Fiddler Crabs Exchange Sex for Protection
Male Crabs Protect Neighbouring Females in Return for Sex
Nov 9, 2009
Offering sex for favours is not merely a human attribute, it seems. Australian biologists have observed female fiddler crabs allowing strong male neighbours to mate with them in return for protection from other males.
Fiddler crabs, Uca annulipes, are so called because the male’s large fiddle-shaped claw. Despite this huge weapon, fiddlers are all small crabs – the largest being slightly over two inches across.
They are found along sea beaches and brackish inter-tidal mud flats, lagoons and swamps.The crabs live in dense, mixed sex colonies and both sexes defend territories that contain a burrow. These burrows are vital as shelter from high tides, predators and desiccation from the hot sun.
Males fight one another with their huge claws, often inflicting nasty injuries, in an attempt to wrest a burrow from other males. Female fiddler crabs don’t try to evict a crab from their burrow, but instead, search for an empty one.
Females at Risk from Young Male Crabs
Unlike the male, the female crab has no weapons with which to defend her territory. They are not at risk from dominant males, but they are likely to be attacked by younger or weaker males who may have lost a fight with another male, and even been injured.
For these secondary males, rather than challenging another big male, a better option seems to be to try to take a territory from a female.
The study by Richard Milner, Professor Michael Jennions and team leader Dr Patricia Blackwell of the Research School of Biology at the ANU looked at how female fiddler crabs go about protecting their territories.
Female Crabs Use their Feminine Wiles
Like other females with initiative, the female fiddler crabs use the one weapon they do have – sexual availability.
The ANU team wondered why didn’t dominant males, with their large weapons, drive weaponless females from their territories, without having to risk injury, as they would in fighting another male?
They observed a population of fiddler crabs in Durban Harbour, South Africa during the mating season in November and December 2008, noting whether or not mating crabs were immediate neighbours that shared a territory boundary.
“One explanation is that females gain protection from their male neighbours through neighbour coalitions and/or mate-guarding where females exchange sex for protection,” Professor Jennions wrote in the paper, Safe sex: male – female coalitions and pre- copulatory mate-guarding in a fiddler crab, published in the November 4, 2009 issue of Biology Letters.
Males Benefit from Neighbourhood Pact
Dominant males stand to gain from this neighbourhood pact with a “helpless female”. In effect it doubles his territory, without the danger or energy expenditure of fighting off another male, and of course, the male enjoys frequent sex.
Some males ensured they had female neighbours either side of them, Dr Jennions noted. Not only did that mean they could enjoy more sex, but it also meant less likelihood of dangerous territorial battles.
“Males protected their female neighbours in 95 per cent of instances where the intruder was male, and only 15 per cent of instances where the intruder was female,” he said.
“This suggests that males don’t care who their neighbour is, as long as they are female.”
“Additionally, by ensuring that their neighbour is female, males will benefit through increased opportunity for reproductive success and reduced boundary maintenance costs – because it is less costly to maintain a territory boundary with a female than a male.”
Crabs not the Only Sex for Benefits Exchangers
Dr Jennions explained that female crabs are not the only animals that employ offering sexual favours for a perceived benefit. He cited Adelie penguins, red-winged blackbirds, and pigeons.
However, his team’s research was the first evidence of such sexual exchange among crabs, as to date all the examples had come from birds.
“For example, female red-winged blackbirds (Agelaius phoeniceus) that mate with an extra-pair male are subsequently allowed to forage on his territory,” he wrote.
“In the Adelie penguin (Pygoscelis adeliae), females exchange copulations for highly sought-after stones used for nest building.
"In pigeons, females trade copulations for protection. Females initiate frequent copulations to keep their male partner close, thereby avoiding harassment from other males.”
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and Colourful Fish Mimics Identity Theft.
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