Ecological energetics of the puerto rican tody: heterothermy, torpor, and intra‐island variation

The extremely small size (5–7 g) of todies, together with their taxonomic affinities (Order Coraciiformes) and tropical distribution, suggest that the thermoregulatory physiology of these birds merits detailed investigation. Here, we present results of a study on body temperature regulation and metabolic rate of one species, the Puerto Rican Tody (Todus mexicanus). Basal metabolic rate (BMR) was 3.24 ± 0.59 mL O₂·g⁻¹·h⁻¹ (mean ± 1 sd), ∼33% higher than predicted on the basis of mass, but within the range of BMRs reported for other small birds. However, many other aspects of the tody's thermoregulatory physiology are unusual. (1) Average active‐phase body temperature (Tb) was 36.7 ± 1.2°C, well below the avian norm of 40–41°C. (2) Todies exhibited heterothermy; over a range of ambient temperatures (Tₐ) from 15° to 40°C, body temperatures varied from 27.9° to 42.9°C. At low Tₐ's, Tb of some individuals decreased by as much as 10–11°C during nocturnal hypothermia; these birds remained active and alert, indicating that hypothermia was a controlled response. (3) Todies showed evidence of climatic adaptation in both BMR and body size. Birds inhabiting hot‐climate scrub forest had significantly lower BMRs than did those residing in cooler montane rain forest. Individuals from the xeric scrub site were also significantly smaller in all measures of body size than birds from the rain forest. (4) Seasonal changes in BMR were evidenced by a significant elevation of BMR during the breeding season as compared to the nonbreeding season. (5) During the breeding season, some female todies went into torpor when exposed to low Tₐ's (15°C), with Tb's ranging from 23.5° to 29.3°C. The Puerto Rican Tody is one of the few tropical birds known to enter torpor, is the first representative of the Order Coraciiformes reported to exhibit torpor, and is the first avian species reported to exhibit torpor restricted to one sex.