J. therm. Biol. Vol. 16, No. 3, pp. 157-166, 1991 0306-4565/91 $3.00+ 0.00 Printed in Great Britain.All fights reserved Copyright© 1991 PergamonPress pie STANDARD OPERATIVE TEMPERATURES OF TWO DESERT RODENTS, GERBILLUS ALLENBYI AND GERBILLUS PYRAMIDUM: THE EFFECTS OF MORPHOLOGY, MICROHABITAT AND ENVIRONMENTAL FACTORS WENDY GOODFRIEND,* DAVID WARD? and A z I z SUBACH~ Mitrani Centre for Desert Ecology, Jacob Blaustein Institute for Desert Research, Ben Gurion University of the Negev, Sede Boqer Campus 84993, Israel (Received 5 May 1990; accepted in revised form I0 November 1990) Almraet--1. We compared the standard operative temperatures (Tcs) of two nocturnal desert rodents, Gerbillus allenbyi and G. pyramidum, in summer and winter in the Negev Desert, Israel. We found that T,, was below the lower thermal critical temperature in both summer and winter. 2. We found significant differences in T** among microhabitats. There was no significant effect of orientation on T~. 3. Unlike other studies of small ground-dwelling endotherms, we found that T~ most closely tracked T a. 4. The larger of the two species, G. pyramidum, had a higher T~ under all conditions, which is related to its lower surface area:volume ratio. 5. The results are discussed in terms of the importance of low ambient temperatures to the thermal physiology of nocturnal desert rodents. Key Word Index: Standard operative temperature; environmental temperature; gerbils; Gerbillus pyramidum: Gerbillus allenbyi; taxidermic mount; seasonal activity INTRODUCTION The nocturnal gerbils, Allenby's gerbil Gerbillus al- lenbyi and the Egyptian sand gerbil Gerbillus pyra- midum, are the most common rodents in the sandy habitats of the Negev Desert of Israel (Abramsky et al., 1985). Much is known of their ecological roles in this desert (e.g. Abramsky et al., 1985; Rosenzweig and Abramsky, 1986; Abramsky, 1988; Kotler and Brown, 1988; (and references therein)), although knowledge of their physiologies is restricted to lab- oratory studies of thermal physiology (Linder, 1988). Our aims in this paper are: (1) to describe the standard operative tempera- tures (Te~) of these two species in summer and winter; (2) to compare measurements of operative en- vironmental temperature (T,) and Tes; (3) to examine the effects of pelage, orientation and location in their habitats; and (4) to relate their nocturnal and annual activity patterns to the energetic cost of maintaining Tcs within the thermoneutral zone. Taxidermic mounts have been used to quantify 7", (e.g. Bakken et al., 1981; Buttemer, 1981; Weathers *Present address: 5142 College Gardens Crt., San Diego, CA 92115, U.S.A. ?Author to whom all correspondence should be addressed. :[:Present address: Biology Department, Ben Gurion Univer- sity of the Negev, Beit Hias Campus, Beer Sheva, Israel. et al., 1984). Te is a thermal index equivalent to the steady-state temperature that a metabolically-inert animal would eventually attain under stable con- ditions (Bakken, 1989). Measuring Te facilitates the determination of the importance of factors such as external insulation, body size and shape, that effect heat flow between the animal and the thermal environment. The mechanisms of heat transfer are free and forced convection, solar radiation and conduction. Endotherms must balance heat loss or gain by changing body temperature (Tb), evaporative heat loss, or metabolic heat production (Bakken et al., 1981). By measuring To, which is the animal's equi- librium temperature if physiological processes are not used to control Tb, the metabolic cost of maintaining Tb within the thermal neutral zone can be determined. This is the theoretical basis for the use of taxidermic mounts to measure To. A type of mount that is appropriate for use as a Te thermometer is a cast of the animal's body made of a metal with high conduc- tivity and covered by the animal's integument. Such a model has essentially the same thermal properties as a live animal and is thermally passive (Walsberg and Weathers, 1986). An index of thermal load must allow direct extra- polation from physiological responses measured under controlled laboratory conditions to the tem- perature the animal experiences in the field (Bakken, 1980). Ideally, the index should specify a rate of heat flow under field conditions equal to that pro- duced by a specified temperature under standardized 157