Bioenergetic Correlates of Foraging Mode for the Snakes Crotalus Cerastes and Masticophis Flagellum

Abstract

Foraging mode may influence an array of ecological and bioenergetic characteristics, several of which we compared between two snakes: sympatric sit—and—wait foraging sidewinders, Crotalus cerastes, and widely foraging coachwhips, Masticophis flagellum. During the majority of surface activity, the nocturnal C. cerastes were either coiled on or partially buried in the sand waiting to ambush mobile prey, whereas the diurnal M. flagellum cruised through the habitat searching for active and sedentary prey. Average duration of daily surface activity of C. cerastes (X ± 1 SE = 7.2 ± 0.7 h/d) was nearly twice that of M. flagellum (3.9 ± 0.9 h/d). Body temperatures (Tb's) of active M. flagellum (33.1° ± 0.1°C) averaged higher, and were maintained within a narrower range, than those of active C. cerastes (25.3° ± 0.1°C). Field metabolic rates (FMR, as CO2 production), measured with doubly labeled water were significantly greater in M. flagellum (body mass = 124 ± 12 g [X ± SE]) than in C. cerastes (125 ± 6 g) during the active season (mid—April to mid—October; 0.154 ± 0.017 vs. 0.063 ± 0.005 mL°g—1°h—1), transition seasons (mid—March to mid—April, mid—October to mid—November; 0.058 ± 0.009 vs 0.028 ± 0.002 mL°g—1°h—1), and hibernation (mid—November to mid—March; 0.014 ± 0.002 vs. 0.007 ± 0.001 mL°g—1°h—1). Masticophis flagellum also possessed significantly greater rates of water influx than C. cerastes during the active (19.7 ± 2.9 vs. 7.6 ± 1.1 mL°kg—1°d—1) and transition seasons (7.1 ± 1.8 vs 2.5 ± 0.5 mL°g—1°d—1). Standard metabolic rates (SMR), measured at six Tb's (10°—35°C), of M. flagellum averaged 37 ± 4% greater than SMR of C. cerastes. The monthly metabolic cost of SMR, calculated by integrating 24—h Tb profiles with temperature—dependent regression equations for SMR, averaged 62 ± 4% greater for free—ranging M. flagellum than for C. cerastes. The difference between FMR and SMR is the energy allocated to activities and other energy—demanding functions such as digestion and represented 65 and 76%, respectively, of the yearly metabolic expenditure of C. cerastes and M. flagellum. Annually, M. flagellum spent 2.6 times the amount of energy on activity and other functions as did C. cerastes. Contributing to expenditures above SMR were digestion (19—43% of FMR) and movement (6—18% of FMR). Feeding rate (calculated from water influx) during the active season of M. flagellum (24.1 ± 5.5 g°kg—1°d—1) was more than twice that of C. cerastes (9.4 ± 1.4 g°kg—1°d—1). Over a full year, M. flagellum consumed 2.1 times as much assimilable energy as did C. cerastes, although both species gained similar energy profits (212 and 177 kJ/yr, respectively). The foraging strategy of the cryptic, ambushing C. cerastes balances low energy expenditure with low food intake, whereas widely foraging M. flagellum have greater expenditures but achieve greater energy consumption, as well.