Sex and the Crickets

A new movie set for general release today follows the lives of sexually liberated females. No, its stars are not the glamorous gals from Sex and the City. The film instead throws the spotlight on crickets, offering unprecedented video footage of the insect's mating behavior in the wild and confirming lab studies that have suggested female crickets have just as many different mates as males.

Most studies of insect behavior have been confined to the laboratory, due to the technical challenges faced in monitoring these small creatures in the wild. Although laboratory-based studies have revealed much about the behavior and physiology of insects, it wasn't known how much the controlled environment influenced these findings.

To address this issue, a team of European scientists monitored a population of flightless field crickets (Gryllus campestris) at the bottom of a river valley in northern Spain from the spring of 2006 through to the autumn of 2007. The crickets were under surveillance 24 hours a day, with a network of 64 motion-sensitive, infrared-equipped video cameras capturing a total of 250,000 hours of footage. Tags featuring a highly visible code were glued onto the backs of the crickets as soon as they emerged from their burrows so that they could be individually identified in the footage. Parentage was then assigned by extracting DNA from the tip of a hind leg.

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Caught on tape. In this video, Tom Tregenza of the University of Exeter narrates several scenes from the crickets' lives.
Credit: Tom Tregenza and Rolando Rodriguez-Muñoz

The study, published by Science, showed that the number of offspring per individual varied more among males than females: Whereas the females had between zero and eight young, that number varied from zero to 17 for the males. This finding is in agreement with laboratory studies of other polygynous species, in which males have multiple mates. British geneticist Angus Bateman hypothesized in 1948 that the difference exists because females are limited in their reproductive success by the number of eggs that they can produce, whereas males are limited only by the number of mates that they can attract. But the new study found no gender difference in the number of mates—the females acted promiscuously and mated with multiple partners as well.

The scientists conclude that some males father a disproportionately large share of the offspring in the next generation, not because they're more successful daters but because their sperm beats the competition in females inseminated by multiple partners and because their offspring are healthier. "The number of mates that a male cricket has is an indicator of their reproductive success, but it doesn't paint a complete picture," says study co-author Tom Tregenza, an ecologist at the University of Exeter in the United Kingdom.

Surprisingly, the team found that dominant males—those that were observed to win more fights—have half as many mates as subordinates. In laboratory studies, however, the dominant crickets were found to monopolize mating access to females. This shows the importance of studying insects in their natural habitats, say ecologists. "Unfortunately, laboratory studies always result in the removal of some of the sources of selection that may be incredibly important in wild populations," says evolutionary behaviorist Susan Bertram of Carleton University in Ottawa, Canada. "Studies like this are exactly what the field needs." But Bertram notes that these crickets, which build burrows and remain close to home, are relatively easy to snoop on; charting the sex lives of more mobile species would be harder.