Saturday, 4 June 2011

Red-billed Buffalo-weavers

Red-billed Buffal0-weaver, Naabi Gate, Serengeti NP, Jan 2011
Over on my other blog I've discovered that searches for Red-billed Buffalo-weavers seem to be one of the most common routes to my blog. Very odd, as they're hardly the most inspiring of birds to look at. But despite being so common most people overlook them, they are actually rather interesting, honest! The most interesting thing about them ecologically, is that they are one of very, very few birds to posses a phalliod organ. In fact, the only other one is the White-billed Buffalo-weaver (NB some other birds - notably some ducks and ostrich - do posess pseudo-phalli, but they're not quite the same as the phalliod organ of a red-billed buffalo-weaver.). Interstingly, both males and females have them - but males are much longer (can be over 1.5cm), and those male with territories are longer still. Size obviously matter - infact, females prefer to be inseminated by males with larger organs, so it matters a lot! Unusually for birds, copulations in this species can last several minutes, and the species is very much a polyandrous breeder with both males fathering chicks in the same nest, feeding the young and defending the nest together. In fact, copulations seem to happen a lot both with females but no sperm transfer, and with other males - only a minority of copulations successfully trasfer sperm it seems, and successful copulation in the field seem to be 10 - 20 minutes long. Not bad for a bird!

The nests of this species are called compound nests, as several 'pairs' share the same walls, with typically several nests within the same overall nest structure. I say 'pairs' in quotes, because these are seriously polyandrous birds. Each compund nest is defended by a group of resident males, and each nest within the compond nest is likely to have two or more males assoiated with it (but just the one female). What's more, if you look at the genetics involved in parternity within the nests, you'll find that more often than not the actual father of some of the yound in the nest isn't even one of the resident males, but one of the non-territorial individuals hanging around the edge of the colony, so it seems that a lot of the time the female sneaks off and finds non-territorial males to father young too.
Red-billed Buffalo-weaver nests, near Arusha, Nov 2009

It follows that there's a lot of competition among males to be fathers, even though genetics again tells us that the cooperating territorial males sharing a female are at least sometimes related (though not usually close elatives). What the phalliod organ actually does isn't entirely clear - it's not the route for sperm to flow through - but apparently it needs stimulation before females can be inseminated by the male, and they're the only birds to experience anything that looks rather like an orgasm (but only after repeated copulation), and only at the point of orgasm (who's main effect seems to be pulling the female closer to the male) is sperm transferred to the female, so we think it must be related. What's more, some rather interested scientsts have demonstrated that if you manually stimulate the organ on a bird, you can induce orgasm and sperm transfer (the description in the paper isn't detailed enough to say exactly how the manual stimulation too place, but you can leave that to your imagination). 

Interesting things huh (you can read more about it here? In fact, it goes on - they've got some remarkably evolved sperm too, to deal with the considerable post-copulatory competition between sperm from different males within the female, all trying to fertilise the same egg.Can't find a picture just now, but trust me - theyre turbo-powered!

No comments:

Post a Comment