Cannibalism link to spider hatching success
Eating would-be mates could lead to a bigger brood for spiders, scientists have found.
The egg cases of American grass spiders that ate their suitors were compared with those that did not in a new study of pre-copulatory cannibalism.
More spiderlings of cannibalistic mothers hatched from thicker cases than non-cannibal offspring did, it showed.
It suggests an evolutionary advantage for female spiders that may eat males rather than mating with them.
Spider cannibals
Widow spiders such as the dangerous redback are famously named after their tendency to eat their mate after intercourse.
Some spider offspring are also known to eat their own kind, and even their mothers.
The study is published in Animal Behaviourand thought to be the first to link cannibalism with reproductive success.
Aric Berning from the University of Pittsburgh, lead author of the study said that sourcing the spiders from the funnel web family was simple, because of their urban habitat.
"There were just couple of blocks we went around and would drop crickets on these funnel webs that we saw and spiders would pop out and we'd catch them."
The researchers then introduced the males to the females' spiderwebs and monitored their behaviour.
"We tracked a number of performance metrics," he said.
"The hungrier the female, the longer she had gone without food, the more likely she was to cannibalise her would-be mate. And we also found, the more aggressive the females were, the more likely they were to cannibalise a mate."
But cannibalism was not associated with male condition or size, but rather the offspring that emerged from thicker egg sacs.
The American grass spider is a funnel web spider that thrives in urban hedgerows
Between 20 and about 200 grass spider eggs can be laid in an egg sac.
The offspring develop inside until they are little hatchlings, and then they tear their way out.
The scientists followed this process and found that heavier egg cases were linked with the emergence of a larger number of offspring for cannibalistic females but not for non-cannibalistic females.
There was no link found between cannibalism and the number of eggs laid or the mass of individual eggs or the mass of the egg case.
"Either how good the offspring are at developing or how adept they are at ripping out of the egg case appears to be the mechanism by which they most receive a benefit," said Dr Jonathan Pruitt, an advisor on the study.
"So it appears for the energetic investment for the amount of calories that they (spider mothers) put into these egg cases, cannibals get more bang for their buck.
"They're getting more offspring and they're not lower quality."
One possible explanation for this is that the male could function as a "walking multi-vitamin", said the researchers, the idea being that if an animal eats its own species it will get the nutrients needed to create offspring.
"It might also be that the males cause some kind of hormonal cue, not necessarily a nutrient but a cue that will cause the female to respond in some physiological way," said Mr Berning.
"There are females that cannibalise every single one of their mates," said Dr Pruitt
The team have not yet proposed an exact mechanism to explain it, but if selective sexual cannibalism is advantageous, there is likely to be an optimum level at which to practice it with the extreme behaviours at either end of the spectrum missing out on the benefits.
"There are females that never cannibalise and there are females that cannibalise every single one of their mates, precopula, so they end up dying spinster spiders because of their hyper aggressiveness," said Dr Pruitt.
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