Differences in morphology, performance and behaviour between recently diverged populations of Podarcis sicula mirror differences in predation pressure

Abstract

We investigated the possible role of variation in predation pressure in the phenotypic divergence of two island populations of the Italian wall lizard, Podarcis sicula . In 1971, ten adult specimens from the island of Pod Kopište (Adriatic Sea, Croatia) were transported to the island of Pod Mrčaru, 3.5 km east, where they founded a new population. Although the two islands resemble each other in general physiognomy (size, elevation, microclimate) and in the absence of terrestrial predators, lizards from the newly established population are now on average larger and have shorter hind limbs. They also exhibit lower maximal sprint speed as measured on a racetrack, and fatigue faster when chased in a torus track. In the field, lizards from the original population of Pod Kopište respond to a simulated predatory attack by fleeing at larger approach distances and by running further from the predator than lizards from Pod Mrčaru. These changes in morphology, behaviour and performance may result from the relaxed predation intensity on the latter island. Our analysis of the structural features of the microhabitats suggests that the vegetation on Pod Mrčaru offers more protection to lizards. Also, plasticine models of lizards, laid out on the islands, less often exhibited signs of being attacked by birds on Pod Mrčaru than on Pod Kopište. Our findings provide an example of how changes in (possibly a single) environmental factor may simultaneously produce responses in behaviour, morphology and whole-animal physiology, and this on a surprisingly small spatial and temporal scale.

Island tameness: living on islands reduces flight initiation distance

“…In conjunction with frequent reports of decreased diversity and abundance of predators on islands [3,4,6,17,37], reduced FID in lizards on islands is consistent with the conjecture that island tameness evolves when predation is rare or absent, making benefits of fleeing low or non-existent, whereas costs of maintaining escape behaviour remain when predators are scarce. In lacertid lizards of the Mediterranean Basin, FID and other escape variables are reduced on islets with fewer predators [3,4,13,14]. Similar findings have been reported for iguanian lizards [2,6,17].…”

“…However, evolution of island tameness sometimes proceeds rapidly, as shown by a decrease in FID within 30 years after introduction of a lizard population to an island having reduced predation intensity [13]. The lack of a strong phylogenetic effect in our dataset suggests that these differences in FID can evolve very quickly and are not strongly clade-specific.…”

“…Some recent evidence suggests that FID is shorter where predation intensity is lower on islands than on the mainland or other islands [4,6,13,14]. However, the generality of this mechanism has not been assessed.…”

“…predator -prey distance when the prey starts to flee) is shorter on islands having fewer predators (e.g. [6,13,14]). Despite such studies and the existence of many reports that some island species appear to be tame, the reality of island tameness as a general phenomenon has not previously been examined empirically in any diverse group of prey species, such as lizards, that are distributed widely on both continents and islands.…”

“…To reduce escape costs when predation is relaxed or absent, shortened FID is expected to evolve over time. Several recent lizard studies have shown that FID is diminished in populations on islands where predation is relaxed [2,6,13] or is increased on islands where prey are exposed to feral cats and dogs [6,17]. In the lizard Podarcis pityusensis, which exists only on Ibiza, Formentera and nearby islets, FID increases as predation intensity on the islets increases [14].…”