The mystery of the missing toes: extreme levels of natural mutilation in island lizard populations

Abstract

Summary1. Ecological interactions that involve aggressive confrontations between animals are important in shaping the evolution of morphology, behaviour and life history. However, as such confrontations are rarely witnessed, direct quantification of the intensity of these processes in natural populations is notoriously difficult. While the utilization of the frequency of non-lethal injuries is fraught with difficulties, it may provide information concerning different types of interaction, such as predation, intraspecific aggression and interspecific interference competition. 2. In this paper, we report on an exceptionally large difference in toe loss incidence between two populations of Podarcis sicula lizards living on small, neighbouring islands in the Adriatic Sea. We caught 900 lizards and recorded the number and location of missing toes. Subsequently, we investigated five non-mutually exclusive hypotheses concerning differences in bite force capacity, bone strength, predation intensity, average age and intraspecific aggression that may provide proximate explanations for the observed differences in injury frequencies. 3. Bite force differences differed considerably between the populations, but bone strength was found to be stronger in the populations with a higher frequency of natural scars. Predation pressure clearly differed between the populations, but we found higher injury rates under predation relaxation. 4. Our results indicate that density and consequently an increased intraspecific competition is the most likely explanation for the observed high frequencies of injuries. We suggest that the intensity of toe amputation between lizard populations may be a useable indirect indication for the intensity of intraspecific competition. 5. This study shows how a combination of morphological, physiological, behavioural and ecological measurements can be used to test assumptions implicit to alternative explanations of an observed phenomenon. Such tests can reveal how likely each of these explanations is, even if the processes leading to the phenomenon are difficult to observe directly.

Seeing through the lizard’s trick: do avian predators avoid autotomous tails?

“…We found a significant different degree of autotomy frequency, ranging from 25.5% in the Pod Mrčaru population up to 41% in de Pod Kopište population [25]. This is in accordance with the suspected difference in predation pressure [21].…”

“…Lizard densities on these islands are exceptionally high [25]. Because of the structural complexity of the islands, each model was not visible (from human eye view) from the next, reducing the possible confounding effects of pseudoreplication.…”

Indirect evidence of predation and intraspecific agression in three sympatric lizard species from a semi-arid area in northeastern Brazil

“…Nonetheless, intraespecific aggressive encounters and predation events are not easily observed in natural conditions. Therefore, accessing non-lethal body injuries can be useful to ascertain the frequency of these interactions in lizard populations (VERVUST et al 2009).…”

Toe Clipping of Amphibians and Reptiles: Science, Ethics, and the Law1

“…A colleague pointed out that the equivalent for a human would be the insertion of a soft drink bottle into our bodysomething most of us would probably rather avoid. In contrast, toe loss is a common phenomenon in nature (Middelburg and Strijbosh, 1988;Hudson, 1996;Vervust et al, 2009). The Guide (NRC, 2010) specifically refers to ''small rodents,'' leaving application of this method to amphibians and reptiles completely open to the discretion of individual IACUCs.…”