Effects of feral cats on the evolution of antipredator behaviors in the Aegean island lizard Podarcis erhardii

Background/Question/Methods

Exotic predators such as feral cats (Felis catus) have been the driving factors behind the extinction of many endemic species of island mammals, birds and reptiles. Island endemics appear to be exceptionally susceptible to invasive predators because of small population size and lack of anti-predator defenses. Our goal here is to determine the impacts of feral cats on the resident populations of Aegean Wall lizards (Podarcis erhardii, Lacertidae) in relationship to the expression of anti-predator behaviors.  This information can aid conservation policies by helping evaluate the potential impacts of feral cats introductions to small islands. We estimated lizard population densities in areas with low cat density sites (LCD) versus high cat density (HCD) sites by conducting 100-m transect along drystone walls, on the island of Naxos, and its surrounding islets (Cyclades, Greece). Degree of expression of antipredator behaviors was determined by measuring flight initiation distance (FID) and autotomy rate in the field and in the lab for 10 populations in HCD, LCD and satellite islets without cat presence. We also staged controlled encounters with mounted cat decoys and quantified escaping responses from lizards from these different populations.

Results/Conclusions

Feral cats had a significant negative effect on lizard populations.  Lizard densities at LCD sites were 110.8% higher than at HCD sites (4.90±1.10 vs. 10.32±1.29 indiv./100m wall). We found that lizards had adapted their anti-predator behaviors in response to cat predation by extending their FID, increasing their capacity for tail autotomy, and by staying closer to refugia (dry-stone walls). In laboratory predation simulations, using a cat decoy, lizards from cat-free islets had significantly shorter FIDs than LCD lizards and in particular HCD lizards. However, a number of unique small islet behaviors, presumably evolved in response to lack of predators and to ameliorate chronic conditions of food shortage, appear to render islet lizards strongly susceptible to cat predation. These behaviors include reluctance to use refugia, and investigatory movements towards anything new, including cats. Nonetheless, we found that repeated exposures over three trials led to significant increases in FID for all populations, indicating at least some behavioral plasticity. Our results suggest that although lizards may adapt their antipredator behaviors to cope with introduced predators, this offers at best only partial protection, so that there remains strong concern about their survival in the face of expanding feral cat populations.