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City Lizards Are Hesitant Feeders

Saturday, March 4, 2017 4:32
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Figure 1. Anolis cristatellus male in survey position.

Foraging behavior reflects a trade-off between the benefits of obtaining vital resources and the potential costs of energy expenditure, missed mating opportunities, and predation. Through time, selection should canalize foraging behaviors that optimize fitness within a given environment, but novel habitats, like urban landscapes, may require behavior to change. For example, human-landscape modification often results in significant reductions in structural complexity of habitat as compared to natural areas, potentially leaving individuals with a greater sense of perceived vulnerability as they venture out to feed. Moreover, these landscapes can alter the diversity and density of predators in ways that might leave prey with a greater sense of perceived predation risk.

In a recent paper in Urban Ecosystems, Chejanovski et al (2017) sought to quantify differences in foraging behavior between anoles from urban areas and those from more natural, forested locations. They utilized two trunk-ground anoles: Anolis sagrei in Florida and A. cristatellus in Puerto Rico. In both urban and natural habitats, they located male lizards in survey posture (Fig 1), which indicates an individual is likely searching for food, and placed a tray with mealworms on the ground at a fixed distance from the perch. They measured each lizard’s latency to feed which was the time it took to the lizard to descend from its perch and capture a mealworm.

Because the availability of complex habitat structure and the proximity of predators might both influence foraging behavior, they experimentally manipulated perch availability for A. sagrei and predator presence for A. cristatellus in both urban and natural habitats. For A. sagrei, they provided half the individuals with two extra perches between the lizard’s original position and the food tray. For A. cristatellus, they manipulated perceived predation risk by placing a static bird model on the opposite side of the feeding tray from half the lizards.

Additionally, they measured several other factors that might influence foraging behavior: the number of available perches within a fixed radius of each lizard – increased habitat complexity might result in lower perceived predation risk; perch height of each individual – those that perch lower to the ground may be more motivated to feed and those that perch higher may be satiated; estimates of body temperature by placing a copper model at the original position of each lizard – body temperature can influence locomotor function and this may have consequences for how easily a lizard can escape predation and play a role in its perceived risk. They also measured the density of conspecifics in the immediate vicinity and noted when conspecific individuals captured mealworms from the feeding tray.

Finally, they measured SVL and mass for a representative sample of each population (urban and natural) in order to calculate body condition. Trade-offs between costs and benefits of foraging decisions can be influenced by satiation of hunger, and body condition, which increases with food consumption, may indicate the extent to which individuals are well-fed.

For both species, lizards from urban areas had a longer latency to feed and demonstrated lower overall response rates to food trays; many individuals never attempted to capture a mealworm in the allotted time (20 minutes). For A. sagrei, habitat (urban vs. natural) best explained feeding latency, but perch height and the presence of conspecifics were also important determinants of feeding latency for A. cristatellus. Individuals perching lower had shorter latency, and latency was shorter when a conspecific attempted to feed from the tray. Neither experimental perch availability nor perceived predation risk (bird model) had any influence on foraging behavior. In both species, individuals from the forest were smaller (SVL) and less massive than those from the city. Body condition was higher for urban A. sagrei but did not differ between natural and urban habitats for A. cristatellus.  


Differences in foraging behavior for male A. cristatellus between natural and urban habitats.

Because of the reduced availability of perches and structural complexity in urban habitats, urban lizards could have generally higher perceived predation risk and this might explain their reluctance to feed; however, experimental perch availability did not influence foraging behavior for A. sagrei and an artificial predator had no effect on A. cristatellis. The latter may simply reflect that the experimental predator was stationary and a moving predator may have elicited different results.

It is possible that foraging differences reflect food availability in urban vs natural habitats, and thus motivation to forage. Urban anoles had higher body condition and may be generally better fed than those from the forest; however, the authors found no significant correlation between individual body condition and latency to feed. It is also possible that mealworms represent a novel food source for urban anoles, and this resulted in a hesitance to initiate feeding since many animals are reluctant to approach novel objects/ food (neophobia).

In summary, this study demonstrates that differences do exist in foraging behavior for two distantly related species of anoles between urban and forested habitats. The increased latency to feed observed in urban anoles could be due to perceived predation risk, foraging motivation, neophobia, or some combination. What is left to be determined is the extent to which these behavioral differences might be adaptive in their respective habitats.


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