Effects of Temperature, Sex, and Gravidity on the Metabolism of Small-Mouthed Salamanders, Ambystoma texanum, during the Reproductive Season

—Amphibians that breed in early spring may encounter considerable variation in ambient temperature. In this study, I investigated how the metabolic rate of male and female Small-Mouthed Salamanders (Ambystoma texanum) is influenced by variation in temperature (5–208C) during the reproductive season. As expected, both rates of resting oxygen consumption (VO2) and resting carbon dioxide production (VCO2) increased with increasing temperature. Both measures of respiration were significantly higher in gravid females than in males and postgravid females across all temperatures. Moreover, the VO2 of gravid females increased more with increasing temperature than did those of either males or postgravid females. These findings suggest that variation in ambient temperature may have an important influence on the energetic cost of reproductive activity in males and females in this species and in other spring-breeding amphibians. Many species of amphibians from northern latitudes, including most ambystomatid salamanders, breed in late winter and early spring. Often, it is the occurrence of warm, rainy weather that prompts migration to breeding areas and subsequent reproduction (Minton, 2001). During this migration and while in the breeding areas, amphibians may be exposed to a wide range of temperatures. Thus, these ectothermic animals may be susceptible to marked fluctuations in their metabolism during the breeding period (Rome et al., 1992). Metabolic rate of spring breeding ambystomatids can differ considerably between sexes and with reproductive condition. In particular, there appears to be a considerable increase in the resting metabolic rate of gravid females compared to both males and postgravid females at 158C (Finkler and Cullum, 2002; Finkler et al., 2003). However, it is unclear how variation in temperature might influence this difference in metabolic rate between males and females and between gravid and postgravid females. Fitzpatrick (1973) noted that gravid females of the plethodontid Desmognathus fuscus had higher oxygen consumption rates than did males at 158C but not at 58C. Data on other taxa are lacking. Given that the males of many ambystomatids tend to emerge from hibernation and migrate to breeding areas at temperatures lower than do the females (Downs, 1989; Petranka, 1998; Minton, 2001), differences in temperature at the time of emergence from the hibernaculae and overland migration could contribute to differences in the energetic cost of reproduction beyond gamete formation. The present study examined the influence of temperature on oxygen consumption and carbon dioxide production in male, gravid female, and post-gravid female Small-Mouthed Salamanders (Ambystoma texanum) to ascertain the effect of temperature on the differences in metabolic rate associated with sex and reproductive condition. MATERIALS AND METHODS Male (N 5 7), gravid female (N 5 5), and postgravid female (N 5 5) Ambystoma texanum were collected via dip net from breeding areas in the Salamonie River State Forest, Wabash County, Indiana, and Lost Bridge State Recreation Area, Huntington County, Indiana, between 22 and 29 March 2003. Animals in all three groups were collected throughout the collection period. Animals were housed in pairs in plastic shoe boxes containing moistened paper towels at a constant 158C for two days prior to respirometry testing. Measurements of resting oxygen consumption (VO2) and carbon dioxide production (VCO2) rates at four different test temperatures (5, 10, 15, and 208C) were conducted using a Columbus Instruments Micro-Oxymax respirometry system. Each animal’s gas exchange was measured at all four temperatures over a fourday period, with the sequence of temperatures randomized for each animal. After the mass of each animal was recorded, it was placed into a 250 mL sample bottle connected via tubing to the multiplexer of the respirometer, which was then submerged in a circulating water bath at the appropriate test temperature. After a 2-h adjustment period, the system automatically measured STPD-corrected VO2 and VCO2 in each chamber hourly for a 6-h period, with a refresh interval after 3 h. Hourly measurements were averaged for each animal to provide a single measure of VO2 and VCO2 at each temperature. As this species only rarely ingests food prior to breeding (Plummer, 1977), I assumed that all salamanders were in a postabsorptive state. Following the last respirometry measurement, animals were anesthetized by submergence in 0.67% MS222 solution. Total body length, snout–vent length, and live mass were recorded; then the animals were dissected to remove eggs from the oviducts of gravid females and to harvest organs for another study. The wet mass of the carcasses with eggs removed was recorded. VO2 and VCO2 were analyzed using the MIXED procedure on SAS 8.2. Differences in body size measurements (live mass, total length, snout–vent length, and wet carcass mass) among different groups (male, gravid female, or postgravid female) were analyzed using one-way ANOVAs. Since the sequence of temperatures tested was randomized for each animal, I analyzed the respiration data in two separate sets of 103