An egg story, for Easter. It's the breeding season. Individual 'A' prowls the neighborhood, searching for a desirable member of the opposite sex. 'A' finds 'B', indulges in some flashy and intimidating displays, the two mate, and before long, their nest is full. No sooner does this happen, however, than 'A' takes off, seeking other conquests, leaving 'B' to rear the young alone. So what else is new? Sounds like an old story until you learn that individual 'A' is the female, and 'B' is the male, and they're both elegant crested tinamous.
This unconventional bird (pronounced 'tin-a-moo'), about the size of a small chicken, lives in the desolate, windswept regions of Patagonia. For the past several years, graduate student, Graeme Gissing, from the Department of Wildlife, Fisheries and Conservation Biology at UC Davis, has been observing the elegant crested tinamou in the remote Isla de los Pajaros nature preserve in Argentina. This study, conducted in collaboration with the University of Toronto's Zoology Department, takes a great deal of patience. The birds are extremely shy and elusive and do not give up their secrets easily. Gissing's perseverance has paid off, however, and he has been able to piece together an intriguing tale.
At the start of the breeding season, which lasts about four months, male tinamous set up territories with which to attract females. Through warning calls and fights, they defend these patches against neighboring males. In nature, this sort of competition usually goes along with males being bigger, more flamboyant, and more aggressive than females. Not so in the tinamou.
Females are not only bigger than males, but they join forces to dominate them. As cocks are establishing their territories and fighting among themselves, the hens are forming paired coalitions with each other. These power duos then cruise the territories, searching for a male that catches their eye. When they've found one they like, they set about harassing him with continual threat displays, as if to drive home their dominance over him. After several days, the hen-pecked male starts asserting himself over his tormentors and finally mates with both of them.
That's the last bit of fun a male will have for quite some time. The females lay their beautiful, shiny green eggs together in the same nest. After laying 7 to 8 eggs between them, they sat good-bye to the whole domestic scene and set off in search of another territory-holding male. Meanwhile, back at the nest, their deserted mate incubates the eggs entirely by himself, and keeps a watchful eye over the chicks for 2 months after they hatch.
How and why did such extreme sex role reversal come about? Gissing believes that female sociality as well as males' readiness to act as parents have been key influences in shaping this unusual breeding system. The tinamou's environment is not egg-friendly. An astounding 80-90% of all nests are lost to predators such as foxes, armadillos and skunks. For a female to leave behind any offspring at all, she must produce a succession of clutches as quickly as possible, essentially replacing ones that are eaten.. This means that someone besides the hen has to incubate the eggs, and that someone would be the cock.
So why do females team up with each other? Gissing suggests that communal laying is the fastest way to produce a clutch of eggs that's big enough to be worth a male's reproductive while to incubate. In the meantime, a female pair can go off and produce many more clutches. As a rapid egg-producing strategy, it works. Gissing has seen a female pair produce as many as 60 eggs in one season.
An additional fascinating twist to the story, as Gissing and his colleagues discovered, is that female breeding duos seem to be stable associations that can persist for several years. While there is a long tradition of male parental care and communal egg-laying among tinamous' relatives, the ostriches, rheas and emus, loyal bonds between females is highly unusual in birds, and might even be unique. Gissing and his colleagues are busy investigating the question of relatedness among the females to find out if the breeding pairs are literally sisterhoods.
Research like this tells us not only about the elegant crested tinamou, but also about the evolution of behaviors like male parental care and female domination over males. What conditions gives rise to systems like this? And why, among birds and mammals, hasn't it happened more often?
Dept. of Anthropology, University of California, Davis, CA 95616
e-mail: kjstewart@ucdavis.edu