The physiological ecology of desert horned lizards (Phrynosoma platyrhinos) in the Mojave Desert.

摘要

The unusual autecology of desert horned lizards (Phrynosoma platyrhinos ) is believed to have co-evolved with their dietary specialization on ants. These lizards generally "ambush" their prey at ant nests or trails, but they also occasionally travel appreciable distances between foraging sites, thus employing a more "widely-searching" foraging strategy. Among reptiles, evidence indicates that some ambush predators have lower energy requirements than do widely-searching predators. If desert horned lizards are primarily ambush foragers, and because they consume a low-quality diet of ants, they should have low energy requirements. The necessarily large volume of low-energy food has led to the suggestion that these lizards may be, food-limited. Because P. platyrhinos inhabits desert areas, they may also have reduced water requirements, as do other desert lizards. If desert horned lizards do eat only ants, then their water economy index (ratio of water influx to energy expenditure) should be unchanging throughout seasons and between years.;To address these predictions, doubly labeled water was used to measure seasonal (spring and summer) energy and water intake rates of a Mojave Desert population of P. platyrhinos during drought (two years) and non-drought (three years) conditions. Daily time budgets and daily movement distances were estimated from behavioral observations. Measurements of body length and mass were used to calculate growth rates and body condition indices, and feeding rates were estimated from energy expenditure and water influx.;In a comparison of 48 lizard species, field metabolic rates were significantly lower in ambush foragers than in widely-searching foragers. Both the behavior and field metabolic rate of P. platyrhinos were consistent with that of an ambush forager. Body condition, body length, growth rate, field metabolic rate, and feeding rates were all significantly lower during the activity seasons of drought years, suggesting that P. platyrhinos is occasionally food-limited. Water influx rates were significantly lower than predicted for a non-desert lizard at all times, and were significantly lower during drought summers. Mean water economy index did not vary between summers, but the variability in measured water economy index values indicates that the lizards often ate other prey in addition to ants.