Geographic variation in desiccation tolerance in Southern African Trogidae (Coleoptera)

Abstract

The distributional patterns of taxa are influenced by a great many factors, both biotic and abiotic. Physiological tolerance of climatic extremes is certainly one such factor and is the subject of this study. In this thesis, desiccation tolerance was utilized as a tool to determine the influence of physiology on the distribution patterns of keratin beetles (Trogidae). Since nothing is known of water balance in these beetles, this thesis commenced with a study of water balance in the keratin beetles. The keratin beetles' body water and lipid content, when fully hydrated, their maximum water loss tolerance, their rate of water loss and survival time under desiccation conditions, were found to be comparable to those of other similar-sized beetles from similar habitats. With the exception of two arid dwelling species, keratin beetles were not able to replenish lost water through the catabolism of lipids. Like most other adult beetles, none of the keratin beetles were able to obtain water through cuticular absorption of atmospheric water vapour but they were able to restore a positive water balance by drinking free water. Surprisingly, none of the keratin beetles studied were able to osmoregulate. At half their LT 50 the arid dwelling beetles' haemolymph osmolalities were found to be in excess of 1000 mOsm, without noticeable adverse effect to the beetles. In order to ascertain physiological differences in desiccation tolerance of beetles from different climatic areas, interspecific desiccation tolerance studies were conducted using eleven keratin beetles species from six localities, as well as intraspecific studies using five species from a variety of habitats. Both on an intra- and interspecific level, beetles from arid areas were generally found to be morphologically larger and physiologically more tolerant to desiccation than their more mesic counterparts. Being larger, these arid dwelling beetles also had a higher body water content, were able to lose more water and thus, were able to survive desiccation for significantly longer than the more mesic dwelling beetles. However, the arid dwelling beetles were found to be more sensitive to desiccation temperatures. It was speculated that this sensitivity enables these beetles to survive unpredictable long dry (winter) seasons. A significant positive correlation was apparent between the beetles' body size, their survival time during desiccation and the degree of habitat aridification. Thus, although physiological tolerance to desiccation is an important factor in determining the distribution patterns of the keratin beetles, doubtlessly body size plays a major contributing role.