Locomotor performances reflect habitat constraints in an armoured species
Résumé
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.