammals. These problems are especially acute for small ( <40 g), short-lived mammals such as house mice, whose reproductive lifespan in the wild is often just a matter of weeks.

Undergraduates Paul Quindry, Greg Gunn, Mike Nelson, and Hank David are studying how the various somatic, behavioral, and gametic events leading to sexual maturity unfold during postweaning development. To mimic natural foraging conditions, young mice are raised in specially designed cages where obtaining food is dependent on locomotor activity. A computer controls a pellet dispenser (and collects data), so students can vary an animal's day-to-day foraging effort by changing the number of running-wheel revolutions required to obtain a 45 mg food pellet. We see profound differences in the way males and females deal with the energy-related demands of foraging, growth, and reproductive development. Female mammals bear the enormous energy burdens of pregnancy and lactation, so ovulation is usually inhibited whenever inadequate food energy prevents normal body growth. When necessary, however, female house mice will run tens of thousands of wheel revolutions every day-the equivalent of several kilometers-to fulfill their food needs for growth. In sharp contrast, male house mice are far less inclined to forage extensively because their sexual maturation is virtually independent of food supply or body growth.

The ability to ovulate has proven such a reliable marker of female reproductive health that undergraduates Julie Becker, Tarik Kadir, and Christine van Look recently began studying estrous cycle patterns in wild and laboratory stocks of house mice fed either high-fat or low-fat food. They soon discovered an exciting correlation between adult body weight and ovulation. The heaviest females ovulate every three to five days, while the lightest females ovulate six to eight days. In science, every answer generates many more questions, so even a simple observation such as this has opened the door to a wealth of intriguing new experiments. Our next approach will employ diet and estrous cycle manipulations as a method for studying the bioenergetic modulation of hormone-driven reproductive rhythms.