Morphology, growth, food habits and population characteristics of the Smooth snake Coronella austriaca in southern Britain

Authors


Abstract

A three year ecological study was made of the Smooth snake in the New Forest, Hampshire. The aim was to provide quantitative information on particular aspects of the Smooth snake's life-history characteristics. On three study sites, snakes were captured by hand and individually marked. On capture, each snake was identified, sexed, measured, weighed and examined for the presence of food in its stomach and hind gut. In general, mature Smooth snakes showed sexual dimorphism in morphology and growth rates. This was thought to reflect the influence of body size on female reproduction. It was found that Smooth snakes preyed on both small mammals and lizards, suggesting that this species is an opportunist predator. Diet did not vary between study sites. The population sizes were estimated by direct enumeration and densities of one and two snakes per hectare were found. The sex ratios in each study site population were even, and the age structures suggested that both sexes exhibited similar mortality. Consideration was also given to the estimation of the total population size of Smooth snakes in Britain.

Summary

The morphology, growth, food habits and population characteristics of the Smooth snake Coronella austriaca Laurenti were studied in southern England. On capture, each snake was identified, sexed, measured, weighed and examined for the presence of food in its stomach and hind gut.

In total, measurements of 22 sexually immature and 92 sexually mature snakes were collected. There was little evidence for sexual dimorphism among immature snakes, though males had relatively longer tails than females. Among mature snakes, females had longer snout-vent lengths and shorter tails than males. The body weights of breeding females were heavier than those for both non-breeding females and males. Regardless of age or sex, body weight was positively correlated with snout-vent length. An analysis of 68 growth measurements revealed that, in common with other species of snake, immature specimens had higher growth rates than mature specimens. Growth within one year did not differ appreciably between males and females. Conversely, growth over more than one year was greatest for female snakes. These differences in morphology and growth were ascribed to the influence that body size has on reproductive success. Large females (i) produce large offspring, and (ii) may have an increased chance of courtship and mating. During non-breeding years it is suggested that females offset the cost of reproduction through an allocation of energy reserves to bodily growth. Thus individuals may “choose” to grow rather than to breed in particular years. Hence, females in the resting phase of their reproductive cycles may exhibit high growth rates.

The Smooth snakes preyed on lizards and small mammals. Nestling prey (young rodents and young shrews taken from underground nests) were the most important dietary items. It was suggested that the Smooth snake is less specialized in its choice of prey than has been previously reported. It was considered to be an opportunist predator, selecting prey according to its availability.

Population size was estimated by direct enumeration, and densities of about one and two snakes per hectare were considered representative for this species in Britain. Attention was drawn to the influence that site factors and snake behaviour may have on these calculations. Few immature snakes were captured. It was unclear whether this reflected a higher mortality among these specimens, or whether it was more difficult to find them. The even sex ratio and stable age structure for males and females suggested that mortality was not dependent on the sex of the snakes. Adequate data are lacking for a more comprehensive examination of mortality to be made.

An alternative method to the anecdotal estimation of the total population size of Smooth snakes in Britain has been suggested.

I wish to record my thanks to Dr I. F. Spellerberg for the advice and support he gave throughout the course of this study. I would also like to thank the Forestry Commission and the Nature Conservancy Council for their permission to undertake this work. Financial support was provided by a University of Southampton Post-graduate Scholarship. Miss E. C. Meredith and two anonymous referees provided useful comments on parts of this manuscript.

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