The causes and consequences of the evolution of monogamy in white-tailed ptarmigan

Benson, David

Over 90% of avian species are monogamous. But what is monogamy and what drives the social behaviors involved? In this study I examine these questions and others in a socially monogamous member of the typically polygamous grouse subfamily, Tetraoninae. I studied a population of White-tailed Ptarmigan (Lagopus leucurus ) in the alpine meadows of Logan Pass in Glacier National Park, MT, that historically has had large numbers of immigrant birds entering the population in late summer. I found that food and habitat variables related to predation avoidance do not appear to have potential to drive immigration. Water and associated variables that are important in providing a cool microclimate during the heat of the summer, on the other hand, do. I also argue that this population is in decline. With the advent of DNA fingerprinting, avian behaviorists have recently become aware of the difference between social and genetic mating systems. White-tailed Ptarmigan are primarily socially monogamous, and I found they were near-genetically monogamous as well. I argue that White-tailed Ptarmigan may be physically better able to guard their mates than male passerines due to their large size, the high visibility habitat, and the lack of constraints such as the need to leave the female to forage. I examined hormone levels to evaluate several hypotheses. The challenge hypothesis predicts that in species in which males provide no parental care, testosterone levels are near physiological maxima throughout the breeding season and are not able to elevate in the face of social challenges. White-tailed Ptarmigan males did not provide any parental care, but also did not have testosterone levels that are near physiological maximum during the breeding season. They did not respond with elevated testosterone to natural social situations or experimental “challenges”. Corticosterone, a hormone related to stress in birds, was higher in lone males than in males that are with a female and was elevated acutely in the post breeding by the immediate proximity of humans.

The causes and consequences of the evolution of monogamy in white-tailed ptarmigan

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