Heat Exchange from the Toucan Bill Reveals a Controllable Vascular Thermal Radiator

Abstract

The toco toucan (Ramphastos toco), the largest member of the toucan family, possesses the largest beak relative to body size of all birds. This exaggerated feature has received various interpretations, from serving as a sexual ornament to being a refined adaptation for feeding. However, it is also a significant surface area for heat exchange. Here we show the remarkable capacity of the toco toucan to regulate heat distribution by modifying blood flow, using the bill as a transient thermal radiator. Our results indicate that the toucan's bill is, relative to its size, one of the largest thermal windows in the animal kingdom, rivaling elephants' ears in its ability to radiate body heat.

Evolution of bill size in relation to body size in toucans and hornbills (Aves: Piciformes and Bucerotiformes)

“…Throughout the toucan family, the length and depth of the maxilla increased as a function of body mass at a rate greater than expected under isometry, implying disproportionately large bills per unit body mass in large-bodied toucan species, consistent with the hypothesis that heat dissipation has been an important factor in the evolution of the large bills of toucans (TATTERSALL et al 2009). Since the capacity for radiation of heat from the bill is a function of surface area, it is further expected that bill surface area will increase with body mass at a rate greater than expected under isometry.…”

“…Consistent with a role for the toucan bill in heat-dissipation, TATTERSALL et al (2009) presented evidence that bill length in juvenile and adult Toco Toucan increases as a function of body mass at a rate greater than the isometric expectation; i.e., greater than an exponent of 1/3 expected for a linear dimension (ALEXANDER 1971). Likewise, SYMONS & TATTERSALL (2010) provided evidence that across toucan species bill length increases as a function of body mass at a rate greater than linear expectation, using published data on 34 species of Ramphastidae.…”

“…A further contribution to understanding the function of the toucan's bill was provided by evidence that the bill of the Toco Toucan, Ramphastos toco Statius Muller, 1776, serves as a key surface area for heat dissipation (TATTERSALL et al 2009), which the bird can use to regulate body temperature by controlling blood flow. There is evidence that bills of a variety of avian taxa can function in heat dissipation (HAGAN & HEATH 1980, SCOTT et al 2008, GREENBERG et al 2012a, b, GREENBERG & DANNER 2013, suggesting that heat dissipation may be a plesiomorphic function of the avian bill.…”

“…The adaptive significance of the large and remarkably light-weight bill of members of the Neotropical toucan family (Piciformes: Ramphastidae) has been the subject of much speculation (SHORT & HORNE 2001, TATTERSALL et al 2009). Although VAN TYNE (1929: 39) suggested that the toucan's bill has no "especial adaptive function", a number of adaptive hypotheses have been proposed.…”

“…There is evidence that bills of a variety of avian taxa can function in heat dissipation (HAGAN & HEATH 1980, SCOTT et al 2008, GREENBERG et al 2012a, b, GREENBERG & DANNER 2013, suggesting that heat dissipation may be a plesiomorphic function of the avian bill. In the Ramphastidae, it might be hypothesized that the ancestral heat-dissipation function has become elaborated by the evolution of a highly modifiable vascular radiator (TATTERSALL et al 2009). On this hypothesis, the emergence of this vascular adaptation has been an additional factor favoring the evolution of large bill size in toucans, in conjunction with other selective pressures such as frugivory and signaling.…”