Ana Pérez-Cembranos

Using faecal samples in lizard dietary studies

While the use of faecal pellets is widely accepted as a primary methodological source of data for dietary studies, a recent paper advocated for the use of gut contents. This was due to the fact that faecal samples would give biased results of the diet of arthropod predators, due to a lower representation of soft-bodied prey in faecal pellets. To test this assumption, we compared the spring diet of several populations of two insular lizards from the Balearic Islands (Spain), Podarcis lilfordi and Podarcis pityusensis, using both faecal pellets and gut contents. Our results do not support the supposed bias of dietary analyses based on faecal pellets. Indeed, soft-bodied prey and particularly insect larvae are often equally represented in faecal pellets and gut contents. Alternatively, soft bodied prey are represented in different proportions in gut contents and faecal pellets, but in some cases with higher proportions being observed in the gut contents, and in other cases with higher proportions in faecal samples. We conclude that faecal pellets can be a reliable source of information for dietary studies.

Omnivory of an Insular Lizard: Sources of Variation in the Diet of Podarcis lilfordi (Squamata, Lacertidae)

Through 17 years and from a sample of 7,790 faecal pellets and 26,346 prey items, we studied the diet of the Balearic lizard Podarcis lilfordi in Aire Island (Menorca, Balearic Islands, Spain). We analysed the diet in terms of prey frequencies, as well as by their volume and biomass contributions. The diet of the Balearic lizard was extremely variable through the years, months and areas under study. The dominance of small clumped prey, particularly ants, was confirmed. However, the main contribution by volume corresponded to beetles, with a relevant role for Diplopoda and terrestrial Isopoda during some months and at particular areas of the island. Several prey items were probably captured at the base of shrubs, under stones or inside rock crevices. Therefore, our estimations of electivity would only be reliable for epigeal and flying prey. The capacity of the Balearic lizard to include marine subsidies in its diet, such as coastal crustaceans, is noteworthy. Also, its consumption of carrion from carcasses of gulls and rabbits and leftovers from human visitors is remarkable. Juvenile conspecifics can also be a sporadic food resource, especially during the second half of summer, whereas the consumption of vegetal matter is constant for each whole year. The shifts of vegetal exploitation among areas of the island and months take place according to availability of different plant species at each area or during a given period. Thus, lizards are able to conduct a thorough monitoring of plant phenology, exploiting a large variety of plant species. Omnivory does not imply the indiscriminate inclusion of any edible food in its diet. Rather, the inclusion of several food items means the adoption of a wide range of foraging behaviours adapted to the exploitation of each food resource.

Local enhancement and social foraging in a non-social insular lizard

Even in solitary foragers, conspecifics can provide reliable information about food location. The insular lizard Podarcis lilfordi is a solitary species with high population densities that sometimes aggregate around rich food patches. Its diet includes novel and unpredictable resources, such as carcasses or plants, whose exploitation quickly became widespread among the population. We tested the use of social information by lizards through some field experiments in which they had to choose one of the two pieces of fruit. Probably due to local enhancement, lizards preferred to feed on the piece of fruit where conspecifics or lizard-shaped models were already present. Conspecifics’ behaviour, but also their mere presence, seems to be a valuable source of information to decide where to feed. Lizards also showed a strong attraction to conspecifics, even in the absence of food. Maybe the presence of a group is interpreted as an indirect cue for the presence of food. The group size was not important to females, but males had a significantly higher attraction towards groups with three conspecifics. We discuss some characteristics of P. lilfordi at Aire Island that can explain the development of the observed social foraging, as well as their possible consequences.

The yellow-legged gull as a predator of lizards in Balearic Islands

Lizards and gulls cohabit in several Mediterranean islands. The yellow-legged gull, Larus michahellis, was found to prey several vertebrate species. However, precise information about the interaction between gulls and other vertebrates, particularly with lizards is still scarce. The Balearic lizard, Podarcis lilfordi, shares several coastal islets with the yellow-legged gull. Using two different sources of information, we studied the interaction of both species in Colom Island (Menorca, Balearic Islands, Spain). We studied the diet of the yellow-legged gull and learnt that the Balearic lizard is not a common prey of the yellow-legged gull. On the other hand, we studied the potential predation pressure of gulls on lizards, using plasticine models of lizards. We did two different experiments from which we can conclude that yellow-legged gulls rarely attack lizards and, consequently, cannot be considered a major threat for this endemic lizard species, at least in the population under study. Finally, we obtained evidence that plasticine models can only be employed with caution to assess predation pressure of opportunistic scavengers, much as gulls are. The majority of marks on models were not the consequence of true attacks by gulls, but the result of ground exploratory behaviour of gulls in search of any edible matter. Therefore, contrary to popular belief, in the case of the yellow-legged gull, the proportion of marked models would be an indication of ground-based wandering activity, rather than a result of its predation pressure on lizards.

Diet selection by the threatened Chafarinas' skink Chalcides parallelus in North Africa

Abstract Studies of diet suggest that skinks are opportunistic predators. However, there is little information on actual diet preferences because most studies do not evaluate the availability of prey in the environment. We analysed the diet selection of a population of the little-known skink Chalcides parallelus from the Chafarinas Islands in North Africa. This species is considered highly threatened, and diet selection is an important basic point on which to support the design of conservation plans. We specifically examined diet estimated from faecal material collected from live skinks, and compared diet with availability of invertebrates in the microhabitats used by skinks (under rocks and in the leaf litter). Results indicated that the diet of C. parallelus skinks consisted of the same types of invertebrates that are more commonly found in the habitat such as beetles (54.8% of prey), cockroaches, spiders and snails. This diet could be considered generalist, and probably opportunistic. However, the comparison of proportions of prey types in the diet and those available in the habitat revealed that C. parallelus did not eat prey at random, but selected some particular prey types (beetles, cockroaches, and pseudoscorpiones) while others (ants and isopods) were consumed less than expected by their abundance. Invertebrates available and consumed did not differ in size, but skinks consumed beetles that were smaller than the average size of those available. We discuss how diet preferences could be due to selection of the more profitable or easily captured prey.

Spontaneous magnetic alignment behaviour in free-living lizards

Several species of vertebrates exhibit spontaneous longitudinal body axis alignment relative to the Earth’s magnetic field (i.e., magnetic alignment) while they are performing different behavioural tasks. Since magnetoreception is still not fully understood, studying magnetic alignment provides evidence for magnetoreception and broadens current knowledge of magnetic sense in animals. Furthermore, magnetic alignment widens the roles of magnetic sensitivity in animals and may contribute to shed new light on magnetoreception. In this context, spontaneous alignment in two species of lacertid lizards (Podarcis muralis and Podarcis lilfordi) during basking periods was monitored. Alignments in 255 P. muralis and 456 P. lilfordi were measured over a 5-year period. The possible influence of the sun’s position (i.e., altitude and azimuth) and geomagnetic field values corresponding to the moment in which a particular lizard was observed on lizards’ body axis orientation was evaluated. Both species exhibited a highly significant bimodal orientation along the north-northeast and south-southwest magnetic axis. The evidence from this study suggests that free-living lacertid lizards exhibit magnetic alignment behaviour, since their body alignments cannot be explained by an effect of the sun’s position. On the contrary, lizard orientations were significantly correlated with geomagnetic field values at the time of each observation. We suggest that this behaviour might provide lizards with a constant directional reference while they are sun basking. This directional reference might improve their mental map of space to accomplish efficient escape behaviour. This study is the first to provide spontaneous magnetic alignment behaviour in free-living reptiles.

Occurrence and ecological aspects of the two-fingered skink Chalcides mauritanicus in the Chafarinas Islands in North Africa

Abstract The two-fingered skink, Chalcides mauritanicus, is a fossorial species from North Africa, where it has only been found at a few localities on sandy sea shores. Virtually nothing is known about its ecology. For the first time, we report the occurrence of an apparently large and well-preserved population of C. mauritanicus on the Chafarinas Islands (NW Africa). In addition, we study some aspects of the morphology and ecology (microhabitat, soil and diet selection) of this skink. Adult females were larger than males but had heads of similar size. In this population, we found an equilibrated sex ratio. Adults showed a very high frequency of regenerated tails, suggesting a high predation pressure. Skinks were restricted to a small area of Congreso Island where, far from the sea shore, they selected sandy microhabitats. These microhabitats were characterised by higher cover at the substrate level of bare soil and medium rocks, lower cover of leaf litter and grass, and more sparse subarboreal vegetation than available microhabitats. Soils occupied by skinks had a higher proportion of sand, and a lower proportion of gravel, silt and clay, resulting in a much lower soil compaction than most soils of the island. The bulk of the diet of C. mauritanicus consisted of small beetle larvae, which were consumed in higher proportions than expected by their availability.