GROWTH-RATE AND SURVIVAL IN RELATION TO RELATIVE HEAD SIZE IN VIPERA-BERUS

In gape-limited predators, such as snakes, the availability of potenti al prey is partly determined by swallowing capacity, i.e., maximum ing estible prey size. Relative head size in snakes influences swallowing capacity and has been shown to vary among species, populations, and se xes. Here I examine individual variation in relative head size in a na tural population of adders (Vipera berus) and test for an association between relative head size and two important fitness components, indiv idual growth rate and survival. I found a linear relationship between head size and snout-vent length. Individual differences in relative he ad size, i.e., the residuals from the regression of head length on sno ut-vent length, were equivalent to that expected, on average, from sno ut-vent length differences of up to 90 mm. There was a close associati on between rates of head and body growth and no difference in variabil ity between the two traits. Individual relative head size did not chan ge as a result of different rates of body growth. Snout-vent growth ra te increased significantly with increasing relative head size in males , but not in females. In small males, surviving individuals had signif icantly larger relative head size than those that died, but there was no association between survival and relative head size in large males or in females. I conclude that, by influencing survival and growth rat e, relative head size may be associated with individual variation in l ifetime reproductive success.