The dangers of leaving home: dispersal and mortality in snakes

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Abstract

For animal populations in many parts of the world, direct (albeit often accidental) killing by humans may be a significant source of mortality. Many snakes are killed by people (especially by automobiles) every year, but the determinants of a snake's vulnerability to anthropogenic mortality (and thus, patterns of mortality with respect to sex, age and season) are poorly known. We present data on 652 French snakes of six species (Coluber viridiflavus, Elaphe longissima, Natrix maura, N. natrix, Vipera aspis, V. berus) killed either by natural predators, domestic animals or humans (including roadkills). We used information on seasonal patterns of mortality (plus information on population structure from 338 captures of live snakes) to test the hypothesis that snakes are killed mostly when they disperse from their usual home ranges. This hypothesis generates several falsifiable predictions on the expected correlates of mortality rates; most of these predictions are supported by our data. For example, young-of-the-year snakes are killed primarily in the period immediately after hatching (while they disperse); subadults (which are sedentary) generally experience low mortality rates; adult males are killed mainly during the mating season (especially in species where mate-searching males travel widely); and adult females in oviparous species are killed during their egg-laying migrations. Relative to population density, species that use frequent long-distance movements in foraging experience higher mortality than sedentary ambush foragers. In one species (E. longissima), larger males are more at risk. The success of these predictions suggests that movement patterns of snakes may offer valuable indices of their vulnerability to direct anthropogenic mortality.

Introduction

Human activities can threaten the viability of animal populations through a variety of processes, some of them complex (e.g. subtle modifications of habitat structure) and some of them very straightforward (e.g. over-harvesting). For many animal populations living in densely-settled areas, direct killing by humans may be a significant component of the overall effect of human activities on wild populations. Because this aspect of the interaction between humans and wildlife is unlikely to be as important overall as broader-scale processes such as habitat destruction (e.g. Dodd, 1987, Caughley and Sinclair, 1994), direct killing of wildlife has tended to attract more attention from community-based “conservation” organisations than from professional scientists.

The impact of this kind of direct killing on natural populations is difficult to evaluate, because the degree to which anthropogenic mortality affects population viability depends on a complex series of factors. The absolute number of animals killed is obviously important, but population-level impacts will also depend on the kinds of animals killed (e.g. male vs female, adult vs juvenile, reproductive vs non-reproductive) as well as the timing of mortality (e.g. before or after reproductive activities; whether or not the additional mortality coincides with seasons of nutritional stress: Caughley and Sinclair, 1994). In practice, we have little clear understanding of the patterns of direct anthropogenic mortality on most kinds of wild populations (e.g. Burbidge and Jenkins, 1984), or even of the underlying motivation for such destruction. Killing of wildlife can be motivated by factors such as commercial gain (Fitzgerald et al., 1991, Warwick et al., 1991), hunger (Klemens and Thorbjarnarson, 1995) or fear (Dodd, 1993), but much of it is accidental. For example, highway mortality is common in many areas (Ehmann and Cogger, 1985, Mittermeier et al., 1992, Rosen and Lowe, 1994), and drivers may often be unaware of the presence of the animals that they kill. Malicious killing is likely to be particularly important for a subset of animal species that arouse fear and resentment from the general public. Snakes feature high on this list, with many reports of intentional killing (e.g. Brown, 1993, Seigel, 1986). Indeed, a review on threatening processes important for snake conservation identified malicious killing as a significant issue (Dodd, 1987).

Before we can evaluate the impact of direct killing on natural populations, the first step is to document the patterns of mortality that result from such killing. Such data may provide a useful basis for recommendations directed to the particular systems in which the studies have been conducted (e.g. Fitzgerald et al., 1991, Fitzgerald et al., 1993). Unfortunately, it may be difficult to generalise such results to other systems (areas, species) unless we can also gain an understanding of the processes involved in determining vulnerability of an animal to anthropogenic sources of mortality. Thus, we ideally need not only to quantify the relative degree of risk experienced by different species, and by different sex/age classes within each species, but we also need to identify the reasons why particular taxa and/or particular segments of the population are at higher risk. Only with this level of understanding can we begin to extrapolate results from one system to other areas and taxa.

In the present paper, we describe the results of such a study on patterns of anthropogenic mortality (direct killing) in six species (four genera) of European snakes. Snakes are killed by many different types of “predators”, but by far the most obvious (and thus, easily quantified) source of mortality is the risk of being run over by an automobile. Our data set includes this type of mortality, enabling us to compare general patterns in mortality rates between the victims of “natural” predators vs “roadkills”. Having noted broad similarities, we then use the combined data set to explore predictions from a specific hypothesis on the determinants of vulnerability to predation.

This hypothesis is as follows. We suggest that the prime determinant of “risk” is movement—and in particular, dispersal away from the usual home range. Although snakes may be at risk in the course of many other activities (e.g. basking, foraging, mating: Gibbons and Semlitch, 1987, Gregory et al., 1987), we suspect that their vulnerability is far greater during extensive movement. We base this hypothesis not only on our own experience in working with snakes over many years, but also from published literature suggesting that sedentary snakes are relatively invulnerable to predation whereas moving snakes are at high risk (e.g. Gregory and Stewart, 1975, Slip, 1986, Shine, 1993, Rosen and Lowe, 1994, Aldridge and Brown, 1995, Shine and Fitzgerald, 1995).

Section snippets

Methods

In this paper, we use data gathered on snakes that we found after they had been killed by various “predators”, including birds, native carnivorous mammals, domestic pets (cats, dogs) and people. We have not attempted to separate these agents into “natural” vs “anthropogenic” sources of mortality, because of the complex way in which human activities may modify opportunities for “natural” predators. For example, common buzzards (Buteo buteo) often feed on snakes (Naulleau et al., 1997), and

Discussion

The primary result from our analysis is that major patterns in the vulnerability of snakes to “predation” (including roadkills) can be predicted successfully from a simple hypothesis: the idea that snakes are most at risk when they travel outside their normal home range (Table 6). Within a species, mortality from this source is highest in the age/sex classes that disperse furthest, and at the times that the dispersal occurs (notably adult males in the mating season; neonates/hatchlings

Acknowledgements

The authors thank the small army who helped to collect snakes: O. Chastel, C. Clement, D. Guerineau, C. Notebaert, the very efficient and large Buzzard-team, Noune family, L. Blub, C. Thiburce, the Bilous, the Guillons, D. Lucchini, P. Duncan, C. Mauget, N. Maboul, X. Fichet, Y. Cherel, L. Jouventin, D. Barre, H.P. Eckstein, G. Baty, S. Houte, C. Attie, M.P. Vignault, E. Chamarre, E. Cabon, O. Lourdais, P. and R. Brillant, C. MacArthur, A. Moreau, N. Mabon, F. Jiguet, V. Garcia, M.P. Colace, P.

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