Neuroendocrine Basis of Reproductive Flexibility

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Coombs-Hahn, Thomas IBN 98-08470 Fluctuating environmental conditions require that animals time demanding activities such as breeding to coincide with periods when food is abundant. They achieve this by responding to cues from the environment with neuroendocrine adjustments that affect reproductive physiology and behavior. A great deal has been learned about these processes through studies of temperate zone birds that breed on relatively invariant schedules each spring, but little is known of how opportunistic species capable of breeding throughout the year time breeding flexibly. This study explores how the brain regulates reproduction in a species that breeds on an extremely flexible schedule. Red crossbills are finches that are highly specialized to feed on the seeds of coniferous trees such as pines. These seeds may be sufficiently abundant for the birds to nest at almost any time of year. However they are not produced regularly at particular locations. Consequently the birds must wander nomadically, and the season when they discover abundant seeds may vary dramatically from year to year. Crossbills regularly breed during about 10 months of the year (late December through early October), and apparently occasionally nest even during the remaining two months. Two complementary questions about how crossbills achieve this reproductive flexibility will be addressed in the present study. First, how does reproductive responsiveness to environmental cues change seasonally in crossbills? In other temperate zone birds, the spring increase in day length first leads to reproductive development, but then induces birds to become "refractory" to long days. That is, they cease to respond to the stimulatory effects of long days. This refractory period ensures that breeding terminates in time for birds to replace their plumage and prepare for fall migration. It is ubiquitous among temperate zone birds that have been studied thus far, and restricts flexibil ity by prohibiting breeding for several months in late summer and autumn. It may be absent or much reduced in crossbills. Two experiments will test to see whether crossbills become refractory to the stimulatory effects of day length, and of non-light cues such as food supply and behavior of mates. Second, how do changes in crossbill sensitivity to environmental factors relate to changes in the brain? All other temperate zone birds that have been studied show dramatic reduction in production of brain reproductive hormones during the refractory period. I will examine these hormones in crossbills at several times of year to determine whether the greater reproductive flexibility of crossbills reflects relatively constant hormone production, providing a mechanistic explanation for their extraordinary reproductive flexibility. This study is important because it addresses phenomenological and mechanistic questions in a species with a highly flexible reproductive schedule. The results will clarify the physiological basis of reproductive flexibility and help to reveal whether findings based on studies of rigidly seasonal breeders apply to birds generally, and to reproductive opportunists in particular. Studies of how animals respond to environmental information help us to understand the effects of changing environmental conditions in the short and long term, and therefore have conservation implications. They also answer basic questions about responses to the environment that continue to provide insight into how all animals, including humans, may be affected by their environment.