Self-righting potential and the evolution of shell shape in Galápagos tortoises
Self-righting, i.e. the ability of an animal to turn over on its back after a fall, is a fitness-related characteristic of terrestrial animals.
Turtles can fall on their backs when travelling on uneven ground, during interactions with other animals such as fights or attacks with predators. The variation in self-righting strategy and performance - how quickly an animal turns over – depends on the flexibility of the body, its shape and size and the extension or length of the movable body parts (for example, neck and legs) . Particularly in animals with rigid and armoured bodies such as crustaceans, some insects and turtles, the feet generally cannot touch the ground when lying on their backs.
Researchers were puzzled by the diverse shell shapes of these creatures on different Galapagos islands, and there have been various hypotheses, but no one had the data to test them. This is where HBK came into play and contributed to the evolutionary study of the popular Galapagos giant tortoise.
HBK provided the good old Z6 HBK load cells and the HBK amplifier with accompanying catman software. One of the most likely theories was tested by calculating the centre of gravity within the shell. Thanks to the precise and advanced technology, this was achieved without needing to flip the vulnerable 150 kg animal.
HBK experts, in collaboration with researcher Arie van der Meijden, constructed a special apparatus. Three load cells were placed between wooden plates, positioned equidistantly from each other, forming the corners of an equilateral triangle. Due to the highly mobile and compact data acquisition system and HBK amplifier, it was possible to perform measurements in the field. The data was collected using a laptop and catman software. The data regarding the centre of gravity was obtained by placing the tortoise on the apparatus with its centre of gravity within the triangle formed by the support points. The data was analysed for each support point. HBK Project Engineers Marc van den Biggelaar and Anthony O’Reilly carried out the measurement.
By gradually tilting the apparatus and weighing the weight per load cell, all the necessary information was extracted, and the tortoise only had to remain still for a few seconds. Despite the high moisture levels, uneven terrain, heat, and other wandering tortoises, the measurement was highly successful.
Experiments, carried out on the two live animals, took place at the Rotterdam Zoo following directions of the zoo staff and according to guidelines and regulation of the EAZA (European Association for Zoos and Aquaria). All experiments were performed in accordance with relevant guidelines and regulations, no additional permits were required.