Monogamy in willow ptarmigan: is male vigilance important for reproductive success and survival of females?

Abstract

Willow ptarmigan, Lagopus lagopus, usually pair monogamously, but a small proportion of males are polygynous. During the pre-incubation period males accompany their mates and provide vigilance while females feed. Polygyny may be uncommon because of costs to females that share male vigilance. Time-activity budgets, reproductive success and survival of experimentally polygynous (hens that became polygynous when males were removed) and monogamous females were compared. Monogamous hens were accompanied by their mates 94% of the time and polygynous hens 60% of the time. When polygynous hens were alone they had a tendency to forage less and be alert more than when they were accompanied by the male. Polygynous hens that had low male accompaniment spent less time foraging than monogamous or polygynous hens with higher male accompaniment. Polygynous hens were involved in more agonistic interactions with other hens than monogamous hens. However, polygynous hens did not have lower reproductive success or survival than monogamous hens. Our results were compared with two other experimental studies on willow ptarmigan: one on the same population at a higher breeding density and another on a different population at low density. We conclude that male investment may be important for female survival over the autumn and winter in some years, but monogamy may be prevalent because of constraints to males of obtaining more than one female.