Critical thermal limits in Mediterranean ant species: trade-off between mortality risk and foraging performance

Temporal differences in foraging rhythms, at both daily and seasonal levels, are common among sympatric ant species (Lynch, Balinsky & Vail 1980; Lynch 1981; Herbers 1985, 1989; Lynch, Johnson & Balinsky 1988; Marsh 1988; Fellers 1989; Samways 1990). Although the temporal partition of interspecific foraging shifts is considered to be one of the consequences of interspecific competition, temporal separations in foraging can also result from non-competitive causes such as physiological tolerance to physical conditions (Briese & Macauley 1980; Fellers 1989; De Bie & Hewitt 1990). Differential abilities of species to resist water loss by evaporation and high temperatures may influence the time of the day at which they are active. Selection of particular thermal environments (and consequent body temperatures) on a daily and seasonal basis is a behavioural mechanism leading to certain advantages for animals. Dreisig (1985) considers that the lower limit of the activity range is determined by the costs and benefits of foraging, but it is not a tolerance limit, and activity is possible below it; on the other hand, the upper limit of the activity range is thought to be a tolerance limit determined by the impairment of physiological functions. A number of studies (Marsh 1985a; Cokendolpher & Phillips 1990; Wehner, Marsh & Wehner 1992) have demonstrated that environmental temperatures experienced by worker ants often result in body temperatures that exceed some critical thermal maximum, defined as the temperature at which locomotor ability is so reduced that they can no longer escape lethal temperatures (Cokendolpher & Phillips 1990). Such thermal stress limits worker activity to certain times of day and specific microhabitats, thus restricting movement and foraging activity (O’Neill & Kemp 1990). What are the benefits and costs of foraging at different temperatures? The problem for ectotherms is that, on the one hand, certain temperature levels are necessary for their activation and survival, but, on the other hand, critical temperatures can also be dangerous and ORIGINAL ARTICLE OA 000 EN