Understanding selection for long necks in different taxa

Abstract

There has been recent discussion about the evolutionary pressures underlying the long necks of extant giraffes and extinct sauropod dinosaurs. Here we summarise these debates and place them in a wider taxonomic context. We consider the evolution of long necks across a wide range of (both living and extinct) taxa and ask whether there has been a common selective factor or whether each case has a separate explanation. We conclude that in most cases long necks can be explained in terms of foraging requirements, and that alternative explanations in terms of sexual selection, thermoregulation and predation pressure are not as well supported. Specifically, in giraffe, tortoises, and perhaps sauropods there is likely to have been selection for high browsing. It the last case there may also have been selection for reaching otherwise inaccessible aquatic plants or for increasing the energetic efficiency of low browsing. For camels, wading birds and ratites, original selection was likely for increased leg length, with correlated selection for a longer neck to allow feeding and drinking at or near substrate level. For fish-eating long-necked birds and plesiosaurs a small head at the end of a long neck allows fast acceleration of the mouth to allow capture of elusive prey. A swan's long neck allows access to benthic vegetation, for vultures the long neck allows reaching deep into a carcass. Geese may be an unusual case where anti-predator vigilance is important, but so may be energetically efficient low browsing. The one group for which we feel unable to draw firm conclusions are the pterosaurs, this is in keeping with the current uncertainty about the biology of this group. Despite foraging emerging as a dominant theme in selection for long necks, for almost every taxonomic group we have identified useful empirical work that would increase understanding of the selective costs and benefits of a long neck.

An Evolutionary Cascade Model for Sauropod Dinosaur Gigantism - Overview, Update and Tests

“…raising the neck and supplying it and the head with blood). Model calculations specifically addressing low browsing in sauropods [61], [128] also confirm the hypothesis that the long neck greatly reduced the need for the animal to change its location during feeding. This would have resulted in energy savings of 80% in a Brachiosaurus bearing a nine-meter neck compared to a minimally-necked one [61].…”

“…Model calculations specifically addressing low browsing in sauropods [61], [128] also confirm the hypothesis that the long neck greatly reduced the need for the animal to change its location during feeding. This would have resulted in energy savings of 80% in a Brachiosaurus bearing a nine-meter neck compared to a minimally-necked one [61]. Both studies [61], [128] independently concluded that the energetic advantage of neck length levels off eventually with increasing neck length.…”

“…The Cretaceous was not covered by this analysis [13], which should not be a problem in the current context because sauropod gigantism already had evolved in the Late Triassic and Jurassic. However, recently a specific hypothesis of extrinsic control by Midgley et al [60], invoking raised levels of carbon dioxide during the Mesozoic to account for dinosaur gigantism, was resurrected [61] and awaits further scrutiny.…”

“…While it is clear that a longer neck confers advantages to an animal of any size [2], [61], [128], as shown by studies on extant animals [61], the important point with regard to sauropods is that this advantage favorably scales with body mass. The scaling effect lies in the scaling of acceleration and deceleration of the body because larger animals are less “athletic” than smaller ones because muscle power only increases with the square of linear size whereas mass increases with the third power (see reviews in [130], [131]).…”

“…Several kinds of new model calculations, on the other hand, do support the hypothesized selective advantage ([61], [128], [133], [134], see also [135]). Model calculations addressing high browsing based on Euhelopus and Giraffatitan [133] indicate that the energetic advantage of this design outweighs its metabolic costs (i.e.…”