Conflicts and interactions among reproduction, thermoregulation and feeding in viviparous reptiles: are gravid snakes anorexic?

Constraints on time and energy suggest that animals often will have to forg o one activity in favour of another. In the field, gravid garter snakes, Th amnophis elegans, eat little or nothing, especially late in pregnancy. In a ddition, they spend considerable time basking, presumably to aid developmen t of their progeny. Apparently, therefore, a conflict exists between feedin g and behaviours related to gestation. Alternatively, low feeding levels in the field might reflect reduced ability to catch food while gravid, or ano rexia, attributable either to reduced space in the gut or to physiological suppression of appetite. In this study, by supplying abundant food in the l aboratory, we test the hypothesis that gravid females are anorexic; we also examine the interactions among feeding, thermoregulation and reproductive condition. Typically, gravid females, whether fed or not, spent most of the ir time at the warm end of a gradient, as did fed non-gravid snakes; unfed non-gravid females spent significantly less time at the warm end. However, gravid snakes, even when presented with food ad libitum, ate less than non- gravid snakes, suggesting that they are anorexic while pregnant. Feeding an d thermoregulatory behaviours of gravid females changed around the time of parturition (increased feeding, reduced warming rate). Comparable changes a re seen in animals in the held, and are correlated with changes in movement pattern. Evidently, the tendency to eat little food is carried over into t he laboratory, even when apparent proximate causes of the behaviour are rem oved. Although we cannot distinguish clearly between the two potential caus es of anorexia, the evidence slightly favours physiological suppression of appetite. Physiological suppression of appetite while gravid would ensure t hat the urge to forage rather than thermoregulate does not diminish the cha nces of present reproductive success. By contrast, non-gravid snakes, faced with a shortage of food, lower their body temperature and reduce their met abolic costs.