My idea is about sexual selection and colour vision. Birds often have dazzlingly bright plumage, in the case of species like peacocks or birds-of-paradise, or use colourful object in their displays, such as bowerbirds. Typically it is the male who is showy (potentially attracting predators), whilst the female is more cryptic (hopefully avoiding predators), something that was obvious to the person who named widow birds. Frogs and reptiles can also be brightly coloured, for example the Augrabies agama with their brightly coloured bobbing heads. Mammals on the other hand don’t really go in for bright colours, when you think of sexual selection in mammals it is often massive males fighting, be they bighorn sheep smashing into each other or elephant seals on an Antarctic beach. The obvious exceptions are primates, with bright anogenital swellings in baboons and the suave faces of mandrills.
Why might there be bright colouration for sexual selection in birds, amphibians, fish and reptiles but not in mammals? One obvious answer is that mammals typically have poor colour vision, with an obvious exception of the primates. How can a bright colour display work if the individuals you need to receive it are unable to detect the signal? Mammals seem to have lost the ancestral colour detection systems that all the other vertebrates retain, potentially because they took up a nocturnal niche and relaxed selection meant that it was lost. But since the dinosaurs died mammals have come out of the shadows and many of them are diurnal (like birds etc.), so why no showy colours if they’re out in the sun? It may be that without the ability for natural selection to explore the avenue of colour in mate choice that male-male aggressive competition was the sole mechanism for sexual selection to use. Maybe once you’ve gone far enough down this route it is difficult for evolution to backtrack, i.e. going down a fitness gradient. This is my own little idea, developed on a Sunday night. However, in primates colour vision has reappeared (as it has in a number of other mammal groups too), some researchers think this was to detect ripe fruit, and in this group colour has become part of mate selection.
It’s really interesting to think that an evolutionary quirk, the fact that mammals seemingly lost their colour vision has had a big impact on how their sexual selection systems have evolved and simultaneously their social systems. Evolution can only work with what it has, and this may be a nice example of how the loss of one characteristic can have a large impact on a seemingly unrelated aspect of evolution. But potentially the same selection pressures that lead to losing colour vision may have pushed the development of smell, and mammals are masters of the olfactory realm. Ironically, as we are mammals, it may be our own preclusion for the visual world that prevents us from appreciating the diversity and intricacy of sexual selection via odour in mammals.