Surveillance shows crickets’ gentlemanly side

Male crickets aren’t the unreconstructed brutes they’ve been made out to be – they don’t bully and harass their female companions, but stick around and put their own lives on the line to protect them from predators.

Indeed, the relationship seems to be less a war of the sexes and more like a mutually-beneficial partnership, according to scientists from the University of Exeter.

Until now, researchers believed male crickets stay around after mating simply to stop females taking more partners, as the last one to mate with a female has the best chance of fertilizing her eggs.

But by analyzing more than 200,000 hours of video footage of crickets in the wild, taken over three breeding seasons, the researchers have shown this isn’t true.

They found no evidence of females being coerced into staying put; males didn’t hinder their mates’ movements and were never aggressive towards them. Instead, they tended to stay further away from their shared burrow than the female, and to fight any male that comes close.

Dr. Rolando Rodriguez-Muñoz is lead author of the paper, which is published in Current Biology. He said:

Our results show that relationships between crickets are rather different from what we’d all assumed. It seems that mate-guarding behavior can evolve through cooperation and not just through conflict between the sexes.

Crickets are widely used as a model of this behavior among insects in general, so the discovery has much wider implications.

Single males and females run about the same risk of being killed by predators, mostly birds. But when a male is guarding a female, he tends to let her flee into their burrow first.

This almost guarantees her escape – only one paired female was caught by a magpie over the whole three seasons – but dramatically increases the risk he’ll get eaten himself. The researchers found that paired males were 3.9 times more likely to be killed by predators than single ones, while paired females were 5.9 times less likely.

As well as better protection from predators, the female gets to increase the sperm contribution from her chosen male, who deters rivals from harassing her. She can get away if she wants – males can’t force females to mate with them – but the situation works to her advantage. In return, paired males get to mate more often and father a much greater proportion of the female’s young.

The researchers concentrate on a single meadow in northern Spain, which hosts a population of around 200 field crickets (Gryllus campestris) during most breeding seasons. At the start of each season, as the crickets emerge as adults, the scientists search the meadow and mark all the active burrows they can find.

They then deploy a battery of 96 infra-red video cameras, microphones and other sensors to monitor all the burrows continuously, keeping up the surveillance until all the crickets have died at the end of the breeding season.

The scientists also glue a tiny numbered plate to each insect’s back, letting them easily recognize it and track its activities. The crickets also have their DNA sampled, to create a unique ‘fingerprint’ for each individual that lets the scientists identify which ones have bred successfully each year.

The hard bit is watching the videos. Rodriguez-Muñoz does most of this himself. The cameras are controlled by six computers; he watches all 16 video feeds controlled by a single computer simultaneously at high speed, noting down where cricket activity is visible. He then goes back and watches each instance at normal speed, entering the details of exactly how the insects behave into a specially-designed database.

He’s now been through the first three years’ footage (from 2006 to 2008) and is now about to start work on 2009. He said:

It’s quite hard work. I have probably spent about a year of my life watching videos of cricket.

Given that the experiment is expected to continue for several more years and that even now there are several more years to get through, there’s a lot more to come. According to fellow author Professor Tom Tregenza, also of the University of Exeter:

The footage we filmed and spent months analyzing has given us a rare glimpse into how natural selection really happens in the wild. Although our study focused on one population, it is likely that our findings are applicable to other species across the insect world and could even have relevance for other animals. Perhaps females aren’t getting pushed around quite as much as we thought they were.

In other species the relationship between the sexes is certainly conflict-ridden. These tend to be those in which the female doesn’t benefit from being guarded by a male, and has some ability to resist that guarding.

Here, mating can turn into an evolutionary arms race in which males and females try to outsmart each other – for example, females may want to mate with many males to get the best sperm available, whereas males may try to prevent any others from mating with a female after they have done so.

But this research shows another model is possible – if both males and females get something out of a behaviour, then it can develop in a context of cooperation rather than competition between the sexes.

Until now scientists have thought crickets fit the first model. But the experiments that supported this idea took place in small, enclosed areas and may not have been representative of the insects’ behavior in the wild.

The results suggest lab experiments should be complemented by field studies, although Rodriguez-Muñoz said it’s possible that wild crickets could start behaving like their cousins in the lab if conditions change – for example, if the ratio between the sexes or the number of predators shifted.

October 11, 2011

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