Sex differences in the response to environmental cues regulating seasonal reproduction in birds

Abstract

Although it is axiomatic that males and females differ in relation to many aspects of reproduction related to physiology, morphology and behaviour, relatively little is known about possible sex differences in the response to cues from the environment that control the timing of seasonal breeding. This review concerns the environmental regulation of seasonal reproduction in birds and how this process might differ between males and females. From an evolutionary perspective, the sexes can be expected to differ in the cues they use to time reproduction. Female reproductive fitness typically varies more as a function of fecundity selection, while male reproductive fitness varies more as a function sexual selection. Consequently, variation in the precision of the timing of egg laying is likely to have more serious fitness consequences for females than for males, while variation in the timing of recrudescence of the male testes and accompanying territory establishment and courtship are likely to have more serious fitness consequences for males. From the proximate perspective, sex differences in the control of reproduction could be regulated via the response to photoperiod or in the relative importance and action of supplementary factors (such as temperature, food supply, nesting sites and behavioural interactions) that adjust the timing of reproduction so that it is in step with local conditions. For example, there is clear evidence in several temperate zone avian species that females require both supplementary factors and long photoperiods in order for follicles to develop, while males can attain full gonadal size based on photoperiodic stimulation alone. The neuroendocrine basis of these sex differences is not well understood, though there are many candidate mechanisms in the brain as well as throughout the entire hypothalamo-pituitary-gonadal axis that might be important.

Figure 1

Seasonal changes in (a) paired testis mass (mg) and (b) largest ovarian follicle diameter (mm) in male and female European starlings (Sturnus vulgaris), respectively, collected each month from March 1994 to February 1995 in Baltimore, MD, USA. Points represent mean±s.e. of the mean. Points with different letters differ significantly, while points with the same letter do not. Note how similar the relative pattern of change is in these field-caught male and female starlings. Male starlings, like many other temperate zone songbird species, when held in captivity under artificial photoperiods exhibit changes of a similar magnitude to that observed in the field. In contrast, captive female starlings exhibit a change of a much smaller magnitude.

Figure 2

Comparison of effect of ad libitum food versus mild food restriction on seasonal variation in gonad size in (a) male and (b) female red crossbills (adapted from Hahn et al. 2005).