Increasingly, designers of indoor temperature regulation systems are studying the temperature regulation strategies of poikilotherms (animals whose body temperature varies with the ambient temperature). Reptiles like the collared lizards (Crotaphytus spp.) commonly alternate between basking and burrowing, but for the Rubber Boa (Charina bottae) and others that function best at low temperatures, life is more of an exercise in avoiding heat than seeking it. New Zealand's relictual Tuatara (Sphenodon punctatus) is the most cold-blooded reptile of all, with an optimum body temperature of around 10ºC (50ºF). Amphibians tend to have lower optimum body temperatures than reptiles. Heat from muscle contractions is frequently used in thermoregulation. Sphinx moths (family Sphingidae), for example, vibrate their wings on cool evenings in a pre-launch warm-up. As an animal increases in size, its surface area-to-volume ratio decreases, and heat loss slows down. A boxy, half-ton Leatherback Turtle (Dermochelys coriacea) can increase its body temperature significantly simply by swimming. This is assisted by heat exchangers in its circulatory system that work like those used in many buildings. Such devices are also present in some sharks, and are most developed in tuna of the genera Thunnus and Katsuwonus, which can maintain a temperature in the swimming muscles of their upper tails about 14ºC (25ºF) higher than the ambient water temperature. Lacking expensive metabolic 'furnaces,' poikilotherms can devote practically all of their energy to foraging and reproducing. Because of this, they tend to be more plentiful than their homeothermic analogs. Since internal chemicals often react differently at different temperatures, poikilotherms depend on redundant chemical systems to function properly at different temperatures. Aside from this, though, the advantages of a steady internal temperature are often exaggerated.