BRISBANE: To deter rivals, male cuttlefish can show their courtship display on one half of their body, while pretending to be a female on the other half, Australian biologists found. But the animals know exactly in which situation this tactic works.
These findings are yet another proof of the extraordinary cognitive abilities of cephalopods – a class that includes octopuses and squid. They also open up the field for speculation about the evolution of these animals’ brainpower as a response to their sociality.
“They not only know to use a tactical display, but they know exactly in which context to use it, which is really astonishing,” said Culum Brown, a behavioural ecologist at Macquarie University in Sydney and lead author of the paper published in Biology Letters.
Too many male competitors
The mourning cuttlefish, Sepia plangon, is a relatively common species that occurs in social groups along the east coast of Australia. Like many other cephalopods, they can rapidly change the colour pattern on their bodies, which they use as camouflage, but also to attract mates.
“The populations are male-biased, so there’s a lot of competition between males to access females,” said Brown. “One of the issues that these guys have is they have to convince the girl and mate with her – and she’s choosy! So you have to put on a really good display.”
“A male has to find enough time alone with a female to convince her to mate,” he said. But his startling, stripy display easily attracts the attention of competitors, who will often disturb the courtship.
“They’re under a lot of pressure to pretend that they haven’t found a female while simultaneously convincing her to mate.” They can do that by showing their attractive courtship display only on the side of their body that faces the female, while the competitor on the other side sees a boring camouflage pattern resembling a female.
Knowing when to bluff
They use this tactic quite frequently when only one female and one competing male is present, but never in any other situation.
To study the cuttlefish, Brown and his team analysed photographs of groups of individuals taken over several years within Sydney Harbour and observed animals in captivity.
In nearly 40% of cases when a male was courting a female in the presence of a single rival, the suitor simultaneously displayed his courtship pattern on one side of his body and a camouflage on the other. And on several occasions the tactic lead to successful mating.
But the researchers never observed this behaviour if more bystanders were present or the male was alone with the female. It is obviously too difficult to hide one’s courtship display from more than one rival without being caught.
“There’s a very special circumstance in which that tactic works. And the cool thing about [the cuttlefish] is that they only ever try to use it in that particular context,” said Brown.
The Social Brain Hypothesis
Among primates, a relationship exists between social group size and certain brain areas responsible for cognitive processes. This supports the so-called ‘Social Brain Hypothesis’, which suggests that the brainpower required to keep track of complex social relationships could have been a driving force for the evolution of the human brain.
“There’s no doubt, [cephalopods] are outstanding in terms of their cognitive powers,” explains Brown. “And they do often live in these complex social groups, particularly around mating time when it is obviously important to be manipulating individuals and keeping track of who’s who.”
As the Social Brain Hypothesis works not only for primates but also for birds, why couldn’t cephalopod cognition have developed as a response to the same evolutionary pressures, asks Brown.
“It is actually quite well known that cuttlefish – and also chameleons – can be different colours on the two sides of their bodies,” commented Devi Stuart-Fox, a behavioural ecologist at the University of Melbourne, who investigates the evolution and function of animal colouration.
But usually the purpose of this behaviour is to remain inconspicuous from predators, she said. “What’s new is that they’re doing female mimicry towards members of their own species rather than trying to stay camouflaged for predators. That’s very funky about [this study].”
Abstract of the original paper in Biology Letters
Homepage of Cullum Browns research group