Reproduction in Zebra Finches: Hormone Levels and Effect of Dehydration

Abstract

-Zebra Finches (Poephila guttata) of arid Australia can breed continuously under favorable conditions. During droughts, however, breeding ceases but is said to begin again immediately after rain. We found that testis size in breeding birds did not change during, or between, reproductive cycles. Luteinizing hormone (irLH) levels in plasma, however, were significantly higher in males during early incubation (1.25 ng/ml), and in females during courtship (0.89 ng/ ml), than during other parts of the reproductive cycle. Plasma levels of the sex steroids were highest (1.06 ng/ml for androgen in males, 0.32 ng/ml for estrogens in females) at the same time that irLH levels were highest. Wild-caught Zebra Finches had, at capture, testes similar in size to those of aviary-breeding Zebra Finches, but after three weeks of dehydration (1 ml water/bird/week), testis size was significantly smaller and hematocrit was significantly higher: 54% vs. 50% for birds given unlimited access to water. When dehydrated birds were given unlimited access to water, testes grew significantly. Access to green grass or exposure to high relative humidity (85%) augmented the effects of the water on testis size. Spermatogenic activity in some dehydrated birds was high despite small gonad size; however, interstitial tissue was poorly developed, and plasma levels of sex steroids and irLH were low. We suggest that Zebra Finches normally maintain gonadal tissue in a functioning state unless severely dehydrated. Relief from dehydration is a necessary, but not sufficient, condition for the full expression of reproduction. For species that do not live in temporally uniform environments, the timing of reproduction can be a critical element in reproductive success. In many temperate zone birds, reproductive activity begins during the spring, when increasing daylength provides a dependable cue that indicates the approach of conditions favorable for reproduction (see reviews in Lofts and Murton 1968, Farner 1970, Farner and Lewis 1971, Wingfield and Farner 1980). Compared to daylength, other environmental cues that may play a role in controlling the timing of reproduction have received little attention (Moreau 1950, Marshall and Disney 1957, Marshall 1970, Immelmann 1971, Wingfield 1980, Earle 1981, Storey and Nicholls 1982). In some habitats, daylength may not be a reliable cue for reproduction because the onset of other favorable conditions is independent of the changes in daylength. Desert birds, for example, may encounter suitable conditions for reproduction only for brief periods after rainfall, and in such species rapid initiation of reproduction in response to rainfall, regardless of daylength or season, may maximize reproductive success. Such breeding in response to rain apparently occurs in the Zebra Finch (Poephila guttata, syn: Taeniopygia guttata castanotis) native to the arid interior of Australia. The reproductive state of Zebra Finches is not influenced by daylength (Marshall and Serventy 1958, Oksche et al. 1963, Sossinka 1975). In the mesic parts of their range, or in areas receiving agricultural irrigation, Zebra Finches breed year-round, except in the coldest winter months, with peaks in nesting attempts occurring in spring and in autumn (Frith and Tilt 1959, Kikkawa 1980). In arid central Australia, however, the species breeds following the irregular rains, regardless of the time of year (Immelmann 1965). Davies (1977), in a two-year study of Zebra Finches in Western Australia, found birds breeding in every month except March, April, and August, and suggested that both rainfall and temperature were important in the initiation of breeding. Immelmann (1963, 1965) reported that Zebra Finches copulated within a few hours of the beginning of the first rain following several months of drought, and began nesting and egglaying within one week. Zebra Finches are capable of producing many offspring under favorable conditions because (1) both members of the pair participate in nest building (Immelmann 1963), (2) a pair will nest repeatedly as long as conditions remain favorable (Ser-