By Matt Walker
Editor, Earth News
Female sexual cannibals do not find their male partners particularly tasty.
Relative to their normal prey, male spiders are not very nutritious, and females only eat their mates if they are starving, researchers have found.
That suggests that, among spiders at least, hunger is not the main driving force that compels females to eat their partners after sex.
The finding by scientists at Miami University, US is published in the journal Oecologia.
Cannibalism is a behaviour that has long intrigued biologists.
Yet it remains poorly understood.
Several hypotheses have been put forward to explain it; for example, it helps reduce competition or it occurs as a result of mistaken identity.
However, the most studied idea is that a member of the same species is the perfect meal, as it contains all the nutrients an animal has in its own body, and therefore needs.
Yet there is conflicting evidence about the benefits of becoming a cannibal.
So Dr Shawn Wilder and Professor Ann Rypstra of Miami University in Hamilton, Ohio investigated just how nutritious a meal a male wolf spider is for a female.
This species (Hogna helluo) is abundant in agricultural areas of the mid-western US.
The spiders, which live for up to two years, do not build webs, but lie in wait for their prey.
After around one in three matings, a female wolf spider will eat the male.
Fatty means tasty
Dr Wilder and Prof Rypstra measured how much of a male spider a female typically eats, including the proportion of lipids and proteins consumed from his body.
They compared that to how much a female spider eats of a more usual prey, a house cricket.
A male spider (above) is only partially consumed, compared to an almost completely devoured cricket (below)
Female wolf spiders typically eat 72% of the biomass of a cricket compared to just 51% of male spiders, the researchers found.
They also sucked out much more lipids from crickets, than from their former mates.
Around 20% of a cricket's body is lipids, while just 5% of a male spider's body is lipids.
Overall, the researchers found, eating males does not provide female spiders with the nutrition required to successfully reproduce.
"Our results show that females need a lot of lipid to produce eggs, yet the male spiders have very little lipid in their bodies," Dr Wilder told the BBC.
"Males are a less nutritionally balanced food item for females than other prey items like crickets."
This was backed up by trials which showed the female spiders would often devour a cricket straight after having rejected eating a male spider.
"I was surprised by the results because sexual cannibalism has often been thought of as a nutritionally-motivated decision," says Dr Wilder.
A larger female launches an attack on a male
"Instead, our results suggest that cannibalism may [provide] a low quality meal that may be a last resort to avoid starvation."
Any meal is not necessarily a good meal for a female spider, Dr Wilder explains.
Any time a spider decides to capture prey, it comes at a considerable cost.
"The spider risks injury and spends energy during the prey capture, invests silk and digestive enzymes into the prey and must spend a considerable amount of time extracting nutrients," he says.
So the cost of eating a low quality male may be too high for females.
The only time when hunger may drive a female to eat her former partner is when a female spider is starving, says Dr Wilder.
Exactly what motivates female spiders to eat males remains somewhat of a mystery.
But that fact that males do not make a tasty meal, may help explain why so few species do it, and why it occurs relatively infrequently, he says.