The dangers of leaving home: dispersal and mortality in snakes

For animal populations in many parts of the world, direct (albeit often acc idental) killing by humans may be a significant source of mortality. Many s nakes are killed by people (especially by automobiles) every year, but the determinants of a snake's vulnerability to anthropogenic mortality land thu s, patterns of mortality with respect to sex, age and season) are poorly kn own. We present data on 652 French snakes of six species (Coluber viridifla vus, Elaphe longissima, natrix maura, N. natrix, Vipera aspis, V. berus) ki lled either by natural predators, domestic animals or humans (including roa dkills). We used information on seasonal patterns of mortality (plus inform ation on population structure from 338 captures of live snakes) to test the hypothesis that snakes are killed mostly when they disperse from their usu al home ranges. This hypothesis generates several falsifiable predictions o n the expected correlates of mortality rates; most of these predictions are supported by our data. For example, young-of-the-year snakes are killed pr imarily in the period immediately after hatching (while they disperse); sub adults (which are sedentary) generally experience low mortality rates; adul t males are killed mainly during the mating season (especially in species w here mate-searching males travel widely); and adult females in oviparous sp ecies are killed during their egg-laying migrations. Relative to population density, species that use frequent long-distance movements in foraging exp erience higher mortality than sedentary ambush foragers. In one species (E. longissima), larger males are more at risk. The success of these predictio ns suggests that movement patterns of snakes may offer valuable indices of their vulnerability to direct anthropogenic mortality. (C) 1999 Elsevier Sc ience Ltd. All rights reserved.