Mate choice or sexual selection in nature is a highly complicated matter as the parental contribution and investment of both sexes is totally different. This is because males are able to produce huge quantities of germ cells daily, while females deliver a single (or very few egg cells) only once during their several-day ovulatory cycle. Therefore females invest substantially in risks, time and energy when it comes to reproduction. Their minimum parental investment is internal fertilization with a species dependent gestation or incubation period and – in most species – followed by a period of constant care and nurture.
Females therefore have developed mechanisms that lead to greater selectivity when it comes to choosing a mate. Often, male animals hardly invest in more than the procreative act.
A female, making a poor mate choice, may produce weak offspring – or even no offspring at all – or risks becoming pregnant without committed assistance.
In the case of animals that live in groups or pairs often fatal for the progeny.
In terms of partner selection: males go for quantity, females go for quality!
In the wild it has been observed that peahens choose – we have already stressed that females have to be choosier – the male with the brightest colours, with the longest tail feathers adorned with blue and green ‘eyes’ and the most extravagant mating and display behaviour.
At least this highly dangerous combination of extravagancies is a signal to the females that ‘although I am highly handicapped by my colourful appearance and provocative behaviour, being a “sitting duck” for a large variety of predators, I am still here! So I must be very strong, agile and healthy, and I also have a rich territory for you and your future chicks’. When peahens concentrate on the most brilliant plumage they have a reliable indicator of good health, and a well adapted immune system in particular. It has been observed that peacocks with a duller plumage carry a higher parasite load.
The peacock’s tail, the bright red colours of many galliform birds, the loud song of the goldfinch, the long tail feathers of male widow birds and the stag’s antlers are more than just ‘physical disabilities’. As these attributes often attract predators, they are regarded as ‘handicaps’ that enable that particular male to signal qualities related to ‘survival skills’ and provision of protection, food, shelter, health and high quality genes for its future mate and her or their offspring.
Both, humans and animals exhibit all kinds of species specific handicaps, i.e. characteristics that impose costs or potential costs on solving one or more adaptive problems. Perhaps more importantly, these handicaps serve as useful signalling functions in solving specific adaptive problems such as communicating one’s value as a mate or one’s ability to run fast in a chase.
This phenomenon of using special species dependent signals is called the Handicap Principle. Normally these signals evolve through natural selection and increase the organism’s fitness or reproductive efficiency in a straightforward way. However, these special signals are selected because they ‘handicap’ the individual in a way that ensures a more reliable signal. Otherwise the receiver may not pay attention to the signals and the related biological consequences.
For females, natural mate choice means also getting access to the right Major Histocompatibility Complex (MHC) genes, reassuring a valuable immunological capability. The highly polymorphic MHC genes control immunologic self / nonself recognition; therefore this mating preference may function to provide ‘good genes’ for an individual’s offspring.
In all probability, MHC-disassortive mating preferences, as has been observed in mice, rats, sheep and humans, will function in all vertebrate animals to produce MHC heterozygous offspring to enhance the immunocompetence of an individual’s progeny. MHC based mating preferences may also circumvent inbreeding as MHC genes seem to play a role in kin recognition.
From this perspective, farmers and other breeders of domesticated animals have not performed so badly by concentrating only on milk yield, egg production or feed conversion rate. But of course natural sexual selection is totally different from – and from a biological point of view far superior to – artificial selection because in the latter we decide if an animal will reproduce, with whom and how often. Fortunately, in our part of the world students choose their own mate.
But it has been demonstrated that the use of contraceptive hormones by women disturbs this so well designed and ancient mate selection system substantially.
The bottom line: A hen is just the egg’s way of making another superior egg.