Northern Prairie Wildlife Research Center
Charles J. Amlaner and James H. Withgott, Department of Life Sciences, Indiana State University, Terre Haute, IN 47809 USA
While activity patterns of some animals are guided strictly by light-dark cycles or circadian rhythms, others show more flexible behavior. For ectothermic animals such as snakes, the temperature of their surroundings is a key factor in their physiology, behavior, and ecology. Various other factors, e.g., hunger, reproductive behavior, ecdysis, prey availability, and predation risk, also influence a snake's patterns of activity, often in conflicting and unpredictable ways. Studies of sleep patterns in such animals with variable and irregular activity may yield insights into the functions of sleep.
Methods: As part of a study on the behavior and ecology of black rat snakes (Elaphe o. obsoleta), 14 snakes were surgically implanted with temperature sensitive radio-transmitters and re-released into the wild for long-term behavior monitoring in the Cuacjhita National Forest, Arkansas in 1992 and 1993. Each snake was located by radio-triangulation several times weekly. The snake's behavior and body temperature were recorded along with data on the surrounding habitat. Motion-sensitive radio-transmitters implanted in 11 additional snakes enabled us to detect activity at night in addition to recording body temperature.
Results: Snakes showed higher body temperatures when actively moving (N = 45, 28.3 + 0.6°C) than when inactive (N = 514, 24.9 + 0.2°C); t = 4.37, df = 557, P < 0.0001). When inactive, snakes were usually hidden from view in retreat sites including logs (N = 154 locations); tree cavities (N = 119); underground caverns such as those created by tree roots (N = 110); and piles of dirt, leaf litter, or rocks (N = 98). Inactive snakes were found resting in the open only 46 times (9% of locations), and in most cases were completely concealed under vegetative cover. Activity patterns of snakes were highly variable, but were best correlated with ambient temperature. Snakes were inactive and sought shelter at temperature extremes. Nocturnal activity peaked in July and August, when daytime temperatures were high. In July, snakes were found to be active at night on one-third of all nocturnal locations. Snakes sometimes rested in sheltered sites for many days without moving, occasionally for as long as 3-4 weeks. Diurnal activity peaked in May and early June, when daytime temperatures were favorable. Individual snakes differed in their overall activity levels, but nearly all showed both diurnal and nocturnal activity.
Discussion: Black rat snake diurnal activity peaked in spring when most reproductive behavior takes place, and snakes became more nocturnal in late summer when daytime temperatures increased. Data from other aspects of our research show these snakes to regularly prey on arboreal birds' nests, and attacking nests at night may decrease risks of predation for climbing snakes. Small mammals, the snakes' principal prey, tend to be active at night and rest in burrows in the day, and it remains unclear when they are most vulnerable to snakes. However, we can speculate about the basic importance of this predator-prey relationship as a significant factor in organizing and possibly constraining the snakes sleep pattern to primarily diurnal and crepuscular periods. Along with ambient temperature, factors such as prey type and availability, predation risk, and reproductive drive interact in complex ways and pose tradeoffs for the organism. Since these factors vary with time, often unpredictably, evolution may have favored flexibility in activity patterns.