Shy Hermit Crabs Have More Luck with the Ladies

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Nice guys finish last? Not for the hermit crab – shy males produce more sperm and are more successful with the ladies.

While we accept the individual differences in behaviour between humans without question, terming it ‘personality’, the idea personality in animals has been met with great scepticism. However, evidence for consistent behavioural differences between individuals, known as personality or behavioural syndromes, is now widespread, the question remaining is why? What is the benefit of personality? Some scientists suggest that different personalities represent different life strategies relating to risk taking and investing in the future, others suggest that personalities exist because environmental conditions are variable and different strategies fair better under different conditions.

Like humans, hermit crabs (Pagurus bernhardus) show distinct personalities – some crabs are bold and others shy. Based on a lot of theory and experimental work in other species such as fish and birds, scientists at Plymouth University expected that the bolder hermit crabs were operating a ‘live fast, die young’ policy, investing heavily in reproduction and taking many risks to get laid. This is thought to commonly be the case when personality traits represent distinct life history strategies, and has previously been demonstrated in Atlantic silverside (Menidia menidia) and domestic dogs (Canis lupus familiaris). Under this paradigm, shy hermit crabs were thought to represent poorer quality males, whose best hope at reproduction was to grow slowly and build up resources for future reproduction. Which strategy you take may be determined through individual quality or condition during early life, characterstics that are influenced by both genetics and the environment. However, when they looked at reproductive quality (spermatophore size) and body condition in relation to risk taking personality traits in the hermit crab, they found quite the opposite pattern. Shy hermit crabs tended to have larger spermatophores (more sperm) and higher levels of the protein hemocyanin. Hemocyanin carries oxygen in the blood and individuals with higher levels are therefore fitter and healthier. Crabs with higher hemocyanin are also more able to fight rival males, further adding to their success with the ladies.

Crabs with higher hemocyanin levels were not necessarily the largest males, but they did tend to also have larger spermatophores. The authors suggest this pattern may be more widespread in nature than previously thought, and similar relationships have previously been reported in Codling moths (Cydia pomonella) and three-spined sticklebacks (Gasterosteus aculeatus). Larger males were less consistent in their risk taking behaviour, suggesting that overarching personality types are also somewhat plastic, modifying behaviour in response to different environmental conditions.

So well-endowed hermit crabs, rather than operating a live fast, die young strategy for life, appear to be carefully conserving their ample resources; avoiding predators and fights with rival males. Far from being confident bearers of positive traits, the risk-taking males are simply those who have nothing to lose. This pattern therefore represents a trade-off, hermit crabs who remain in their shell ‘startled’ for longer after being disturbed will have less time to forage and find a mate, but will be exposed to fewer risks. In the hermit crab, the individual’s condition mediates this trade-off, with high quality males choosing to safeguard their valuables for later reproduction. In this case, large spermatophores represent an investment that has not yet provided it’s return – until the male hermit crab finds a suitable, willing female, his reproductive resources are irrelevant. And since hermit crabs are pretty safe inside their shells , it’s hardly surprising they’ve tended towards a cautious strategy for life.

The traits measured in this study represent overall life-history strategies of different individuals, but can be partitioned into traits that affect day to day performance, important in growth and survival (that’s the hemacyanin) and those that affect reproductive success (that’s the spermatophore). That these traits were strongly positive correlated suggests that personality traits do fall into discrete, correlated life-history strategies known as ‘behavioural syndromes’, as has previously been hypothesised.

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Their only real problem when tucked up inside the shell is breathing – or gas exchange more precisely. The water around their bodies inside the shell slowly runs out of oxygen, forcing them to peep out and get access to fresh water. Just like putting your head under the pillow, eventually you’ve got to come out.

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