title: The bare head of the Northern bald ibis (Geronticus eremita) fulfills a thermoregulatory function.
abstract: BACKGROUND
  Dark pigments provide animals with several adaptive benefits such as protection against ultraviolet (UV) radiation and mechanical abrasion, but may also impose several constraints like a high absorbance of solar radiation. Endotherms, with relatively constant and high body temperatures, may be especially prone to thermoregulatory limitations if dark coloured and inhabiting hot environments. It is therefore expected that adaptations have specifically evolved because of these limitations. Bare, highly vascularised head skin may have evolved in birds with dark plumage from hot geographical regions because of favouring heat dissipation. Using the Northern bald ibis (Geronticus eremita) as a model species, we measured the surface temperature (Tsurf) of the head, the bill and the black feathered body of 11 birds along ambient temperatures (Ta) ranging from 21 to 42.5 °C employing thermal imaging.
  
  
  RESULTS
  While Tsurfof the bill and the feathered body was only slightly above Ta, head Tsurfwas considerably higher, by up to 12 °C. Estimated values of heat loss followed similar variations. We also found that the red colour intensity of the head of ibises increased with head Tsurf, suggesting that birds are capable of controlling blood flow and the thermoregulatory function of the head.
  
  
  CONCLUSIONS
  These findings are consistent with the hypothesis that bare skin has evolved in dark pigmented birds inhabiting hot environments because of their ability to dissipate heat.
authors.affiliation: Department of Evolutionary Ecology, Doñana Biological Station - CSIC, 41092 Sevilla, Spain.
authors.forename: Ismael
authors.identifier: 0000-0002-6523-8592
authors.initials: I
authors.lastname: Galván
authors.affiliation: Department of Zoology, University of Córdoba, 14071 Córdoba, Spain.
authors.forename: Daniel
authors.initials: D
authors.lastname: Palacios
authors.affiliation: Department of Evolutionary Ecology, Doñana Biological Station - CSIC, 41092 Sevilla, Spain.
authors.forename: Juan José
authors.initials: JJ
authors.lastname: Negro
keywords: Bare skin; Heat stress; Northern bald ibis; Pigmentation; Thermal imaging; Thermal radiators