Spatial and seasonal dietary patterns of the desert horned lizard (Phrynosoma platyrhinos): harvester ant specialist or generalist ant feeder?

A suite of factors including prey availability and prey defenses influence prey selection by consumers and ultimately define an animal’s feeding strategy (e.g., generalist or specialist). Here we examined the relationship between availability and selection of ants by the desert horned lizard ( Phrynosoma platyrhinos Girard, 1852) to test the hypothesis that P. platyrhinos are specialist predators on harvester ants, and to investigate which factors influence the prey preference of lizards. Variation in ant availability and lizard diets was assessed using 83 plots established along a Great Basin shrub–steppe bajada in northwestern Utah, USA. Across the study site, 14 of 20 ant species were represented in the lizard diet. However, 70% of that diet consisted of just two species. In contrast to previous studies, P. platyrhinos did not show a disproportionate preference for harvester ants. Instead, larger ants of any species (including harvester ants) were preferentially selected over medium-sized and smaller ants. These findings suggest that desert horned lizards demonstrate an opportunistic feeding strategy in which lizards select ants based on their body size and abundance. More generally, these results emphasize the importance of assessing predator response across naturally varying gradients in prey availability to evaluate feeding specialization.

Kinematics, maximal metabolic rate, sprint and endurance for a slow-moving lizard, the thorny devil (Moloch horridus)

Metabolic physiology, morphology, activity patterns, performance traits and movement kinematics are thought to have coevolved in lizards. We examined links between these parameters for the thorny devil (Moloch horridus), a morphologically and ecologically specialised agamid lizard (body mass ~30 g). It has a maximum sustainable metabolic rate (VO2max) of 0.99 mL O2 g–1 h–1 while running at a velocity of 0.11�m�sec–1 at 35°C. This VO2 is typical of that for other lizards (except varanids), but its burst speed (1.21�m�sec–1) is slower than for a typical agamid (e.g. Ctenophorus ornatus at 3.59 m sec–1) and its endurance is appreciably higher. The kinematic pattern of hind-limb movement for M. horridus is different to that of a 'typical' similar-sized agamid, Ctenophorus ornatus, which is a fast-moving lizard that shelters in rock crevices. It is also different to the ecologically equivalent Phrynosoma platyrhinos. The slow and erratic ventilation of M. horridus (2.3 breaths min–1) at its maximum sustainable aerobic running speed occurs when it stops running. This might be a consequence of the hypaxial muscles being used for both lung ventilation and locomotion, which might be impairing pulmonary ventilation when running, but might also contribute to its high endurance. M. horridus is metabolically typical of agamids, but its body shape, movement patterns and locomotory performance traits are different, and might have coevolved with its specialisation for eating ants.