For monogamous sparrows, it doesn't pay to stray (but they do it anyway)

May 22, 2012

It's quite common for a female song sparrow to stray from her breeding partner and mate with the male next door, but a new study shows that sleeping around can be costly.

The 20-year study, which is reported in The , found that offspring conceived outside sparrows' social pairs go on to have lower than within-pair offspring. The findings throw a monkey wrench into theories about why ostensibly monogamous animals might be inclined to cheat.

Most bird species display some form of monogamy. Bonded pairs stay together for a breeding season, a few seasons, or sometimes for life. But beneath this veneer of monogamy, there's plenty of hanky-panky in most species. Why promiscuity exists in is "one of the biggest remaining enigmas in evolutionary ecology," said Jane Reid, a research fellow at the University of Aberdeen and one of the study's authors.

One hypothesis for this is that when a female strays she makes it count by mating with a male of higher than her social mate. The result is higher-quality offspring that have a better chance of carrying a female's genes into . This study, however, turns that explanation on its head.

The researchers studied a population of in Mandarte Island in British Columbia, Canada. Each year starting in 1993 the team drew small blood samples from nearly every hatchling in the population and used to see who fathered each bird. They found that 28 percent of all chicks were fathered by males other than a female's socially paired mate. Thirty-three percent of broods had chicks that were fathered by multiple males.

The researchers tracked both within- and extra-pair offspring throughout their lives. They found that extra-pair offspring had 40 percent fewer offspring of their own, and 30 percent fewer grandoffspring, compared to within-pair offspring.

"These results are remarkable because they are completely opposite to expectation," Reid said. "They show that females suffer a cost of promiscuity because they produce worse offspring through extra pair mating. Rather than answering the question of why females should mate promiscuously, [these results] have blown the question wide open."

Explore further: Birds 'weigh' peanuts and choose heavier ones

More information: Rebecca J. Sardell, Peter Arcese, Lukas F. Keller and Jane M. Reid, "Are There Indirect Fitness Benefits of Female Extra-Pair Reproduction? Lifetime Reproductive Success of Within-Pair and Extra-Pair Offspring." The "American Naturalist" 179:6 (June 2012).

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1.4 / 5 (9) May 22, 2012
Imagine the promiscuity of Hollywood sparrows.
5 / 5 (1) May 22, 2012
"Rather than answering the question of why females should mate promiscuously, [these results] have blown the question wide open."

The female sparrows are maybe just out for fun ...
2 / 5 (2) May 22, 2012
These results do not make any sense. Why would "promiscuity produce worse offspring"?
1.5 / 5 (8) May 22, 2012
These results do not make any sense. Why would "promiscuity produce worse offspring"?

Perhaps momma bird is out screwing around when she should be home feeding her young, or training them to stand by their future mates.
1 / 5 (7) May 22, 2012
These results do not make any sense. Why would "promiscuity produce worse offspring"?

Are these promiscuous females copulating with subordinate male bachelors? Perhaps this helps keep up the diversity of the gene pool by retaining some genes from the wimps.
1 / 5 (1) May 23, 2012
Would be interesting to see same study done with humans. One could compare for example IQ-numbers of children when they reach 18.
1.5 / 5 (10) May 23, 2012
Scientists insist on treating animals as though they have no inner life. Why would a sparrow want to copulate with a stranger? Well, why would a human want to?
1 / 5 (8) May 27, 2012
The study found that the majority of sparrows were raised in broods with only one father, and that they went on to have greater reproductive success than those raised in broods with multiple fathers.

This leads to the conclusion that the majority of sparrows choose the most successfull stratagy. Darwinism is alive and well. How is this completely opposite to expectation?

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