Regulation of heat exchange across the hornbill beak : functional similarities with Toucans?

Show simple item record Van den Ven, T.M.F.N. Martin, R.O. Vink, T.J.F. McKechnie, Andrew E. Cunnigham, S.J. 2016-07-05T10:54:46Z 2016-07-05T10:54:46Z 2016-05-18
dc.description S1 Data. Numerical data. Numerical data used in preparation of Figs 1 and 4; Tables 1, 2, 3 and 4; S1, S2 and S3 Figs. (XLSX) en_ZA
dc.description S2 Fig. Heat loss (% of total) in Southern Yellow-billed Hornbills. Heat loss as a proportion of total body heat loss (%) plotted against air temperature (Ta) of torso, gular skin, the beak as a whole and lower mandible of the beak in Southern Yellow-billed Hornbills (Tockus leucomelas). Data above the panting initiation temperature (Ta = 37.4 ± 2.1°C) has not been included in this graph since evaporative heat loss has not been assessed and this makes total heat loss after initiation of panting incomplete. (TIF) en_ZA
dc.description S3 Fig. Relative humidity (%) and water vapour pressure (kPa) in the temperature cabinet. Relative humidity (%) and water vapour pressure (kPa) in the temperature cabinet in response to air temperature (°C). Data are combined from all the individual experiments. Error bars represent SE. (TIF) en_ZA
dc.description S1 Text. Additional methods for heat transfer calculation (DOCX) en_ZA
dc.description S1 Video. Thermal imaging sequence of the Southern Yellow-billed Hornbill. Thermal imaging sequence of the Southern Yellow-billed Hornbill (Tockus leucomelas) during the experiment. (MP4) en_ZA
dc.description.abstract Beaks are increasingly recognised as important contributors to avian thermoregulation. Several studies supporting Allen’s rule demonstrate how beak size is under strong selection related to latitude and/or air temperature (Ta). Moreover, active regulation of heat transfer from the beak has recently been demonstrated in a toucan (Ramphastos toco, Ramphastidae), with the large beak acting as an important contributor to heat dissipation. We hypothesised that hornbills (Bucerotidae) likewise use their large beaks for non-evaporative heat dissipation, and used thermal imaging to quantify heat exchange over a range of air temperatures in eighteen desert-living Southern Yellow-billed Hornbills (Tockus leucomelas). We found that hornbills dissipate heat via the beak at air temperatures between 30.7°C and 41.4°C. The difference between beak surface and environmental temperatures abruptly increased when air temperature was within ~10°C below body temperature, indicating active regulation of heat loss. Maximum observed heat loss via the beak was 19.9% of total non-evaporative heat loss across the body surface. Heat loss per unit surface area via the beak more than doubled at Ta > 30.7°C compared to Ta < 30.7°C and at its peak dissipated 25.1Wm-2. Maximum heat flux rate across the beak of toucans under comparable convective conditions was calculated to be as high as 61.4Wm-2. The threshold air temperature at which toucans vasodilated their beak was lower than that of the hornbills, and thus had a larger potential for heat loss at lower air temperatures. Respiratory cooling (panting) thresholds were also lower in toucans compared to hornbills. Both beak vasodilation and panting threshold temperatures are potentially explained by differences in acclimation to environmental conditions and in the efficiency of evaporative cooling under differing environmental conditions.We speculate that non-evaporative heat dissipation may be a particularly important mechanism for animals inhabiting humid regions, such as toucans, and less critical for animals residing in more arid conditions, such as Southern Yellow-billed Hornbills. Alternatively, differences in beak morphology and hardness enforced by different diets may affect the capacity of birds to use the beak for non-evaporative heat loss. en_ZA
dc.description.department Zoology and Entomology en_ZA
dc.description.librarian am2016 en_ZA
dc.description.sponsorship The Department of Science and Technology, Republic of South Africa and the award number is 40470. en_ZA
dc.description.uri en_ZA
dc.identifier.citation Van de Ven TMFN, Martin RO, Vink TJF, McKechnie AE, Cunningham SJ (2016) Regulation of Heat Exchange across the Hornbill Beak: Functional Similarities with Toucans? PLoS ONE 11(5): e0154768. DOI: 10.1371/journal.pone.0154768. en_ZA
dc.identifier.issn 1932-6203
dc.identifier.other 10.1371/journal.pone.0154768
dc.language.iso en en_ZA
dc.publisher Public Library of Science en_ZA
dc.rights © 2016 van de Ven et al. This is an open access article distributed under the terms of the Creative Commons Attribution License. en_ZA
dc.subject Beaks en_ZA
dc.subject Heat transfer en_ZA
dc.subject Size en_ZA
dc.subject Respiratory cooling en_ZA
dc.title Regulation of heat exchange across the hornbill beak : functional similarities with Toucans? en_ZA
dc.type Article en_ZA

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