Anolis cristatellus in survey posture (photo by K. Winchell)
Foraging decisions are the result of a complex decision-making process involving intrinsic factors (physiology, body condition, cognitive ability, sex, ontogeny, etc.) and environmental factors (food availability, structural habitat, presence of predators and competitors). In short, it comes down to the tradeoff between the benefits of energetic gain and the potential costs of predation risk, missed opportunities for reproduction, and expended energy. However, little is known about the specifics of this process – what information are lizards considering when making this decision? By conducting manipulative field experiments on Anolis cristatellus in Puerto Rico, Drakeley et al. (2015) attempt to elucidate what environmental factors influence the decision to forage.
The authors conducted field experiments involving feeding trays in the wild. The Puerto Rican crested anole is a trunk-ground anole and a sit-and-wait forager. When receptive to feeding, it perches head down in “survey posture,” a behavior it reduces when satiated. Aside from movement associated with foraging and social interactions, this species typically remains stationary on a perch. Because of this, the authors were able to easily locate a focal individual and count the number of conspecifics present, using natural variation instead of manipulating the number of animals present.
In the first experiment, they manipulated the food quantity to determine how foraging decisions differ when food is plentiful versus scarce and how this is influenced by the presence of competitors. They found that lizards foraged faster when there were more conspecifics present and food was scarce. When no lizards were near the feeding tray and the feeding tray was full, the focal animal took longer to approach the tray to take the mealworms compared to when there were many conspecifics nearby. Interestingly, this was not related to overall local density, but rather to the number of conspecifics in the immediate vicinity. Therefore the decision to forage likely involves an instantaneous assessment of the local conditions rather than knowledge of the long-term population trends. The authors also considered several other factors and found that although body size was related to foraging latency (larger lizards were quicker to the feeding tray), no other environmental factors were relevant (temperature, humidity, perch height, perch diameter, local density of conspecifics).
Figure 1 from Drakeley et al. (2015). Latency to feed was correlated with the number of conspecifics present and abundance of food.
In the second experiment, the authors chose focal animals farther from the feeding trays and considered distance as a proxy for predation risk. The farther the lizard was from the tray, presumably the greater exposure it had to predators as it moved towards the tray. They found that under this scenario, when risk was elevated, there was more latency in the approach of the food tray. This effect was driven mainly by the increased use of intermediate perches rather than a direct approach across open ground. Increased latency to feed was observed regardless of how abundant the food was or how many conspecifics approached the tray, supporting the conclusion that this effect was because of the perception of greater predation risk (i.e. movement over a longer distance). They also found that larger lizards had a lower latency to feed (approached the feeding tray more rapidly) and lizards not in the foraging position had a longer latency to feed.
In summary, it seems that anole foraging decisions are quite complex. Lizards appear to weigh the risk of predation taking cues from conspecific behavior and abundance versus the abundance of food to make instantaneous decisions to approach a novel feeding source.
Drakeley M, Lapiedra O, Kolbe JJ (2015) Predation Risk Perception, Food Density and Conspecific Cues Shape Foraging Decisions in a Tropical Lizard. PLoS ONE 10(9): e0138016. doi:10.1371/journal.pone.0138016