F Rosa Rubicondior: Altruism and Spiders

Saturday 8 October 2016

Altruism and Spiders

A female dark fishing spider (left) and its male counterpart, which sacrifices itself as a food source immediately after mating. A new study from the University of Nebraska-Lincoln and Gonzaga University has found that this cannibalism can benefit the male's offspring.
Photo: Karina I. Helm
Ultimate sacrifice: Spider's post-sex cannibalism aids offspring | University of Nebraska-Lincoln.

Altruism is one of those evolutionary traits that creationists claim not to be able to comprehend for a couple of reasons:
  1. It seems to require some sort of morality and a knowledge of outcomes, and they normally find it hard to think why anyone would do something for someone else if there is no promise of a reward. At least that is normally behind their claim that you need their god to be moral.
  2. How can a trait which is detrimental to the individual in that it causes loss or harm, be advantageous in the Darwinian sense, and so be passed on differentially in preference to a trait for selfishness?

Well, a paper published two days ago in Current Biology shows exactly how this can happen provided the sacrifice results in an increased chance of success for the future generation. This examples involves a species of spiders, more precisely, a spider in which the males is not only cannibalised by the female after mating, but in which it seems to actively facilitate its own destruction.

Unfortunately every thing apart from the abstract sits behind a paywall and the copyright holders, © 2016 Elsevier Ltd., want £24.04 for the right to reproduce even that here, so this blog post if based on information in the press release from University of Nebraska-Lincoln.

The researchers, Steven K. Schwartz, William E. Wagner Jr. and Eileen A. Hebets, believe they have found evidence to support the paternal-effort hypothesis which says that post-mating sexual cannibalism is likely to occur if this results in an increase in the number or fitness of the offspring.

By using the dark fishing spider, Dolomedes tenebrosus and by comparing the results from matings where the female eats the male after copulation, the female eats a cricket of similar size to a male and where the female eats nothing by way of a post-coital meal, they found that there was little difference between eating nothing and eating a cricket. However, eating a male results in more offspring which are also larger and longer-lived than either of the other two scenarios.

Females that cannibalized their mates produced nearly twice as many spiderlings as females that were denied their post-sex dessert, the study reported. The spiderlings also grew nearly 20 percent larger, and survived about 50 percent longer, than those whose mothers did not eat their mates.

Schwartz also decided to test whether consuming a cricket, rather than the male spider, confers the same advantages. Though the crickets were the same size as the male spiders, the study found no evidence that cricket-snacking delivers any substantial boost to the females' offspring.

"It's only when a female eats the male that we see these benefits," said Schwartz, a faculty member at Gonzaga who conducted the study while a doctoral student at Nebraska. "So there's something unique, something special, about the males.

"There might be a nutrient, or maybe a cocktail of nutrients, that is somehow concentrated in the males' bodies. We don't know what that is, but there is something going on there."

After a male transfers his sperm to a female through a reproductive organ known as a pedipalp, a bulb inside the pedipalp suddenly "goes off like an airbag," Schwartz said. The bulb remains inflated while the male's body curls up, and his heart would stop beating within hours if the female were not there to dine on him first.

Although it should be immediately obvious from a gene-centred view of evolution that any behaviour which produces more copies of the genes for that behaviour in the species gene pool will succeed over genes which have no such result. However, given that female dark fishing spiders can and do mate with multiple males, it might seem that the altruistic male spider could be benefitting the offspring of those males who don't sacrifice themselves - which would give a clear advantage to these males instead, especially since they, theoretically at least, could survive to mate with other females.

The authors suggest a reason why this altruistic behaviour might have evolved despite this:

Though the extreme behavior seems especially counterintuitive given that females will mate with multiple males, Schwartz and his colleagues have proposed several explanations for it. Because the female dark fishing spider is about 90 percent larger than the male, any attempts to resist cannibalization would probably – and probably did – prove futile. With no reasonable alternative, males may simply be "making the best of a bad situation" by offering themselves up as an offspring-enriching meal, Schwartz said.

The behavior could also have evolved in response to first-male sperm precedence – the fact that the first male to mate with a female gets to fertilize far more of her eggs than do the males that follow. Though research has yet to confirm first-male precedence in the dark fishing spider, Schwartz said that males seem to prefer virgin mates. And if the species' reproduction does adhere to first-male precedence, the value of surviving to mate with other females would diminish in short order.

"If you're a male and can find a virgin female, that's the best scenario, because you're not in sperm-competition with other males," Schwartz said. "But say a week goes by. (Even if) you survive and find another female and go on to mate with her, the probability of that next female being previously mated goes higher and higher as (time passes). So even though it's likely that (surviving) males would encounter multiple females, it's less likely that they'd be virgin."

I wonder if a intelligent (sic) design creationist can suggest a better explanation for this extreme form of altruism? Answers below, please.

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