Non-photoperiodic regulation of reproductive physiology in the flexibly breeding pine siskin (Spinus pinus)

Abstract

In order to time reproduction to coincide with favorable conditions, animals use environmental cues to up- and down-regulate the reproductive axis appropriately. Although photoperiodic cues are one of the best studied of such environmental cues, animals also attend to others such as temperature, food availability, rainfall and social cues. Such non-photic cues are expected to be particularly important for tropical species and temperate-zone species that exhibit flexible or opportunistic breeding schedules. In this study, we investigate the use of non-photic cues, specifically food availability and social cues, to time the initiation of reproductive development in the pine siskin (Spinus pinus), a temperate-zone songbird with a flexible breeding schedule. Following winter solstice, males were housed on a 12L:12D photoperiod with either access to a preferred food, a potential mate (social cue), or both. Control birds received only maintenance diet and no mate. Access to a preferred food had a significant positive effect on testis size and circulating luteinizing hormone (LH). However, we found no effect of social treatment on reproductive development. The effect of the food treatment on reproductive development did not appear to result from effects on body mass or fat, as neither measure differed across treatments. The food treatment influenced not only reproductive physiology, but also reproductive behavior in this species, as access to seeds had a positive effect on affiliation of pairs. This study demonstrates that food is a potent stimulus for the initiation of reproductive development in pine siskins.

Figure 1

Testis length of male pine siskins receiving only maintenance diet (triangles) or maintenance diet supplemented with mixed seeds (circles), and housed singly in a cage (open symbols) or housed with a female (filled symbols). Mean ± 1 standard error is plotted.

Figure 2

Plasma LH levels of male pine siskins receiving only maintenance diet (triangles) or maintenance diet supplemented with mixed seeds (circles), and housed singly in a cage (open symbols) or housed with a female (filled symbols). Mean ± 1 standard error is plotted.

Figure 3

Plasma testosterone levels of male pine siskins receiving only maintenance diet (triangles) or maintenance diet supplemented with mixed seeds (circles), and housed singly in a cage (open symbols) or housed with a female (filled symbols). Mean ± 1 standard error is plotted.

Figure 4

Body mass and subcutaneous fat scores of male pine siskins receiving only maintenance diet (triangles) or maintenance diet supplemented with mixed seeds (circles), and housed singly in a cage (open symbols) or housed with a female (filled symbols). Means and standard errors are plotted. There were no effects of treatment on mass or fat. A significant difference (p < 0.05) between two time points is indicated by an asterisk.

Figure 5

Mean (± standard error) affiliation score of paired pine siskins receiving maintenance diet only (open bar, n = 9 pairs) and maintenance diet supplemented with mixed seeds (grey bar, n = 9 pairs). An asterisk indicates a significant difference (p < 0.05) between treatment groups