Saturday 4 July 2015

Bats prefer rashers

As Emma Teeling at UCD is fond of asserting, a fifth of all the species of mammal on this planet are bats.  They make a living a remarkable diversity of ways.  The nine species of native Irish bats [ialtóg or sciathain leatháir {leather-wing}] are all insectivores but, like different zebra and their grasses, they clear the sky of different species of flying insects. The common Irish name of Myotis daubentoni, for example, is ialtóg uisce - the water bat - for its characteristic coursing over waterways hoovering up midges.  Just as you can study the DNA in antelope poo to determine what grass species dik-diks and springboks eat, you can analyse the droppings of bats under their roosts and see what insect wing-cases come through. That's all messy low-tech sifting and doesn't require DNA technology.

But not all bats are insectivores: the megachiroptera eat a lot of figs and nectar for example and there are fish-eating bats which scoop a square meal direct from the water rather than piffling about after thousands of tiny nibbly-bits. And of course there is Desmodus rotundus, the vampire bat [there are two other species which also diet on blood] which is in the process of re-adapting its fore-limbs to walking as well as flying.  They land close to their prey and hirple across the intervening ground before sinking their teeth into a pig's udder.  A National Geographic video is a [highly editted] anecdote; where is the data on what are the preferred prey for vampire bats? A team led by Rogério Gribel from National Institute of Amazonian Research in Manaus, Brazil sifted through really fresh vampire-bat shit to work out the species which contributed the DNA remaining after a pass through the bats' digestive system. It had to be really fresh and refrigerated because DNA degrades rather quickly into an unanalysable sludge.

Turns out that 60% of the bloody stool was from from chicken Gallus gallus with pig Sus scrofa a distant second at 30%, the other species: dog Canis familiaris and cattle Bos taurus made up the remaining 10% of the DNA.  They looked for human blood but didn't find any in 150+ samples from 20 different Amazonian villages.  That's all very useful and interesting - and a bit myth-busting about vampire-bats getting an armful each night from some chap sleeping in a low-hanging hammock.  But I liked the step further when they estimated the abundance of the various potential prey in the area and calculated that bats had a distinct preference for pigs which are much rarer on the ground than their contribution to the samples would suggest. So the National Geographic video cited above wasn't too far from the truth.That tells rather a lot about the behaviour of the bats and suggests that "keep a pig" is a way of keeping your own blood in its own leg.

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