J. Zool., Lond. (A) (1986) 209, 461-476

A. HAILEY AND P. M. C. DAVIES

(With 6 figures in the text)

The snakes Natrix natrix and N. maura were studied in a small Spanish river; the former was a widely foraging predator of frogs, the latter a sit-and-wait predator of fish. Natrix natrix was diurnal and terrestrial, N. maura was active by day and night and semi-aquatic; their thermal niches are described and compared. The activity metabolism of these snakes was investigated to show the direct (activity) and indirect (thermal) effects of lifestyle. At high temperatures, aerobic scope was greater in N. natrix, an adaptation for greater activity. Aerobic scope of N. maura was less temperature sensitive, an adaptation to activity at low temperature and within a broad thermal niche. Lactate production and burst speed were less different. Activity metabolism of low and high latitude N. natrix were compared. The latter had elevated and less temperature sensitive aerobic scope. The functional interpretation of this pattern is noted, but no supporting evidence was found from published data on activity body temperature mean and range in natricine snakes.