Ontogenetic shift of antipredator behaviour in Hermann’s tortoises

Abstract

Armoured animals generally exhibit two main antipredator responses: They either flee or stay motionless, withdrawn in their protective armour. The transition between these two threat reactions can be affected by the degree of armour sturdiness. Tortoise shell stiffness gradually increases through ontogeny due to ossification. Additionally, neonates do not benefit from parental protection. Thus, juvenile survival could rely strongly on behavioural adaptations. This experimental approach addresses the effects of age (size), morphology, sex and population of origin on the transition between the two strategies. Predator attack was simulated by overturning individuals on their backs. Juveniles displayed bolder threat response comparing to adults. They also spent shorter periods of time withdrawn in shells and inspecting surroundings. Immature tortoises from all localities had high self-righting success, contrary to adults. The deterioration of righting success coincides with age of sexual maturation. Prompt switch from hiding to fleeing strategy in threatened juvenile tortoises implies that natural selection acts strongly on their swiftness and agility. Sexes did not diverge in antipredator displays. Self-righting speed correlated with shell shape in both juveniles and adults. Morphological measurements used in this study affected self-righting speed only in adults. These effects were accompanied with a general negative effect of increase in body size. Further studies should explore how frequency of predator encounters (i.e. experience) shape antipredator behaviour of tortoises. This could have conservation implications, especially for efficient releasing of animals from captive breeding programs.