The objectives of this paper are to organize our concepts about the environmental regulation of reproduction in mammals and to delineate important gaps in our knowledge of this subject. The environmental factors of major importance for mammalian reproduction are food availability, ambient temperature, rainfall, the day/night cycle and a variety of social cues. The synthesis offered here uses as its core the bioenergetic control of reproduction. Thus, for example, annual patterns of breeding are viewed as reflecting primarily the caloric costs of the female's reproductive effort as they relate to the energetic costs and gains associated with her foraging effort. Body size of the female is an important consideration since it is correlated with both potential fat reserves and life span. Variation in nutrient availability may or may not be an important consideration. The evolutionary forces that have shaped the breeding success of males usually are fundamentally different from those acting on females and, by implication, the environmental controls governing reproduction probably also often differ either qualitatively or quantitatively in the two sexes. Mammals often live in habitats where energetic and nutrient challenges vary seasonally, even in the tropics. When seasonal breeding is required, a mammal may use a predictor such as photoperiod or a secondary plant compound to prepare metabolically for reproduction. A reasonable argument can be made, however, that opportunistic breeding, unenforced by a predictor, may be the most prevalent strategy extant among today's mammals. Social cues can have potent modulating actions. They can act either via discrete neural and endocrine pathways to alter specific processes such as ovulation, or they can induce nonspecific emotional states that secondarily affect reproduction. Many major gaps remain in our knowledge about the environmental regulation of mammalian reproduction. For one, we have a paucity of information about the annual patterns of breeding and about the mechanisms controlling these patterns in the most common mammals on the planet-the small to average-sized mammals living in the tropics. We probably have only a shallow conceptualization of the way available energy and nutrients control reproduction and, likewise, we may have only a narrow view of the potential kinds and uses of seasonal predictors. Finally, we have little appreciation of the way environmental cues interact with each other to control reproduction.

Ultraviolet light has been used to examine urine marks deposited by adult male house mice on filter paper on the floors of their cages during overnight tests. Both the urination frequency and the pattern in which urine was deposited on the filter paper depended upon social rank. Dominant males vigorously marked their entire cage floor, whereas subordinate males typically voided urine in only two to four pools in the corners of their cages.

Seasonal reproduction is common among mammals at all latitudes, even in the deep tropics. This paper (i) discusses the neuroendocrine pathways via which foraging conditions and predictive cues such as photoperiod enforce seasonality, (ii) considers the kinds of seasonal challenges mammals actually face in natural habitats, and (iii) uses the information thus generated to suggest how seasonal reproduction might be influenced by global climate change. Food availability and ambient temperature determine energy balance, and variation in energy balance is the ultimate cause of seasonal breeding in all mammals and the proximate cause in many. Photoperiodic cueing is common among long-lived mammals from the highest latitudes down to the mid-tropics. It is much less common in shorter lived mammals at all latitudes. An unknown predictive cue triggers reproduction in some desert and dry grassland species when it rains. The available information suggests that as our climate changes the small rodents of the world may adapt rather easily but the longer lived mammals whose reproduction is regulated by photoperiod may not do so well. A major gap in our knowledge concerns the tropics; that is where most species live and where we have the least understanding of how reproduction is regulated by environmental factors.Keywords: seasonality; gonadotropin-releasing hormone; foraging conditions; energy balance; photoperiod; the tropics The world's climate has changed radically from hot to cold and wet to dry and back again throughout its 4.5 billion-year history. When mammals first appeared 250 million years ago, the world was warming and drying out and there was only one landmass in existence, the supercontinent of Pangea. Some parts of Pangea experienced extreme seasonal cycles of climate and food availability, while others did not (Crowley 1994). Thus, some of the first mammals probably reproduced only seasonally, while others reproduced throughout the year. As Pangea broke up and the new continents spread around the world, the Earth's climate continued to shift from one extreme to the other and the expanding numbers of mammals continued to adapt reproductively. The Cretaceous mass extinction of 65 Ma opened the door to massive adaptive radiation and today more than 4000 species of mammals can be found surviving and reproducing in a huge diversity of habitats, most characterized by some degree of seasonal variation. The world's climate is changing rapidly now and there is a concern that many species may face extinction if they cannot evolve new seasonal strategies (Bradshaw & Holzapfel 2006). The objectives of this paper are threefold: first, to consider what laboratory experimentation has taught us about the neuroendocrine pathways that link seasonal factors to the reproduction of mammals; second, to relate the knowledge gained in the laboratory to the kinds of challenges mammals actually face in natural habitats; third, to use the information generated by the first two objectives to consider how mammals might or might not adapt s...

This book presents evidence that infection is cyclical with the seasons, and that this phenomenon is mirrored in cycles of immune function. The book identifies the mechanisms by which immune systems are bolstered to counteract seasonally-recurrent stressors, such as extreme temperature reductions and food shortages. Stress, infectious diseases, autoimmune diseases, and human cancers are examined, and the role of hormones such as melatonin and glucocorticoids is considered. The book begins with an overview of seasonality, biological rhythms and photoperiodism, and basic immunology, and then discusses seasonal fluctuations in disease prevalence, immune function, and energetics and endocrinology as they relate to immune function. The clinical significance of this issue is also addressed, as such seasonal changes may play an important role in the development and treatment of infections. This first monograph to examine seasonal immune function from an interdisciplinary perspective will serve practitioners as well as advanced undergraduates and graduate students in biology, immunology, human and veterinary medicine, neuroscience, endocrinology, and zoology.

Mice produce litters containing many pups, and the female fetuses that develop between male fetuses have significantly higher concentrations of the male sex steroid testosterone in both their blood and amniotic fluid than do females that develop between other female fetuses. These two types of females differ during later life in many sexually related characteristics. Thus, individual variation in sexual characteristics of adult female mice may be traceable to differential exposure to testosterone during prenatal development because of intrauterine proximity to male fetuses.

1. More is known about the western European house mouse, Mus (musculus) domesticus than any other non-human mammal. If laboratory and field information is combined, an extremely valuable understanding of the species' bioeconomy could be obtained. 2. The seven stages of mouse life-history are surveyed (up to birth, nest life, sex life, social structure, population statics and stability, senescence, and death), and the interactions between the changing phenotype and the environment are described. 3. These interactions can be used to build up a model of the opportunities and compromises which result in the fitness of individual mice. It is not yet possible to quantify such a model, but this should in principle be achievable.

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Prepubertal female rats were maintained continuously at 45% of their expected 50-day body weight by restricting their food intake. Uteri and ovaries declined in weight under these conditions. No evidence of pulsatile LH release was seen when these animals were examined at 50 days of age. Allowing unlimited access to food at this time caused rapid pubertal development. LH pulsing began in some females within 12 h; strong LH pulsing was seen in most females within 24 h, and all ovulated after only 2 1/2 or 3 1/2 days of ad libitum feeding. These were fertile ovulations, accompanied by mating and resulting in pregnancy. Administering GnRH in a pulsatile manner to 50-day-old, food-restricted animals also yielded full pubertal development. Uteri and ovaries gradually increased in weight, and ovulation occurred in 3 1/2 to 5 1/2 days. These findings support a contention that the major reproductive deficit resulting from food restriction relates to the control of GnRH secretion. In toto they also suggest a close metabolic coupling between some dimension of nutrient and/or energy processing and the GnRH pulse generator in the normally growing female.