Hill-topping in British butterflies: incidence and cues in a cool, windy climate?

摘要

Hill-topping by butterflies is evidently an unusual activity in the British Isles. There is a record of such behaviour for Vanessa cardui L. on the Wrekin, Shropshire (3 individuals at 407 m summit) by Joy (1996) and one for Vanessa atalanta L. on a low hillock (82 m) in north Anglesey by Hardy & Kinder (2000). Both of these nymphalids have apparently been recorded hill-topping earlier by Moffat in Ireland (1922). Vanessa atalanta has also been observed on the summit of Bidean nam Bian (1150 m), north-east of Oban, Scotland, on 13 September 2002 (P. Ormerod, pers. comm.). Hill-topping was first described by Shields (1967) in North America and has been thoroughly studied since then, mainly in the United States (e.g. Baughman, Murphy & Ehrlich, 1988,1990; Brown & Alcock, 1990) but it has also been observed elsewhere (e.g. McFarland, 1976; Pinheiro, 1990; Tennent, 1995). The behaviour is far from being fully understood (Ehrlich & Wheye, 1986; Brown & Alcock, 1990; Singer & Thomas, 1992). Fundamentally, hill-topping behaviour is considered to facilitate mating in populations occurring at low density (Scott, 1968). Males typically occupy the summit and await the arrival of females that, once mated, descend to lower elevations to lay eggs. Males eitherpatrol the summit or establish territories that they defend. Although the key to the activity may well be low population density, clearly suitable conditions are required to occupy hilltops, especially if at any great altitude. Sunny, hot, calm conditions are particularly propitious for hill-topping and the opposite - cloudy, cool and windy conditions - are likely to deter the behaviour, if only because males will be too cool to locate and acquire mates at the summit. As such, it is very possible thatthe limited observations on hill-topping among Lepidoptera in Britain reflects the actual rarity of the behaviour in this region and that poor summer weather conditions are responsible.