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The phenomenon of males comparing their attributes to those of other males is millions of years old, according to a new study. Flying reptiles and fin-backed ancestors of modern mammals developed body traits for the purpose of sexual selection.

Dr Joseph Tomkins, from The University of Western Australia and colleagues from the Universities of Hull and Portsmouth in the UK have found that prehistoric pterosaurs evolved elaborate headcrests to help them attract the best mates while the pelycosaurs, a group of our own distant ancestors, developed fantastic sails along their backs to oust sexual competitors.

The purpose of the exaggerated crests and sails found in many fossil animals has long been controversial, though many have claimed they helped regulate the animals' body temperature, or in the case of the pterosaurs, they helped the reptiles to steer when in flight.

The new research published in the latest edition of The American Naturalist shows how the relative size of the head crest compared to the body of the pterosaur was too big for it to have been dedicated to thermoregulation or temperature control.

These findings suggest that the elaborate crests and sails became so grand because of sexual competition: bigger crests and sails were more attractive to prospective mates, so they became more exaggerated over successive generations.  Some pterosaurs had crests five times bigger than their skulls.

Numerous species, known only from the fossil record, display exaggerated and often bizarre morphological structures.  These include the plate-backed Stegosaurus, head-crested hadrosaurs, pterosaurs such as Pteranondon and the enigmatic sail-backed eupelycosaurs Dimetrodon and Edaphosaurus.

Lead author Dr Joseph Tomkins, from UWA's Centre for Evolutionary Biology, said: "Our analysis suggests that male Pteranodon either competed with each other, in battles for dominance using their crests - in a similar way to animals with horns or antlers - or alternatively, that females assessed males on the size of their crests, in a similar way to peahens choosing among a group of displaying males."

The scientists used the laws of physics to predict the scaling relationships.  Animal metabolism - the process behind heat generation - can be plotted against body size.  The researchers found that in each case the scaling of the crests of the sails were too extreme to have a dedicated body temperature control function.

Dr Stuart Humphries from the University of Hull said: "One of the few things that haven't changed over the last 300 million years are the laws of physics, so it has been good to use those laws to understand what might really be driving the evolution of these big crests and sails."

Dr Dave Martill from the University of Portsmouth said: "Pterosaurs put even more effort into attracting a mate than peacocks whose large feathers are considered the most elaborate development of sexual selection in the modern day.  Peacocks shed their fantastic plumage each year, so it's only a burden some of the time, but pterosaurs had to carry their crest around all the time."

Dr Joseph Tomkins concluded: "The sails of the Eupelycosaurs are among the earliest known examples of exaggerated secondary sexual traits in the history of vertebrate evolution; indeed the sail of Dimetrodon is one of the largest secondary sexual traits of any animal."

See the paper entitled:  ‘ Positive Allometry and the Prehistory of Sexual Selection' .

Images

1. Image of Pteranodon longiceps from the National Geographic Sea monsters series.

For copyright information regarding this image contact Susan Henry

2. Image of Dimetrodon

Media references

Janine MacDonald (UWA Public Affairs)  (+61 8)  6488 5563  /  (+61 4) 32 637 716
Claire Mulley (University of Hull)  (+00 11 44) 14 8246 6943
Mobile:  07809 585965
Kate Daniell (University of Portsmouth)  (+00 11 44) 23 9284 3743