Effects of Day Length and Temperature on Gonadal Development, Body Mass, and Fat Depots in White-Crowned Sparrows, Zonotrichia leucophrys pugetensis

Abstract

We tested the effects of ambient temperature (5°, 20°, and 30°) on photoperiodically induced reproductive functions in male and female white-crowned sparrows, Zonotrichia leucophrys pugetensis.Transfer from short days (9L 15D) to long days (16L 8D) resulted in rapid testicular growth and partial ovarian development in all three temperature treatments. There were no differences in sizes of testes and cloacal protuberance following 30 or 70 days of exposure to long days at the different temperatures. However, brood patch and follicular development were enhanced in females at 30° compared with the 5° and 20° groups. Many of these females exposed to 30° had large yolky follicles by Day 70. This enhancement was evident only when females were housed in the same room with males, however. Despite the effects of high temperature on ovarian development, there were no differences among groups in plasma levels of follicle-stimulating hormone or luteinizing hormone, suggesting that differential ovarian development may have been mediated by gonadal sensitivity to gonadotropins rather than by differential secretion of these hormones. We examined circulating levels of corticosterone (B) and both tri-iodothyronine (T 3) and thyroxine (T 4) as possible regulators of this differential ovarian sensitivity to gonadotropins. Plasma B levels showed transitory increases in males at 5° and 20°, but were suppressed in males at 30°. Titers of B were not influenced by temperature treatments in females. Circulating T 4increased following photostimulation in both sexes, but this increase was reduced at 5°. T 3concentrations in plasma were highly variable and not influenced by either photoperiod or temperature in males, but were significantly lower in females exposed to 30° by Day 70. Thus, B and T 4levels do not appear to help explain differential ovarian development, but circulating T 3levels cannot yet be excluded as a regulator of ovarian sensitivity to gonadotropins. Long days resulted in no change, or a gradual decrease, in body mass and fat deposit in males and females, and temperature regimes had no further effects on fattening or body mass. Thus, reproductive development under long days appears to be resistant to naturally relevant temperature extremes in male Z.l. pugetensis,whereas follicular development (i.e., yolk deposition in follicles leading to ovulation and onset of nesting) can be enhanced by high temperature. Reasons for the dimorphism in this response are unknown, but may be explained by the role of females in determining onset of final ovarian maturation and nesting in relation to favorable environmental conditions. In a second experiment, in which the sexes were isolated from one another, we determined the effects of the same treatments on Z.l. pugetensis.Again there was no effect of temperature on photoperiodically induced testicular growth, and the enhancement of follicular development in females at 30° was greatly reduced in the absence of males. We also continued this experiment up to 116 days of treatment to investigate effects on onset of photorefractoriness (spontaneous gonadal regression) and onset of prebasic moult. In both sexes it was clear that low temperature (5°) retarded gonadal regression and high temperature (30°) advanced it. Similarly, the prebasic moult score was greater at 30° and less at 5° in both sexes. There were no effects of temperature on plasma levels of LH at Day 116 of treatment, but plasma levels of T 4were higher in the 5° group of both males and females sampled at Day 116. Clearly, the effects of temperature can have different effects on gonadal recrudescence, onset of breeding (yolk deposition), andtermination of breeding. Whether these influences of temperature on reproductive function at different stages in the breeding cycle have different mechanisms remains to be determined.