Thermal Adaptation - Increased Bill Size

Endothermic organisms regulate their body temperature by balancing metabolic heat production with heat exchange in their environment (Tattersall et al 2009). Toucans (family Ramphastidae) are endothermic, and have been studied in nature for their large bill size in comparison to body size. Early explanations for large bill size included mating, feeding purposes, and territorial defense (Tattersall et al 2009; Hughes 2014). Techniques of convective heat loss have been discussed previously in this blog, such as gular fluttering, sweating and increased surface area of ears in elephants (comments). This blog post will discuss the growth and thermoregulatory ability of the toucan’s bill, and evaluate its primary ecological function.

The toucan family (Ramphastidae) is entirely distributed in Neotropical areas, with most species inhabiting lowland tropical forests, where high daily temperatures and humidity is frequent (Hughes 2014). The toucan’s body is covered primarily by dense feathers, which remain slightly above environmental temperature, suggesting most areas of body mass are unsuitable for thermoregulation (Tattersall et al 2009). A 2014 study conducted on the development of physical structures relative to body mass, found that maxilla length and depth (upper part of bill) increases at a greater than isometric rate more than any other physiological structure (e.g. cranium length and width, sternum, synsacrum, femur and tibiotarsus) (Hughes 2014). A 2009 study found that the proximal region of the bill (closest to head) dissipated heat at lower environmental temperatures (>16°C - 25°C), with blood flow increasing to distal regions of the bill as temperatures rose toward 30°C and higher. This study also determined that heat loss from the bill can account for 25 – 400% of heat production in adult toucans, depending on environmental variables such as altitude, wind speed, ambient temperature etc. The thermoregulatory capabilities of the toucan bill also far surpass that of elephants ears’ estimated at a range of 9-91% of heat production (Tattersall et al 2009).

Toucans live in tropical ecosystems, with high humidity and high daily temperatures. Studies have shown they are dependent on rapid growth of the maxilla to dissipate heat (Hughes 2014). Their ability to control blood flow between proximal and distal regions of the beak enables relative homeostasis in a varied climate (Tattersall et al 2009). Toucans have evolved with large bill size in order to maintain optimal internal temperature, suggesting that thermoregulation is its primary function. However, this research does not disprove that mating, feeding or any other environmental pressures have selected for large bill size during the evolution of the toucan.

References:

Hughes, A. 2014, ‘Evolution of bill size in relation to body size in toucans and hornbills’, Zoologia, Vol. 31, pp. 256-263

Tattersall, G. Andrade, D. Abe, A. 2009, ‘Heat Exchange from the Toucan Bill Reveals a Controllable Vascular Thermal Radiator’, Science, Vol. 325, No. 5939, pp. 468-470