In ectotherms, most physiological processes are strongly influenced by body temperature (Huey 1982; Heinrich 1993). Sensitivity to temperature has the potential to influence the behaviour, ecology and, ultimately, fitness of an individual. The extent to which ectotherms can tolerate changes in their ambient thermal environment is thus critical in determining their distribution and abundance.
There are two broad strategies by which an ectotherm can adapt to its thermal environment:
1. Be a thermal generalist (eurythermal) and evolve a physiology that is relatively insensitive to temperature change and which confers a broad range of temperature tolerance.
2. Be a temperature specialist (stenothermal), with a physiology adapted to a relatively narrow range of temperatures. There are then two alternative strategies possible, either
(a) a low body temperature optimum close to ambient temperatures, or
(b) a higher body temperature optimum, maintained above ambient, which then requires mechanisms to achieve this.
Most poikilothermic animals are eurythermal generalists, biochemically adapted to function at the temperatures to which they are subjected in the field (Heinrich 1977), and many small ectotherms have little scope for maintaining their body temperatures at levels that are different from ambient (Watt 1991). However, broad temperature optima for enzymes are limited by the temperature sensitivity of molecular bonding, and temperature independence in ectotherms is associated with comparatively low rates of aerobic metabolism (Heinrich 1977). An effective metabolic architecture requires the integration of many reactions, each of which may have a different temperature sensitivity. Over a broad range of body temperatures, precisely adjusted integration of steps in a pathway, necessary for high metabolic efficiency, may not be possible (Watt 1991). This explains the trade-off between breadth of performance and maximal performance (Huey & Hertz 1984; Huey & Kingsolver 1989).
In predictably cool environments high metabolic rates can be achieved with high levels of enzymes adapted to a narrow range of low temperatures (Watt 1991). Lower body temperature optima are seen in Antarctic terrestrial invertebrates (Block & Young 1978; Block & Somme 1983). However, there are synthesis costs to high enzyme levels, and there is a very high risk in adopting the low-temperature specialism strategy in unpredictable environments. Many enzymes are deactivated only a few degrees above their optimum, so any organism that evolved low-temperature specialism would run the risk of severe physiological stress or death in the higher temperatures potentially occurring in an unpredictable environment. Antarctic fish are well adapted to temperatures of − 1·5 °C to + 1 °C but die at ambient temperatures of as little as + 6 °C (Heinrich 1977).
The alternative thermal specialist strategy is to evolve a high-temperature optimum. This reduces the risk of overheating, but as most enzymes rapidly become inactive as temperatures fall below their optimum it is necessary to evolve ways of raising the body temperature when ambient temperatures are lower than optimal. These may be morphological, such as the colour polymorphism and pubescent body covering of Colias butterflies (Watt 1968; Jacobs & Watt 1994); physiological, such as the use of flight muscles to generate heat in bees (Heinrich 1972); or behavioural either by selection of warm microsites (Sudd et al. 1977) or by basking to absorb incident solar radiation (Chappell & Whitman 1990).
Orthopteran life-history traits are very sensitive to temperature, as seen in the laboratory (Whitman 1986) and in the field (Richards & Waloff 1954; Atkinson & Begon 1988a; van Wingerden, Musters & Maaskamp 1991), where this sensitivity has strong demographic consequences (Dempster 1963). Orthopterans are members of a predominantly tropical order (Uvarov 1977), with those species occurring in Britain close to the northwestern limit of their range where they experience a relatively cool and highly unpredictable climate. Because of this extreme variability in the thermal environment the low-temperature specialist strategy is not viable. British grasshoppers therefore lie somewhere along the continuum between thermal generalists and high-temperature thermal specialists. Behavioural thermoregulation is the principal means of controlling body temperature, basking in direct insolation to raise body temperature and shade seeking to avoid lethal high temperatures (Young 1979; Willott 1992).
We predict that those species that are high-temperature thermal specialists will have a distribution that is more restricted to the South and East, where temperatures are higher and sunshine more predictable, or be restricted to a more limited range of habitats than those that have developed a more generalist strategy.
To test these predictions four species of grasshoppers were raised at a range of ambient temperatures and key life-history traits were monitored. The results are integrated to determine an index of fitness at each temperature using the model described by Grant, Hassall & Willott (1993). Those species that are more specialist are identified by their susceptibility to reductions in temperatures below 35 °C and by them having higher temperature thresholds for growth and development. The sensitivities of the species to temperature change are discussed in relation to their behavioural thermoregulatory abilities (Willott 1997) and known distribution throughout the British Isles (Marshall & Haes 1988).