Seasonal fat content fluctuations in a gyrinid beetle: comparison between an arctic and a temperate zone population (Coleoptera: Gyrinidae)

Abstract

Seasonal fluctuations in fat content (percentage of dry body weight) of adult Gyrinus opacus were compared between a lake population at the northern range limit of the species in Greenland (G) and a rock pool population in south-central Sweden (S) near its southern range limit. From equally low values at emergence in summer, beetles of both populations accumulated substantial fat reserves, especially during their first weeks as adults. Hardened S-beetles were fatter than G-beetles at time of onset of hibernation. For late emerging (soft) G-beetles, however, the situation was severe because of an immediate freeze-up of the lake. There was a similar winter fat decline in both populations. During the first month after hibernation there was a significant decrease in fat content of the S-beetles, because they were still utilizing their fat deposits from the previous autumn. This trend was broken at the first emergence peak of chironomids in spring. Hibernating G-beetles used their lesser fat storage to the same extent, and led to some reproductive females reaching extremely low levels of fat. Still lower fat content and smaller body size of beetles in a closely situated small lake to G in early spring shows a between locality variation related to differences in habitat quality at the northern range limit.