Baby cupboard spiders that can鈥檛 find enough food resort to devouring their siblings.
Cannibalism has been observed in several spider species, but it typically involves adult females eating males during or after sex. In one species, the black lace-weaver, 聽in the ultimate sacrifice.
Now, at the Netherlands Institute of Ecology and his colleagues have discovered that a common species called Steatoda grossa engages in a different kind of cannibalism 鈥� one that involves eating siblings.
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The species, which occupies dark spaces in houses in many countries including the UK, US and Australia, is more commonly known as the cupboard spider, brown house spider or false widow.
Newly hatched offspring 鈥� called spiderlings 鈥� typically stay with their siblings in their mother鈥檚 web for several weeks before going out on their own. Harvey, who was breeding the spiders in the lab, noticed that when he kept groups of spiderling siblings together in Petri dishes, 鈥渢heir numbers would decrease quite rapidly, and some would start to get quite big, suggesting they were eating their siblings,鈥� he says.
To investigate further, Harvey and his colleagues kept groups of spiderling siblings from 10 different mothers in Petri dishes, with each dish containing the offspring of a different mother, and either fed them freshly killed flies once a week, every three weeks or gave them no food. The researchers measured the rate of cannibalism by counting how many spiderlings were left per dish until only one remained because it had eaten all its siblings.
They found that spiderlings with no food were the fastest to eat their siblings, and those fed every week were the slowest. This suggests they must reach a certain level of hunger before considering an attack on their siblings, says Harvey.
鈥淭here鈥檚 a risk to it because if you鈥檙e going to attack a similar-sized sibling, you might lose,鈥� he says. Evolutionarily, it is also disadvantageous to eat siblings because it prevents them from passing on shared genes, he says.
However, if food becomes really scarce, spiderlings may be forced to think, 鈥渓isten, I need to eat or I鈥檓 going to starve to death, and I鈥檝e got a nice sibling sitting next to me, so I鈥檒l eat them鈥�, says Harvey. If a spiderling successfully consumes one of its siblings, it then grows bigger and is better placed to attack more siblings, he says.
He believes sibling cannibalism among S. grossa is probably rife in our homes because 鈥渋f [the spiderlings] are living in a cupboard or a closed space, they鈥檙e not going to be able to disperse very far, and their sources of food, like flies, are going to be hard to find,鈥� he says.
Ethology
