What do some Tamarind monkeys, Tibetan tribespeople and Tasmanian native hens have in common?
They all feature groups in which two males bond with the same female.
University of Queensland Zoology Department Associate Professor Anne Goldizen began studying this form of mate-sharing among Tasmanian native hens eight years ago after completing her PhD on the same arrangement among Saddle-Back Tamarind monkeys in the Peruvian jungle.
"Both these animals exhibit polyandry in which two or more males share one female. This is highly unusual behaviour especially among birds which tend to be monogamous. The only known example among humans is in Tibet where two or more males, usually brothers, will share one wife. This is done to stop family land being divided up," Dr Goldizen said.
Funded by the United States National Science Foundation, the United States National Geographic Society, a three-year $150,000 Australian Research Council (ARC) large grant and a three-year, $60,000 ARC small grant, the study sheds new light on how evolutionary pressures can change mating strategies.
Dr Goldizen's study suggested reasons for polyandry evolving among animals varied according to environmental pressures. For example, Saddle-Back Tamarind monkeys have a high rate of birthing twins.
"Due to their small frame, female monkeys cannot carry two babies on their backs as well as feed. The mate-sharing arrangement allows the males to carry babies while the female eats," Dr Goldizen said.
However, a scarcity of good territory could be the environmental impetus for mate-sharing among Tasmanian native hens, according to Dr Goldizen's study.
"Male birds tend to be bigger and stronger so having two in the group is a big advantage in obtaining and retaining the most prized sites - those with both ready access to fresh water for drinking and tall grass or shrub to protect chicks," she said.
"Chicks leave the nest after one day and follow adults around, being fed insects. When a predator such as a crow approaches, the chicks must be able to shelter quickly under tall grass or scrub. If the chicks have to travel long distances between this shelter and water, they are more vulnerable to predators.
"Securing the best sites is vital for chick survival. Males have therefore evolved to overcome natural instincts to fight with each other for females and instead co-operate to get the best territory with their one female partner."
In 1998, Dr Goldizen received a one-year, $15,000 ARC small grant to examine mate-sharing among moorhens in south-east Queensland.
Honours student Keesha Mackenzie is investigating moorhen populations at Cooby Dam near Toowoomba.
The need to secure the best territory was again thought to be the reason for mate-sharing, Dr Goldizen said.
"We've found moorhen mating behaviour centres on groups containing two to four males and two to four females. The males mate with all females and then all birds incubate the eggs and rear the young. These birds have evolved to be hormonally in sync so a nest can be shared at the same time," Dr Goldizen said.
However, she said the study had revealed some cases of females within the same group tossing each other's eggs out of the nest and this could explain why some clutches never yielded chicks.
Another study by PhD student Fiona Manson will examine whether mate-sharing is a feature of Buff-Banded Rails - the same family as Tasmanian native hens - on Heron Island where good territories were plentiful.
"We suspect this population will be largely monogamous because of the lack of territorial pressure. Cover for chicks is plentiful as are insects, the birds' main food source," Dr Goldizen said.
To complete the Tasmanian native hen study, Dr Goldizen, her husband Botany Department researcher Alan Goldizen and research assistant David Putland studied groups of Tasmanian native hens foraging in pastures on Maria Island, off Tasmania's east coast.
The researchers found while around half of the approximately 500 birds lived and mated in simple, monogamous pairs (one male, one female), the other half exhibited mate-sharing, most commonly two males and one female.
They also observed some breeding groups containing two females sharing a male or two females and two males.
To conduct the study, the researchers trapped and banded birds then observed their behaviour during the breeding season between September and December each year.
Study results have recently been published in several journals including the Journal of Animal Ecology, AUK and Animal Behaviour. PhD student Jason Buchan recently presented the study's findings at the International Behaviour Ecology Conference in California.
For more information, contact Dr Goldizen (telephone 07 3365 4824).
