Testosterone, estrogen, and breeding behavior in an Arctic bird, the Lapland longspur

Abstract

Successful breeding in the Arctic requires precisely timed changes in reproductive behavior and physiology, and usually only one reproductive attempt per year is possible. Hormone and behavior patterns of Arctic breeders may therefore be different from those seen in lower-latitude species with longer breeding seasons. During 1991-1996, I investigated relationships of hormones and breeding behavior in an Alaskan population of Lapland longspurs (Calcarius lapponicus), socially monogamous passerine birds that breed only in the Arctic. Male Lapland longspurs had a very high, brief peak of circulating testosterone (T) for the first few days on the breeding grounds, during which they performed aerial song displays over a "territory" with indistinct boundaries, and were not aggressive toward simulated territorial intrusions (STIs). T then fell to intermediate levels as males guarded their mates for approximately one week, singing less and exhibiting high aggression to STIs. Finally, T, song, and aggression declined to low levels during incubation. I investigated these patterns further by implanting free-living birds with hormones (controls received empty implants). During mate-guarding, males with implants of testosterone blockers (ATD and flutamide) had decreased aggression and normal song. However, during incubation, T-implanted males had increased song and normal (low) aggression. Parental care of T-implanted males was initially low, but recovered to near-normal levels as nestlings grew. All experimental groups had normal nestling growth and normal nest success. Finally, estradiol-implanted females did not show changes in sexual behavior, nest-building or parental care, but did tend to beg more from their mates. Overall, Lapland longspurs have some hormone-behavior patterns that seem different from those seen in lower-latitude species, namely: (1) Males may have two functional levels of testosterone: at intermediate levels of T, aggression is T-dependent and song is T-independent, but very high levels of T stimulate increases in song. (2) Neither sex is very responsive to reproductive hormones after the young hatch. These patterns may be related to the Laplands' very brief courtship, brief reproductive opportunities, and the need to concentrate on parental care of their single brood.