Homeothermy and Thermal Niche Expansion

Many species encounter wide ambient temperature ranges, the result of moving through their milieu, migrating or changing climatic conditions that occur over various temporal timescales (days, seasons, or evolutionary). Homeothermy permits exploitation of a range of thermal environs by freeing Tb from ambient temperature. In turn, thermal niche expansion, coupled with elevated Tb which improves temperature sensitive processes (e.g., locomotory performance, digestion, neural function, and metabolic rate) and the advantages of an improved metabolic rate provide the potential to optimize foraging, growth, and reproduction.

The thermal niche of pelagic tuna, billfish, lamnid, and large sharks has expanded because of homeothermy associated with elevated Tb. Improved temporal resolution resulting from retinal warming and temperature stability in endothermic fish such as the swordfish provides benefits over their prey which will have eyes equilibrated with the temperature of the water in which they swim and thus lower temporal resolution, diminishing the ability to avoid predation. Leatherback turtles by their enormity are homeothermic and consequently, like the warm pelagic fish, can travel vast distances across ocean basins to forage in high latitudes or dive to depths in cold waters. At the other extreme, small insects are capable of homeothermy and elevated Tb in order to enable flight for foraging, defense of territory, and mate selection during cool periods. Many insects, together with a host of reptiles rely on behavioral approaches to improve heat transfer between themselves and their thermal environment in an attempt to regulate Tb.

Decreasing Ta is associated with the increased metabolic costs of thermogenesis in endothermic homeotherms. Continual increases in metabolic rate in response to cold can become unsustainable, particularly if resources are limiting. In response to extrinsic constraints many endotherms respond by lowering their thermal set-point and entering a regulated hypometabolic state (e.g., torpor). Others choose thermally buffered microenvironments (e.g., burrows) or behaviors such as huddling or communal roosting to reduce heat loss, conserve energy, and preserve Tb. A worthy example of the latter approach is found in the red squirrel that remains active during winter (Ta =— 26 °C). The red squirrel minimizes energy expenditure during winter by ensuring easy access to a food hoard and a well-insulated nest in which it remains inactive for most of the time, venturing outside only for short periods on the warmest winter days.

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