|Citipati, an animal unusual in combining a large, bony head crest with feathers. While the display feathers here are speculative, similar structures are known from the oviraptorosaur Caudipteryx. Art by Niroot Puttapipat.|
'Mutual sexual selection' is pretty self-explanatory. While 'sexual selection' is a one-sided process, with one sex selecting for highly dimorphic traits in the other (Hone himself gives the "endlessly repeated" example of peacocks), 'mutual sexual selection' is, well, mutual. As such, it is likely that both genders of the animal concerned will have ornamentation, or similar sexually selective traits. As is noted in the paper,
"An instructive example is the crested auklet, Aethia cristatella...in which both sexes bear feather plumes on their heads [and] both sexes prefer mates with longer crests" (p. 3)The authors contend that palaeontologists are largely ignoring this idea and failing to realise its changing status in behavioural ecology. They propose that since "there are many circumstances under which male mating time and effort are limited", and given the varying quality of females, it makes sense that males should be selective rather than simply trying to copulate with everything in sight (pp. 11-12).
In formulating this hypothesis, the authors run through a number of others that have been proposed down the years. Some, such as the evolution of crests as weaponry or thermoregulatory devices, can obviously be ruled out for a lot of species. However, what's probably going to raise people's heckles is the authors' rejection of the 'species recognition' hypothesis, that is to say the idea that (some) crested dinosaurs and pterosaurs evolved their displays so that members of the same species could recognise one another. As the authors point out, this idea doesn't explain why "lambeosaurine hadrosaurs required large crests for species recognition, when...members of [the] closely related iguanodontian lineage did not" (p. 10). They also note that a lot of animals today don't require such obvious signals to be able to differentiate between even very similar species. For example:
"...tyrant flycatchers notorious for showing little to no morphological variation exhibit clear boundaries between species, despite sympatry" (p. 9)Mutual sexual selection also neatly solves a problem as regards ceratopsians - that although sexual dimorphism has been proposed for certain species, "the proposed degree of sexual dimorphism is weak" (p. 5) with all mature individuals in a species seemingly being near-equally well-adorned with fancy head ornaments. The same has been found to be true of certain pterosaurs and theropods. As far as theropods go, the authors note the prevalence of crests in relatively basal clades (like the coelophysoids and ceratosaurs), but hypothesise that feathers might have replaced head crests as a sexual display in more advanced coelurosaurs and especially maniraptorans (the clade that includes dromaeosaurs, troodonts, oviraptorosaurs and birds). The known presence of display feathers on animals like Caudipteryx and Epidexipteryx would appear to back up this claim (pp. 12-13).
Of course, one problem with this is that some oviraptorosaurs have well-developed crests, but were presumably fully feathered. While acknowledging this as an "anomaly", the authors point out that the clade is very unusual among coelurosaurs in this respect. Furthermore, they also contend that modern birds that possess bony head crests - like cassowaries and certain hornbills - are also very unusual in having them (p. 13).
Noting that ornithodirans (dinosaurs and pterosaurs) likely relied heavily on vision - with a great deal of evidence backing this up - Hone et al also propose that
"...the evolution of the flight-capable feather and of flight itself may well have its roots in the evolution of ornithodiran sociosexual display." (p. 14)It's an idea that's been proposed before, but here it's presented in the context of mutual sexual selection. Could it be that, in maniraptoran dinosaurs, it was a case of both sexes trying to impress each other that sped along the evolution of the flight feather?
Obviously, this is really just scratching the surface of what's in the paper and, knowing me, I've probably cocked up somewhere along the line (cf. some of the Planet Dinosaur reviews). I'd urge you to get hold of a copy of the paper for all the information - it's actually very accessible for laymen (I should know!).