A potential mate influences reproductive development in female, but not male, pine siskins

Abstract

The role of photoperiod in avian reproductive timing has been well studied, and we are increasingly recognizing the roles of other environmental cues such as social cues. However, few studies have evaluated the extent to which males and females of the same species respond similarly to the same type of cue. Moreover, previous studies have rarely examined how variation in the quality or nature of a given social cue might modulate its effect. Here, we examine the sensitivity of male and female pine siskins (Spinus pinus) to a potential mate as a stimulatory cue for gonadal recrudescence, and we investigate whether variation in the relationship between a bird and its potential mate modulates the effect of that potential mate. Birds were initially housed without opposite sex birds on a 12L:12D photoperiod with ad libitum food. After gonadal recrudescence had begun males and females were randomly paired with an opposite sex bird or housed alone. An additional group of males was paired with estradiol-implanted females. In males, these social treatments had no effect on testis length, cloacal protuberance length, luteinizing hormone (LH) levels, or testosterone levels. In females, presence of a potential mate had a significant and positive effect on ovary score, defeathering of the brood patch, and LH levels. Among paired birds, the degree of affiliation within a pair corresponded to the extent of reproductive development in females, but not males. Thus, reproductive timing in females appears to be sensitive to both the presence of a potential mate and her relationship with him.

Keywords: Affiliation; Birds; Breeding; Environmental cues; Luteinizing hormone; Pair formation; Reproductive timing; Sex differences; Social cues.

Fig. 1

Testis length (mm) of male pine siskins housed with an estradiol-implanted female (closed circle), an unmanipulated female (open circle), or without a female (filled triangle). Mean ± 1SE is shown for each time point. There was no significant difference between treatment groups. In pine siskins, a completely regressed testis is typically 1mm in length, and a mature testis is typically 6-7mm in length (Hahn et al., 2004; MacDougall-Shackleton et al., 2006).

Fig. 2

Measures of reproductive condition in female pine siskins housed with a male (mate, closed circle) or without a male (no mate, open circle). (A) Ovary score, (B) percent of the brood patch defeathered, and (C) plasma LH differed significantly between treatment groups (p < 0.05). Mean ± 1SE is shown for each time point. A completely regressed ovary would be given an ovary score of 1, and a reproductively mature ovary would receive a score of 5 or greater.

Fig. 3

Ovary scores for female pine siskins and the degree of affiliation with their mate. Circles indicate values for individual birds. The parameter estimate from the linear mixed model is represented by the line.

Fig 4

Plasma VLDL levels of female pine siskins and the degree of affiliation with their mate. Each female was sampled on two dates; the paired samples for each female are connected by a line. The dashed line represents the parameter estimate from the linear mixed model. Smaller values of the transformed VLDL variable reflect higher raw VLDL concentrations.