Since they gave up walking routinely on four legs, bats' hind limbs have become very spindly. Which made Daniel Riskin wonder how the mammals'fragile-looking rear legs hold up as they get around. Having previously investigated the way that some bats walk on all fours, Riskin was curious to find out more about something that almost all bats do; land upside down to roost. `This is something you don't normally see,' says Riskin. `They fly past in a cave and suddenly they're on the ceiling. It's that fast,' he adds. Curious to find out how landing bats manoeuvre themselves into a hanging position, Riskin and his colleague John Ratcliffe travelled to Le Biodôme de Montréal, home of several bat species, to find out how Artibeus jamaicensis bats flip up to roost(p. 945).
Setting up a force plate in the corner of a ceiling to measure each landing bat's impact, Riskin and Ratcliffe filmed the animals as they tried to perch and saw that the bats somehow flipped their hind legs up, grabbing hold of the screen covering the force plate with their hind legs. But the force plate was too small and the film too grainy for Riskin to get clear measurements of the bats' manoeuvres. So when Riskin joined Sharon Swartz's lab, he built a larger force plate to use with three synchronized digital cameras to record bat landings at high speed.
Constructing a plastic lined enclosure with the force plate mounted at the highest point, Riskin encouraged tree dwelling dog faced fruit bats, Cynopterus brachyotis, to alight on the force plate as he filmed their landings. But something was wrong; instead of grasping hold with their rear legs, Cynopterus landed on all four. Replaying the slowed down movie, Riskin could see that the animals flew up vertically towards the force plate, before tipping backwards (until they were horizontal in the air on their backs), ready to grasp the netting covering the force plate with all four feet. Riskin admits that his first reaction was surprise. `I thought they were doing it wrong,' he says, `but then I realised: oh, it's a different bat'. And when he looked at the force plate recordings, he could see that the bats were really slamming into the force plate, impacting with forces ranging from 1.1 up to 10.5 times their own body weights.
Travelling down to Maryland to work with Cynthia Moss's leaf nosed bats,Riskin was able to test out two smaller, cave dwelling, species' landing techniques. Both Carollia perspicillata and Glossophaga soricina grasped hold of the force plate's netting with their hind feet. Like Cynopterus, they approached the roost site vertically, but then their technique deviated from that of the larger bats. Both Carolliaand Glossophaga twisted their bodies so that they athletically lifted their feet up along one side of their body until the feet were above their heads and they could grab hold of the netting. Riskin also noticed that the animals' landings were significantly softer than those of Cynopterus,with the smaller bats registering forces up to twice their own body masses.
Riskin suspects that the differences between the three species' landing forces probably reflect their different life styles. `If you smash into a leaf it's OK, but if you smash into a cave, that will hurt,' explains Riskin.