Rather than hibernate during the cold Canadian winter, hoary bats prefer to undertake an impressive migration to their wintering grounds in Southern USA and Mexico before returning home to Canada during the spring. These bats are not the only winged animals to undertake such mammoth journeys, with many birds also migrating similar distances to escape harsh winters. However, as Liam McGuire from the University of Western Ontario, Canada, points out, ‘We know quite a lot about bird migration, but migratory bats haven't been studied to any extent at all.’ Do bats use the same strategies as birds to fuel their long-haul trips? McGuire decided to investigate, and travelled to New Mexico, USA, and Saskatchewan, Canada, to capture bats at the beginning and end of their spring migration (p. 800).

On his return to the lab, McGuire compared the mass of different organs and the fatty acid composition of adipose and muscle tissue in the two different groups of bats, helped by his co-advisors, Brock Fenton and Christopher Guglielmo. They found that migrating bats had higher levels of fatty acids that can be easily burnt to fuel flight and that they had larger exercise organs (for example the heart and lungs). However, to the team's surprise they had smaller digestive organs. McGuire explains that in avian migrants, reduction of digestive organs only occurs when they travel long distances without the chance to eat and refuel – they don't want to lug around excess baggage. However, hoary bats have plenty of pit stop opportunities on their route and McGuire says that he ‘expected that the bats would have increased the size of their digestive organs so that they could take advantage of opportunities to refuel quickly’.

Does this mean that bats are abstaining for the duration of their migration like their fasting feathered friends? McGuire doesn't think so. He found that most bats had in fact eaten something prior to their capture. Instead, he finds it helpful to think of the bats' fat energy store like a car's petrol tank, which they fill up at the beginning of their migration, but rather than letting it run completely dry they drain it a little every day and top it up frequently with small meals. Therefore they don't need increased digestive capacity. This fits with their nocturnal nature, points out McGuire, who suspects that, ‘They may fly at night and as the sun's coming up they go down to where they're going to spend the day, grab a quick bite to eat, before roosting somewhere’.

McGuire also found that there were differences between the sexes, with female bats reducing more of their lean body mass (mass that isn't fat) to increase the amount of relative body fat that they carry. Furthermore, when McGuire looked specifically at the types of fatty acids that make up the muscle membrane, he found variation in the ratios of omega-3 and omega-6 fatty acids between males and females. McGuire suspects that these differences mean that males use torpor – reduction of body temperature and metabolic rate – during their daily roosts to conserve energy. Females cannot profit from torpor to save energy, as during the spring migration they are pregnant and cannot drop their body temperature without harming their future offspring. They thus need to fill up their ‘tank’ even more than male bats before the spring migration.

Migration represents a real challenge for these bats. However, it seems that they plan differently to birds to cope with this demanding journey, using strategies that are tailored to individual bats' needs – namely, a dislike of daylight and the possibility of being pregnant.

L. P.
M. B.
C. G.
Phenotypic flexibility in migrating bats: seasonal variation in body composition, organ sizes, and fatty acid profiles
J. Exp. Biol.