Thermoregulatory Responses of Bridled and Juniper Titmice to High Temperature

Abstract

Bridled Titmice (Baeolophus wollweberi) and Juniper Titmice (B. ridgwayi) occur sympatrically in southeastern Arizona, with Bridled Titmice preferring habitats that are more heavily vegetated, moister, and cooler than those occupied by Juniper Titmice. To assess whether these differences in habitat preference have physiological correlates, we measured the oxygen consumption, evaporative water loss, and body temperature of post-breeding titmice at ambient temperatures between 24-44°C. Bridled Titmice were less tolerant of heat than Juniper Titmice and had significantly higher rates of metabolic heat production and evaporative water loss, but not body temperature, at ambient temperatures above 40°C. These differences were entirely attributable to the Bridled Titmouse's smaller body size (10 versus 15 g), and the differences vanished when rates were expressed per unit metabolic mass (mass raised to either the 2/3 or 3/4 power). Within the thermoneutral zone, the rate of evaporative water loss (EWL) was significantly lower in Juniper Titmice than Bridled Titmice, even after accounting for the difference in body size. Reduced EWL is characteristic of species from hotter, drier habitats and suggests that physiology plays a role in these species' habitat preferences.