Endothermy ecological and evolutionary aspects

Facultative endothermy is an option available only to adult insects with functional flight muscles. Other than preflight warm-up and flight, the flight muscles are used for endothermy during terrestrial activity (Bartholomew and Casey 1977), for stridulation in katydids (Stevens and Josephson 1977), and also for brood incubation and regulation of nest temperature in social insects (for review see Heinrich 1993). Colony homeostasis provides not only an incubator for the brood, but also a thermal refuge in which individuals can exploit temperature gradients to regulate their own Tb (Section 6.6.2). Both social and individual thermoregulation depends on flight muscle activity of adults.

In some beetles MHP can be independent of flight or preparation for flight, and examples of terrestrial endothermy have been recorded in large, nocturnally active beetles (Bartholomew and Casey 1977). Synchronized mating activity in male rain beetles (Pleocoma spp., Scarabaeidae) is characterized by large temperature excesses maintained during both flight and ground searching (Morgan 1987). Competition for the dung of mammalian herbivores has been an important factor in the evolution of endothermy in ball-rolling dung beetles, which maintain elevated Tb while making or rolling balls, and in which contests for possession of already constructed balls are usually won by the beetle with higher Tb which has most recently ceased flight (Heinrich and Bartholomew 1979). Also explicable in terms of competition for dung is the temporal partitioning of flight times in dung beetle communities, and 11 species of the genus Onitis show distinctive daily flight behaviour which is dependent on light intensity (Caveney et al. 1995). Smaller species of Onitis (<0.4 g) are less likely to fly at night or at dawn owing to excessive radiant heat loss. The unusually low conductance of another scarabaeid, the foliage-feeding Sparrmannia flava (0.8 g) makes its thermal physiology more like that of larger beetles, and its activity peaks at 3-4.30 am in the Kalahari desert (Chown and Scholtz 1993), so maximizing predator avoidance. Endothermy enables nocturnal activity in moths and beetles, and this is a major difference from its significance in bees.

Constant body temperature is considered an advantage because biochemical systems evolve to function best at a single temperature. Endothermy in flight is an inevitable biophysical consequence of the relationship between body size and high rates of energy expenditure, and preflight warm-up is necessary because muscles become adapted to operate at high temperatures (Heinrich 1977). The evolution of metabolic architecture is complicated by temperature change, because the temperature dependence of enzyme reactions in a pathway will differ, and so precise regulation of the metabolic pathway will be possible only in a narrow temperature range (Watt 1991). For many insects there is a common Tth ceiling around 40oC, near the upper lethal temperature. When capacities and loads are compared at the biochemical level it is evident that, during flight, enzymes in the flight muscles of honeybees operate closer to Vmax than in other muscles which have been studied (Suarez et al. 1996). When combined with high Ta, high rates of MHP can lead to flight temperatures which come close to upper thermal limits. This is especially true of bees with mating systems involving patrolling or territorial behaviour by males, such as hovering C. pallida (Anthophoridae) in a Californian desert with Tth at 47oC, within 2-3oC of lethality (Chappell 1984).

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