Locomotor performances reflect habitat constraints in an armoured species

Abstract

Physical limits of speed performances impose strong selective pressures on animals, influencing important functions such as predator avoidance and foraging success. Armoured herbivorous species such as tortoises represent a peculiar case: features that optimise encounter rate during reproduction, the ability to reach favourable sites for thermoregulation and resting, foraging or nesting sites may be more important than running speed. To explore this issue, we measured three characteristics related to travelling ability but that are independent of running speed: (1) muscular strength, (2) time needed for overturning and (3) jumping from a high step as proxies of the ability to overcome various obstacles. Additionally, reaction times in tortoises placed in a normal or overturned position were measured as a proxy of antipredator response. More than 400 adult Hermann’s tortoises from six populations were tested in the field during two seasons. Measures of travelling ability and antipredator response varied markedly among populations, and thus with environmental characteristics such as habitat type, terrain ruggedness and presence of predators. Tortoises from rugged and hot habitats (e.g. Mediterranean macchia) were the most successful and the fastest to accomplish the tests. Overturned tortoises were more reluctant to start moving compared to those in normal position, but this effect was absent in the two localities free from predators. Sex and season had limited effect on the measured performances. Overall, locomotor features essentially varied with environmental constraints. Future environmental studies should explore whether the observed differences among populations are linked to genetic adaptation or phenotypic plasticity.

Significance statement

Locomotor performances crucially influence habitat use, foraging and reproductive success, thus directly affecting individual fitness. Although running speed is often considered as a main indicator of agility, in armoured terrestrial vertebrates some other agility components might be more important. Heavy, rigid armour imposes trade-off between protection of soft body parts and locomotor performances and consequently habitat use. This study compiles three tests of locomotor performance which might be important for overcoming various obstacles present in mosaic habitats of tortoises. Testing adult Hermann’s tortoises from six populations with various habitat characteristics, we found significant inter-population differences. The ability of tortoises to complete the tests positively correlates with climatic conditions and topography of their habitats. Additionally, presence of predators in the habitats dramatically affects tortoise’s antipredator behaviour. Measured locomotor performances and antipredator behaviour showed surprisingly low level of sexual dimorphism.