Stuck with rigid armour in a jungle of grass and shrubs? How do tortoises manage?

Abstract

Hermann's tortoises cover long distances in their mosaic habitats. Movements often take place through dense vegetation, from grasslands to spiny bushes (e.g. in macchia). While pushing their way through, protuberant parts of tortoise's shells sometimes get stuck on vegetation. In that specific situation tortoises have to release themselves to be able to move further in a preferred direction. Furthermore, when disabled to move, tortoises can succumb from predation, overheating, starvation or dehydration. For testing of behavioural patterns involved in releasing from vegetation we used dynamometer attached to unstretchable rope with a loop on the end. This loop was fitted between protuberant front part of the plastron and front legs of examinees. During three minute long test we measured maximal muscular strength and noted all direction changes. We tested 452 adult tortoises (216 females and 236 males) at six localities in the central Balkans. Tortoises used two different tactics in releasing from the experimental setup. Group of tortoises which inhabit herbaceous habitats pulled persistently and forcibly in one direction. Others, living in Mediterranean macchia, changed directions swiftly, with weak occasional yanking of the apparatus. «Mediterranean» strategy was the efficient one in our experimental design. Reverse movements were highly useful for releasing. Vegetation cover present in the habitat probably imposes specific behavioural responses. In herbaceous habitats, tortoises live in dense meadows thus mostly stuck on grass. Tearing grass blades is probably more efficient releasing technique comparing to time consuming directions changing. Oppositely, in macchia, tortoises stuck on a branch of a spiny shrub resilient to tearing, can overcome it only by changing the direction until the obstacle fall out. Females and males generally showed similar behavioural patterns, although females used slightly more force in releasing trials. Probably experiential learning has the major role in shaping of this important behavioural response.