Individual boldness is linked to protective shell shape in aquatic snails

Abstract

The existence of consistent individual differences in behaviour ('animal personality') has been well documented in recent years. However, how such individual variation in behaviour is maintained over evolutionary time is an ongoing conundrum. A well-studied axis of animal personality is individual variation along a bold-shy continuum, where individuals differ consistently in their propensity to take risks. A predation-risk cost to boldness is often assumed, but also that the reproductive benefits associated with boldness lead to equivalent fitness outcomes between bold and shy individuals over a lifetime. However, an alternative or complementary explanation may be that bold individuals phenotypically compensate for their risky lifestyle to reduce predation costs, for instance by investing in more pronounced morphological defences. Here, we investigate the 'phenotypic compensation' hypothesis, i.e. that bold individuals exhibit more pronounced anti-predator defences than shy individuals, by relating shell shape in the aquatic snail Radix balthica to an index of individual boldness. Our analyses find a strong relationship between risk-taking propensity and shell shape in this species, with bolder individuals exhibiting a more defended shell shape than shy individuals. We suggest that this supports the 'phenotypic compensation' hypothesis and sheds light on a previously poorly studied mechanism to promote the maintenance of personality variation among animals.

Keywords: boldness; morphology; personality; phenotypic compensation; shell shape.

Figure 1.

(Left) A shell describing the morphological characters, examples of an elongated and less protected shell and a rounder and thicker shell with a wider aperture (right). (Online version in colour.)

Figure 2.

(a,b) Density plots of the shell shape in bold and shy Radix balthica. Shell shape characteristics are expressed as principal component scores (a) (PC1) and (b) (PC2) with the visual shapes under the x-axis.