Risk of Predation in Male and Female Adders, Vipera Berus (Linné)

Risk of predation in male and female adders, Vipera berus (Linné)

Claes Andrén

University of Göteborg, Department of Zoology,Box 250 59, S-400 31 Göteborg, Sweden

Abstract. In a pilot test with live adders, Viperaberus, more males than femaleswere attacked by visually guided predators after, but not before, the synchronized spring sloughing in males. In three 24 h experiments and one five days experimentthe same proportion of male and femaleadder modelsin basking position was attacked by predators. I spring more males than femaleswere exposedto predators, but in the summer the oppositewas true. However,in the wholeseason females were exposedto predationa longer period than males. As sex ratio re- mains constant, male activityin spring, with high mobilityand fighting,may be more risky than the exten- sive basking in pregnant femalesduring summer.

Introduction

Snakes generally exhibit little sexual dimorphism (Gans and Tinkle, 1977). However, in the adder males and females differ markedly in colour pattern, which is most pro- nounced after the spring and summer sloughings (Bernström, 1943; Andrén, 1981). In this study I use live adders and adder models to test if risk of visually guided predation is greater in either sex. I also test if activity pattern "exposure to predators" is different in free living male and female adders.

Study area and methods

Field work was carried out in a free living population of adders on the island Stora Hdstholmen (57°46'N) SW Sweden, and complemented with a polit test in a 25 m2 outdoor field terrarium in Göteborg. Stora H5stholmen has an area of 54 ha of which one third is bare rocks partly overgrown by heather. Low and flat areas are vegetated by grass and herbs, and on wet ground the grass is highly tussocked. Possible adder predators are hooded crow (Corvus cornix (Linn6)), raven (Corvus corax (Linn6)), magpie 204

(Pica pica (Linn6)), buzzard (Buteo buteo (Linn6)), rough-legged buzzard (Buteo lagopus Brünn) and temporarily visiting foxes (Vulpes vulpes (Linn6)). In 1970 a pilot test on visually guided predation was performed in the outdoor field terrarium with seven male and six female live adders. The number and sex of adders being removed by predators, Pica pica and Corvus cornix, during five days before and after male sloughing in spring were noted. The importance of colour pattern in relation to visually guided predation was ex- perimentally tested on Stora Hästholmen in May 1980. Fifty plastic models, 65 cm long and shaped as adult adders, were put out around the island. Fifty per cent of the snake models were painted in the colour pattern of sloughed males and the rest were painted to resemble sloughed females. Male and female morphs were allocated ran- domly to predetermined experimental sites. I marked head and tail tip position with small sticks and under the snake, invisible from above, I placed a small piece of meat as a reward for predators attacking the model. Models which had been moved several centimetres and with the piece of meat removed were classified as subjected to predatory attack. Three experiments of 24 h and one of five days were run. Field data were collected on Stora Hdstholmen in 1974. During 44 days I followed the same standard route and noted position, sex, sloughing status, and behaviour of all adders observed. Only non-parametric statistical test, sign tests, have been used (Siegel, 1956).

Results

A pilot test in the outdoor terrarium on avian predation was made during a period of five days on non-sloughed live adders. In this test two males and two females were attacked and removed from the terrarium. During five days after the synchronized male sloughing four out of five remaining males were killed by birds, while none out of four remaining females were attacked. The predation on models of male and female adders located on basking spots is shown in Table 1. In three 24 h experiments taken together, and one experiment of five days, there were no differences in the proportion of male and female models attacked by visually guided predators. The number of male and female adders observed (predator exposures) in different activity periods during 44 days is given in Table 2. During spring basking and mating periods significantly more males than females were exposed. In the feeding period from mid May to end of August the opposite was true, significantly more females than males were seen. During the autumn basking period no difference was found in male and female exposure behaviour. The expected number of snake observations (predator exposures) during the whole active season, if all days were observation days, were 919 in males and 1,497 in females, which is significantly different (p < 0.0001).