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

Hughes, Austin L.

doi:10.1590/s1984-46702014000300007

Cited by 9 publications

(8 citation statements)

References 30 publications

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“…It would therefore be expected that as birds increase in size, the synsacrum must also increase in size to continue to support the bird's weight adequately. In one study of the evolution of bill size, Hughes () found that synsacrum length increased with positive allometry relative to body mass and this was attributed to the enhanced need for weight support in the overall heavier hornbills. No similar relationship was seen in toucans, and Hughes () suggested that this may be due to their substantially smaller body masses and the fact that they spend much less time on the ground than hornbills.…”

Section: Discussionmentioning

confidence: 99%

Effects of phylogeny and locomotor style on the allometry of body mass and pelvic dimensions in birds

2017

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The pelvic girdle provides physical support and attachment for the hind limb musculature. In birds there is variability in pelvic morphology across different orders and this has been used as evidence for various types of locomotion. However, the morphological variation of pelvic bones has yet to be studied systematically in birds. Therefore, we investigated basic allometric relationships among female body mass (as a size proxy) and various pelvic measurements in a phylogenetic context. We also examined in detail the inter-relationships among various pelvic measurements. Also considered were the effects of broader taxonomic relationships at the level of order, with further examination of the influence of style of terrestrial locomotion on the allometric relationships. Positive relationships were initially found among all pelvic linear measurements and female body mass (FBM). The relationships among measures of pelvic width and FBM were isometric, whereas those between pelvic length and FBM showed positive allometry. Also, FBM explained more of the variance in pelvic length than in width. Similarly, the angle of the pelvis had a significant negative relationship, but FBM only explained a very low proportion of the variation in pelvic angles. In general terms, ancova showed that the effect of FBM was smaller than the effect of locomotor style in this species set. Both the synsacrum and pelvic girdle play roles in weight support and therefore scale in proportion to body weight accordingly. All three parts of the pelvis (ilium, ischium and pubis) are attached around the acetabulum and also provide muscle attachment points, so it might be expected for them to scale in a similar manner. Increased angulation of the pelvis is linked to orders which employ their hind limbs in feeding, perching and running, although FBM also explains a very low proportion of the variation in pelvic angle. Muscle attachment and the confines on morphology presented by locomotion explain much of the variation exhibited by the relationships among pelvic measurements.

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Section: Discussionmentioning

confidence: 99%

Effects of phylogeny and locomotor style on the allometry of body mass and pelvic dimensions in birds

2017

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“…Hornbills (Bucerotiformes: Bucerotidae) are widespread in the Afrotropical and Indomalayan regions, with members of this taxon occupying habitats ranging from arid savannas to humid tropical forests [ 10 ]. Like toucans (Piciformes: Ramphastidae), hornbills have disproportionately large beaks and are a candidate for a similar capacity of heat exchange (however see Hughes [ 11 ]). Large beaks might be beneficial in various ways (dietary, thermoregulatory, sexual selection, etc.…”

Section: Introductionmentioning

confidence: 99%

Regulation of Heat Exchange across the Hornbill Beak: Functional Similarities with Toucans?

et al. 2016

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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 25.1 W m-2. Maximum heat flux rate across the beak of toucans under comparable convective conditions was calculated to be as high as 61.4 W m-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.

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“…As a consequence, the adaptive significance of any individual trait is likely to be best understood in the context of other traits with which it is associated to form such a phenotypic complex. The results of the present study illustrate the usefulness of measures of the relative size of avian skeletal elements for deriving simple indices of body plan (such as principal components) that can reduce complex, multidimensional patterns to a linear scale (Hughes 2013(Hughes , 2014.…”

Section: Discussionmentioning

confidence: 61%

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2015

The Auk

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

Cited by 9 publications

References 30 publications

Effects of phylogeny and locomotor style on the allometry of body mass and pelvic dimensions in birds

Effects of phylogeny and locomotor style on the allometry of body mass and pelvic dimensions in birds

Regulation of Heat Exchange across the Hornbill Beak: Functional Similarities with Toucans?

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