Variation in female reproductive tract morphology across the reproductive cycle in the zebra finch

Background

In seasonally breeding birds, the reproductive tract undergoes a dramatic circannual cycle of recrudescence and regression, with oviduct size increasing 5–220 fold from the non-breeding to the breeding state. Opportunistically breeding birds can produce multiple clutches sequentially across an extended period in response primarily to environmental rather than seasonal cues. In the zebra finch, it has been shown that there is a significant reduction in gonadal morphology in non-breeding females. However, the scale of recrudescence and regression of reproductive tissue within a single breeding cycle is unknown and yet important to understand the cost of breeding, and the physiological readiness to breed in such flexible breeders.

Methods

We examined the reproductive tissue of breeding female zebra finches at six stages in the nesting cycle from pre-breeding to fledging offspring. We quantified the wet mass of the oviduct, the volume of the largest pre-ovulatory follicle, and the total number of pre-ovulatory follicles present on the ovary.

Results

Measures of the female reproductive tract were highest during nesting and laying stages and declined significantly in the later stages of the breeding cycle. Importantly, we found that the mass of reproductive tissue changes as much across a single reproductive event as that previously characterized between birds categorized as breeding and non-breeding. However, the regression of the ovary is less dramatic than that seen in seasonal breeders. This could reflect low-level maintenance of reproductive tissues in opportunistic breeders, but needs to be confirmed in wild non-breeding birds.

Sex steroid profiles and pair-maintenance behavior of captive wild-caught zebra finches (Taeniopygia guttata)

Here, we studied the life-long monogamous zebra finch, to examine the relationship between circulating sex steroid profiles and pair-maintenance behavior in pairs of wild-caught zebra finches (paired in the laboratory for >1 month). We used liquid chromatography-tandem mass spectrometry to examine a total of eight androgens and progestins [pregnenolone, progesterone, dehydroepiandrosterone (DHEA), androstenediol, pregnan-3,17-diol-20-one, androsterone, androstanediol, and testosterone]. In the plasma, only pregnenolone, progesterone, DHEA, and testosterone were above the limit of quantification. Sex steroid profiles were similar between males and females, with only circulating progesterone levels significantly different between the sexes (female > male). Circulating pregnenolone levels were high in both sexes, suggesting that pregnenolone might serve as a circulating prohormone for local steroid synthesis in zebra finches. Furthermore, circulating testosterone levels were extremely low in both sexes. Additionally, we found no correlations between circulating steroid levels and pair-maintenance behavior. Taken together, our data raise several interesting questions about the neuroendocrinology of zebra finches.