Ambient Temperature Activity of Horned Adders, Bitis caudalis: How Cold Is Too Cold?

Abstract

Most physiological processes are greatly affected by temperature; therefore animals thermoregulate to maintain a body temperature (Tb) that approximates that which is optimal for physiological performance (reviews by Avery, 1982; Huey, 1982; but see Webb and Shine, 1998). As an ectotherm, a snake relies on external heat sources to maintain its preferred Tb. Although heat sources are readily available during the day, the nocturnal environment has few, if any, heat sources. Even though the nocturnal environment poses a significant thermal handicap, numerous snakes species are nocturnal (Greene, 1997). Because of the lack of heat sources, activity of nocturnal species should be sensitive to ambient temperature (Ta). The physiological and behavioral means by which snakes adapt to thermal challenges associated with nocturnality has not received extensive study. Many performance measures including locomotion, digestion rate, tongue flick, and strike velocity are maximized at relatively high Tb (e.g., Stevenson et al., 1985; Ayers and Shine, 1997; Dorcas et al., 1997; Webb and Shine, 1998). These body functions are near maximum levels during the day but are significantly reduced at night (Stevenson et al., 1985). Activity at night would not only require a snake to use physiological processes associated with foraging (e.g., locomotion, tongueflick rate, strike velocity) when they are suboptimal but could also negatively impact temperature-sensitive physiological processes, which are not associated with activity (e.g., digestion). Nocturnal activity of rubber boas, Charina bottae, led to a Tb below that which would occur if the snake remained in a nighttime refugium (an inverted Tb pattern; Dorcas and Peterson, 1998). A nocturnally active snake typically has a Tb that approximates Ta (Dorcas and Peterson, 1998); therefore, Ta may be more critical to performance of nocturnal species than of diurnal species, where radiation and conductance can lead to a Tb much greater than Ta (Peterson, 1987). Because of the thermosensitivity of processes critical to foraging, thermal factors might constrain the timing, place, and duration of predatory bouts (Ayers and Shine, 1997). Such constraints can lead to seasonal differences in activity period. For example, copperheads, Agkistrodon contortrix, are diurnal during cool seasons (spring and fall) but are nocturnal during the warm summer months (Sanders and Jacob, 1981). Thermal constraints can also lead to ontogenetic differences in activity period. Adult diamond pythons, Morelia spilota spilota, forage nocturnally, whereas juveniles are diurnal (Ayers and Shine, 1997). We used a controlled laboratory environment to test the