Many animals (potentially including humans) pair with partners of equivalent attractiveness. This ‘assortative’ mating works such that the best looking individuals pair up, leaving the remaining, less attractive individuals to pair.
In order for this to happen, the animal in question need to have a means by which they can assess how attractive the oppose sex is.
In many species, male have ‘secondary sexual traits’ which are large or obvious features which function only to attract females (in most animals is the males doing the attracting and the females doing the choosing). For example, many male birds have bright colouration while the females are drab in colour to be camouflaged and less obvious to predators. These colourful ‘ornaments’ serve as cues that females of the species can use to determine which male is the most attractive.
However, in a few species both males and females have secondary sexual traits. For example, both male and female king penguins Aptenodytes patagonicus have two patches of yellow feathers on their heads, colourful feathers on their chests as well as orange and UV spots on their beaks.
Three reasons have been suggested for why in some animals, like king penguins, males and females share the same sexual ornaments. First, it is possible that the ornaments are used by both sexes as a signal of the quality of the individual. This is more often the case in species where both sexes invest a lot in childcare: it’s as important for males to be choosy about who they mate with as it is for females. However, it is also possible that males and females share a trait because although on the whole males and females are pretty different, they still share a lot of genes. This means that sometimes one sex will get a trait they don’t need because the other sex needs it, just because of these shared genes. Male nipples in humans might be an example of this – they don’t serve a function and may just be a constraint of evolution (in short, for females to have nipples males have to have them too). Finally, it is possible that the ornaments have nothing to do with selecting mates for attractiveness at all. Instead, they could be a signal of something completely different, like dominance, which could be used by individuals to determine who gets the best spots for feeding or nesting.
To see whether king penguins used each other’s colourful ornaments when deciding whether to pair with each other, Keddar and colleagues from Centre d’Ecologie Fonctionnelle et Evolutive in France observed penguins in the Antarctic.
King penguins come ashore to breed in the spring, where they live temporarily in giant colonies on sandy or cobbled beaches. The penguins who are still ‘dating around’ hang out on the outskirts of the colony, displaying to each other. If they decide to mate (and thus become ‘definitive’ partners) then they move into the centre of the colony and produce an egg. However, other penguins will form ‘temporary’ partnerships where they separate before producing an egg and may switch to a new partner.
The researchers found that the penguins did mate assortatively, with paired males and females having beak spots that were more similar in colour to each other than you would predict by chance (generally more colourful). However, this only applied to the colour of their beak spots and not the other colours on their feathers. Furthermore, the females in definitive pairs (the more serious relationships) had more UV and coloured spots on their beaks than females in temporary pairs.
So, it seems that these animals may use at least one feature of each other’s appearance (their beak spots) to choose who to mate with and thus mate with individuals of equivalent attractiveness. However, it is also possible that the paired males and females both had brighter beaks for another reason. For example, it could be that females are the ones still doing the majority of the choosing and it’s just the ones with the more colourful cheek spots that are better at grabbing the best males before the other females get to them. This could feasibly be the case, given that there are more males around than females, meaning the females can afford to be more choosy than the male penguins.
Keddar, I., Altmeyer, S., Couchoux, C., Jouventin, P., & Dobson, F. S. (2015). Mate Choice and Colored Beak Spots of King Penguins. Ethology 121, 1048–1058. doi: 10.1111/eth.12419