While some people enjoy throwing themselves out of aeroplanes or abseiling down cliffs, other folk are happier staying at home with a nice cup of tea. These differences in personality type have fascinated scientists for decades and, more recently, these interests have spread to the study of animal behavioural traits. Much of the emphasis on animal personality types has been centred on the bold–shy continuum and trying to understand why certain individuals are willing to take more risks than others. Being bold can confer many benefits and bold individuals have been found to have higher foraging rates and more mating opportunities. One of the downsides, however, to exhibiting more risk-taking behaviour is the higher chance of being preyed upon. A recent study, by Johan Ahlgren and colleagues at Lund University, Sweden, has demonstrated a strong link between bold risk-taking behaviour and shell morphology characteristics in the snail Radix balthica. It turns out that it is your ability to protect yourself against predation that dictates whether you are likely to indulge in risk taking or not.
The study, published in Biology Letters, focused on a small air-breathing aquatic snail species, commonly found in ponds and lakes. The authors had noticed that there was a large variation in the size and shape of the snail shells, even between individuals of the same age. Wondering how this might interact with personality traits and behaviour, they set out to collect snail eggs from a series of ponds close to the campus. Upon hatching, each snail was then individually tagged with a minute numbered tag to allow identification. An individual was then carefully transported to a personality assessment arena, where the time to emerge from the shell following a fright was used as a measure of boldness. To test the consistency of this personality trait, the trials were repeated again 1 week later. Then the team deployed a rather novel approach to measure each shell's shape – placing a snail onto a flatbed scanner, they scanned it to analyse the shells and their associated characteristics.
Each snail had a distinct shy or bold personality type and that trait was highly consistent and repeatable for each individual between trials. When the team linked their boldness scores to the shell morphological traits, they found that bolder snails have a more rounded shell shape with a wider shell aperture. These shell characteristics offer enhanced protection against predation as they have a higher crushing resistance, meaning the snails are safer from shell-crushing predators such as fish. So it seems that the safety provided by a rounder shell results in those individuals being more willing to take risks and display bold-type behaviour. What is of particular interest is that these bold individuals must be aware of the increased protection that such a shell type confers and have adjusted their behaviour accordingly.
This study implies that shell morphology influences the personality type of snails and suggests that these bolder individuals are able to offset the increased risk of being preyed upon that comes with being braver through having a tougher shell. The findings provide strong support for the ‘phenotypic compensation’ hypothesis, whereby individuals that demonstrate bolder personality traits are better equipped with anti-predator defences than more shy individuals. Such a finding suggests that morphology may be a strong contributing factor when it comes to maintaining personality type – and variation – among animals. For us, it's less likely that our morphology interacts quite so strongly with our propensity to display risky behaviour and it's far more likely that those who are risk averse just prefer being cosy in the warm.