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| Long-tailed tit, Aegithalos caudatus Source: ebird.org |
The long-tailed tit, Aegithalos caudatus is one of my all-time favourite birds. And now they illustrate a fascinating aspect of evolution - how something evolved for one purpose can be co-opted and used for another. In this case, the constant twittering they make to maintain flock cohesion and to identify members of their extended family groups, is used to avoid incestuous breeding.
You can come across these lovely little birds in woods, especially in autumn and winter, when suddenly you realise the trees are alive with little twittering black, buff-orange and white birds, constantly on the go as they move through the trees foraging for small insects in groups of maybe a couple of dozen, constantly twittering and calling. Then, just as suddenly as they arrived, they're gone.
They are extremely sociable birds to the extent that if one pair lose a brood or one doesn't find a mate, they will cooperate with a sibling to help raise their brood, in an example of evolved altruism. The brood will stay together for the winter, oten joining with other family groups, probably those of the parents' siblings, only breaking up to pair and breed in the spring.
The brood will consist of between 8 and 12 or more eggs laid in an exquisite domed nest often concealed in the middle of dense thorn and made entirely of grey lichen held together with cobwebs and lined with up to 1500 small feathers. As the chicks grow, the nest stretches and expands to contain them.
We recorded the calls of males and females in many pairs of long-tailed tits and found that the calls of breeding pairs were less similar than the calls of close relatives that they could have bred with. Call similarity within breeding pairs was, instead, similar to that observed among distant relatives or unrelated birds
This sociability and because both sexes disperse over the same local area, there is a high probability of close-kin paring or incest, which would tend to lead to inbred genetic weakness. However, as a team of researchers from the University of Sheffield discovered recently, they avoid this by recognising the calls of close kin - something facilitated by the way they stay together as a social group throughout the winter.Dr Amy Leedale, Lead author.
PhD student
University of Sheffield,
PhD student
University of Sheffield,
The results of this research was published yesterday in PNAS behind an annoying paywall - again! However, Sheffield University have kindly provided a press release describing the work.
Inbred animals typically suffer from reduced survival and reproductive success, so incest is usually avoided. But, in species where young stay close to where they were born, relatives are often encountered as potential mates, increasing the risk of harmful inbreeding.
Long-tailed tits often breed close to home, allowing kin to help raise each other's chicks, but also incurring a risk of incest that reduces the reproductive success of offspring. The research, led by Dr Amy Leedale from the University of Sheffield's Department of Animal and Plant Science, found that despite this risk, close relatives are actively avoided when pairs form each spring.
Long-tailed tits use distinctive calls to recognise close relatives so that they can help raise their offspring. The authors suggest that these calls also explain how the birds avoid inbreeding.
As I said in the introduction, this is an example of how something evolved for one purpose, in this case, probably to help the group keep together and to recognise the offspring of siblings, can be adapted for a different function - in this case to avoid mating between close kin with all the genetic harm that might entail in the long run. Obviously, throughout the evolutionary history of this species, any tendency towards incest would have been selected against whilst anything that helped avoid it would have been selected for. The result is a genetic barrier to incestuous matings.
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