Scientists at the Konrad Lorenz Institute of Ethology at the Vetmeduni Vienna compared the preen gland chemicals of black-legged kittiwakes with genes that play a role in immunity.

Kittiwakes that smell similarly to each other also have similar genes for immunity. Since the birds prefer to mate with unrelated mates, the scientists have now found the likely mechanism by which they recognise relatedness.

It has long been understood that reproducing with close relatives may have profoundly negative effects on offspring.

Biologists have discovered in some species that breeding individuals have evolved ways to detect their genetic similarity with those of prospective partners.
Female mice choose unrelated over related males as mates. Females achieve this by comparing the smell of the urine of each male and comparing it with their own odours. The urine odours reflect the genetic composition of each mouse.

More specifically, the odours are correlated with a special group of genes called the "major histocompatiabilty complex," or MHC, which helps individuals resist diseases.
Thus, by pairing with MHC-dissimilar mates, breeders produce offspring with a more diverse collection of disease-resistant genes.
Richard H Wagner and behavioural geneticist Wouter van Dongen of the Konrad Lorenz Institute of Ethology, a part of the Veterinary Medicine University Vienna, have been collabortating with French colleagues on a long-term study of a cliff-nesting gull, the black-legged kittiwake, breeding in Anchorage Bay, Alaska.
When birds groom themselves with their bills, they spread chemical compounds from their preen glands throughout their plumage.
These chemicals produce odours that appear to be unique to each individual, providing an olfactory fingerprint. The team suspected that, just as in mammals, these odours may be used by kittiwakes to assess their relatedness to other individuals.
To test this idea, the researchers collected both DNA samples and preen gland odour samples from nesting kittiwakes.

The project then entailed two kinds of laboratory work: while Sarah Leclaire at the University of Toulouse conducted the analyses of the preen gland chemicals to characterise the odour signatures of each individual, van Dongen analysed the MHC of the kittiwakes in the Vienna lab.
The results showed that individual kittiwakes that smell  similarly to each other (ie have similar preen gland chemicals) also have similar MHC genes.
Closer relatives therefore have more similar odours than distantly related individuals. This suggests that birds may be able to compare their own odour with those of potential mates, and to choose unrelated individuals as breeding partners. The study is published in Nature's Scientific Reports.

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