Females Seek Multiple "Valentines" in Some Species

February 11, 2005

Many people will be looking forward to a candlelit table for two on Valentine's Day. But how many men would reconsider their romantic intentions if they knew they could end up holding the baby?

Well, many males in the animal kingdom have no choice in the matter. Their mates have a "wham, bam, thank you, man" attitude. Once the mating is over, the female is off, leaving dad to raise the kids. In the rare mating system called polyandry, the female breeds with several males, which then raise the offspring alone.

During courtship, the female often takes the lead, staking her breeding territory and fighting with other females over potential mates. After mating, the female departs to find new sexual partners.

Polyandry is largely confined to fish and birds, especially waders and shorebirds.

For mammals, which generally bear live young, polyandry just isn't an option. It's only the males that have the opportunity to play the field and become absent parents, says Ian Hartley, a lecturer in behavioral and evolutionary ecology at Lancaster University in England.

"In mammals, once the male has inseminated the female, the female is literally left holding the baby because it develops inside her," he said. For birds, which lay eggs, this isn't the case.

Even so, polyandry is rare in the bird world. Hartley says more than 90 percent of birds are socially monogamous (pairings remain intact during the breeding season, at least). "Polyandry accounts for about two percent of bird species, so it's pretty unusual," he added.

Why this mating system exists in some species and not others is poorly understood. New research, published last month in the scientific journal Ethology, helps to explain how it evolves. The study's author is Malte Andersson, professor of animal ecology at Göteborg University in Sweden.

Three Steps

Andersson suggests that three evolutionary steps are crucial for polyandry to develop:

1. Males take on care of the eggs.

2. Females become able to lay more eggs than a single male can accommodate.

3. Females compete with each other to lay clutches for several males.

Male-only care of young can occur for a variety of reasons. For instance, studies suggest that egg production can become difficult for female birds when food is scarce, so if males take over incubation duties, it is likely to boost the pair's breeding success.

"In fishes, a male can usually combine parental care with defense of a territory or nest," Andersson adds.

For instance, male stickleback fish will fertilize and guard the eggs of different females that visit the males' aquatic nests.

The second step toward polyandry, increased female fecundity, can occur when breeding habitats are especially rich in food, allowing female birds to lay clutches more rapidly. But the amount of chicks she can produce is limited by the number of eggs the parents can incubate.

In waders and shorebirds, which have large eggs for their body size, clutches are usually limited to a maximum of four eggs. So instead of increasing her clutch size, a better option might be for a female to have offspring with several different males.

For waders like the red-necked phalarope, which gorge themselves on midge flies during a short but intense breeding season in northern countries such as Iceland, polyandry is an ideal solution.

"Males are okay to raise the chicks alone, because they come out of the egg with feathers on, and within a few hours they are walking around and feeding," Hartley said. "The chicks don't need a lot of parental care—they just need shepherding, keeping out of the way of predators, and protection from the elements."

Andersson says similar evolutionary pressures help to explain the sexual antics of the pipefish, a close relative of the sea horse.

Eggs are transferred to the male, which then fertilizes them and then carries them in or on his body. Studies show female pipefish have a higher reproductive rate than males, with some males only able to take half the clutch of their larger mates. So the female passes over what's left to other males.

Gulf Pipefish

Genetic analysis of the gulf pipefish showed as many as four males being simultaneously "pregnant" with eggs from the same female.

The final step in the development of polyandry is for females to compete with each other over mates.

Fights between spotted sandpipers, a North American wader, can get especially nasty. When a female tries to take over a harem of males, she may attempt to puncture a rival's eye or break her leg. The males just sit there, waiting impassively for a winner to emerge.

Female dominance in the mating game is also reflected in the size and appearance of polyandrous birds. Because female red-necked phalaropes are the ones that actively seek to attract a mate, they are larger and have more brightly colored plumage than males.

Usually, the reverse is true for birds.

The most dramatic sex-role reversal is seen in the tropical freshwater birds called jacanas. Females often dwarf the males, and conflicts over territories and mates can get particularly violent.

Hartley provided an example of their ruthlessness. "When a female jacana gets predated or dies, females from next door will swallow up her territory and take on males that come with it," he said. "If the male is sitting on a clutch of eggs, the female that takes over will destroy the eggs because they're not hers. She will then force the male to sit on a new clutch that she lays."

Hartley compares this behavior to infanticide by male lions when they take control of a new pride.

So when it comes to polyandry, females definitely rule the roost.

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