The Struggles of a Pollinating Bat

Note the dusting of pollen...

When we think of flowers being pollinated, for most people, the first things that are likely to come to mind are insects, especially bees. But other creatures, including mammals, can also be a part of the pollination process, and this is particularly true of bats. Nor are we talking about some occasional event that merely gives the flower some sort of backup - for some the presence of pollinating bats is crucial to their survival.

Even aside from the obvious importance that this gives bats to the wider ecosystem, this can also have direct economic importance to we humans. For example, sour pitayas are an important cash crop in parts of Mexico. Similar to the much sweeter dragonfruit (although not closely related), they grow on a particular type of cactus that is native to the country but is also commonly cultivated. As it turns out, this cactus relies on bats for pollination. While they are not essential, crop yields drop by over a third when the bats are prevented from reaching them, which would clearly be devastating for a Mexican farmer who may be living on the edge of profitability to start with.

The main bats that provide this service are long-nosed bats (Leptonycteris spp.). There are three species in this genus, one of which doesn't live in Mexico, and one of which is now rare and endangered. That leaves us with one key species: the lesser long-nosed bat (Leptonycteris yerbabuenae). First identified as a subspecies of the southern long-nosed bat in 1940, this was promoted to full species status in 2005. It lives across much of Mexico, and as far south as Honduras and El Salvador. In the summer, they can migrate as far north as Arizona and New Mexico although they don't appear to be year-round residents there.

The reason that these bats are important pollinators is that they feed, not on insects, but on nectar. This is far from unique among bats, but the importance of this particular species is that it focuses on flowering cactuses. The long, narrow snout from which they take their name holds a long tongue specially adapted for lapping up nectar from deep inside flowers and they regularly end up covered in pollen while feeding. Pitayas are by no means their favourite food, since they primarily feed on agaves and the giant tree-like seguaro cactuses of the region, but they are nonetheless a part of their diet and perhaps the way in which they have the greatest economic impact on humans.

Perhaps partly because sour pitaya isn't as significant in the developed world as some other cash crops, and partly because bats are often overlooked, there has been little study on how these animals live their lives and how that might relate to their diet. We can say, however, that their chosen lifestyle is not an easy one. The issue here is that nectar is not a great foodstuff taken on its own. It's very high in sugars, so the raw calories are there, but it's rubbish as a source of protein or fat. Flowers relying on pollination by bats produce much greater amounts of nectar than those relying on insects and a nectar-feeding bat can consume up to 150% of its own body weight a night and they still struggle.

Part of the problem is that flight takes a lot of energy. Indeed, it's been argued that nectar-feeding bats need to stay active in order to avoid hyperglycaemia and, of course, those energy demands are even higher during reproductive periods, especially for females. As with any small mammal, the greatest physiological demand comes when suckling their young and it doesn't help that nectar isn't rich in calcium, either.

Recently, however, a study was published that looked at these specific issues in the case of the lesser long-nosed bat. They looked at the bats living in three caves in Baja California, catching them with harp traps outside their caves and tagging them with RFID radio transmitters. (A harp trap, incidentally, looks rather like a harp with a bag at the base - the bats fly into the strings or swerve to avoid them, and fall into the bag. This avoids entangling the bat, and is more effective than the more popular mist nets if you can be confident that the bats will be flying through a specific narrow area).

The results showed that females evidently have a harder time of things than males. While the males tended to leave the roost first - about a quarter of an hour earlier each night, on average - they were far less active once they had done so, spending less of their time feeding or searching for food. However, this changed significantly depending on the availability of food. When nectar was scarce, the females beat the males out of the roost by a full 75 minutes, evidently wanting to make the most of the night. 

On the other hand, when the cardon cactus was in flower, the females were even more eager to leave the cave, doing so a full 90 minutes before the males. That's the most common cactus in the study area, but more importantly, it also happens to be when the bats give birth (presumably not by coincidence). For males, the most energetically costly time would be the mating season, six months earlier, but that seemed to make little difference to their feeding habits. While many male mammals put a staggering amount of energy into competing for mates, it seems that if you're a bat, it's not that big of a deal, and not even close to the effort required to care for young.

In fact, female bats with young pups spent more time outside of the roost, and were more active, than those that did not. Pregnant bats were also more active than other females, something that probably has a lot to do with it being harder to fly in that condition, as well as diverting resources to the growing baby. 

Indeed, while the authors had assumed that mother bats would return to the roost sporadically throughout the night, in order to suckle their young, they actually did so less frequently than usual, sometimes spending a full 12 hours outside at a time. Earlier studies had shown that this particular bat species can have a daily commute of around 100 km each way, although about half that is more typical. So they probably wouldn't want to have to that multiple times a night.

They get around it by placing their young in creches while they are away, having them huddle together to keep warm. It was also notable that some female bats didn't bother leaving the cave at all during this period, despite it being when nectar was most available. The most likely explanation here is that they were staying behind to look after the creches, including those young that weren't their own - something that has been observed with other bat species. 

The bats in these specific caves weren't economically important - sour pitaya is commonly grown in central Mexico, but not out in the west where the study took place. They are, however, a key part of their local ecosystem, as well as being important to pitaya farmers elsewhere. Understanding some of the struggles they go through to survive in a harsh desert landscape with a limited food supply may give us a better appreciation of a type of mammal that can often be overlooked.

[Photo by Juan Corzado Cortés, from Wikimedia Commons.]