How to Drink Nectar

Orange nectar bat

When most people think of what bats eat, the first thing that likely comes to mind is insects. It probably doesn't make much further thought to remember that fruit bats also exist. And these are the two most common sources of food for bats, although 'fruit' in particular can cover quite a wide range of specific food types. But bats are the second largest order of mammals, after the rodents, and there is considerable variety amongst them.

This is particularly true of the leaf-nosed bats, or phyllostomids. While most formally recognised families of mammals have names almost everyone is familiar with - cats, bears, dolphins, horses, gibbons, etc. - and most of those that don't at least sound like they're actual names - binturongs, tuco-tucos, tenrecs, colugos - bat families tend to lack anything we could reasonably describe as a common name. Instead, we have bulldog-bats, and sucker-footed bats, and disc-winged bats, and so on. 

So it is with the leaf-nosed bats, which are the second-largest family of bats in terms of number of species, beaten only by the vesper bats. The family is usually divided into no fewer than eleven subfamilies, all of which have equally obscure-sounding names, and, in some cases, not even that much. It may not be obvious that, say, the spear-nosed bats are a subgroup of the leaf-nosed bats, but they are. And it's even less obvious that stenodermatines are phyllostomid, but kerivoulines are not.

The leaf-nosed bats are an ancient group, likely originating, as most bat families did, around 30 to 40 million years ago. It's thought that the first leaf-nosed bats ate insects. Their most distinctive feature, the leaf-like protrusion on the tips of their noses, focuses their sonar beams, and that's likely especially handy for something that hunts flying insects at night. But, today, not all phyllostomids eat insects, with some having evolved to pursue larger prey, or to eat fruit, or to suck the blood of other animals.

Or, in some cases, to feed primarily on nectar.

Indeed, nectar-feeding isn't some one-off evolutionary development in a particular phyllostomid lineage. Not only are there a fairly large number of nectar-feeding leaf-nosed bat species, but the practice has evolved twice within the family. We can tell this because it's found in two of the eleven subfamilies, and these don't share a unique common ancestor.

The nectar-drinking subfamily with the largest number of known species are the glossophagines, a group that includes the tailless bats, flower bats, and a host of "long-tongued" bats. With a scientific name that loosely translates as "tongue-feeders", these originated as one of many side-branches to the main group of leaf-nosed bats. 

The second group are the lonchophyllines, which also includes several species referred to as "long-tongued" bats because there are no rules against being confusing when it common names. This arose within a separate, later evolving lineage that includes three or four other families, most of which eat fruit (although they're not the large "fruit bats" that typically come to mind when thinking of such things). 

That nectar-feeding evolved twice, albeit in species that were probably already somewhat herbivorous, is interesting, because it's not that easy a thing to do. Nectar is a good source of sugar for energy, but it's lousy as a source of protein, so nectar-feeding bats will need to supplement their diet with something else to bulk up their bodies; this is often insects, but it can be pollen (which is how bees manage it), depending on the species.

More importantly, however, you have to get at it. This requires hovering in place while you feed, which requires a lot of energy and exposes you to predators until you've finished. So the quicker you can gulp that nectar down, the better. Because they face similar challenges in doing so, the two groups of nectar-feeding bats have independently developed three key features to feed on nectar.

First, they're very good at hovering, which bats generally aren't, since there wouldn't be a lot of point. Secondly, they have long, thin snouts, which they can poke into flowers. And thirdly, they have a very long, narrow tongue which they can stick out to get right to the bottom of the flower, where the nectar is, and drink it up. 

For example, Seba's short-tailed bat, which does occasionally feed on nectar, but would much rather eat pepper, has a tongue 11 to 24 mm long (½ to 1 inch), which is quite reasonable for its size. But the Mexican long-tongued bat, a glossophagine nectar-feeder, can stretch its tongue out to 77mm (3 inches)... which is impressive when you realise the bat is only 10cm (4 inches) long...

If all this reminds you of hummingbirds, then you're not wrong.

Subfamilies of leaf-nosed bats

But here's the thing: the two subfamilies of nectar-feeding bats don't actually feed in the same way. These differences are reflected in the finer details of their physical form, lending weight to the genetic evidence that leaf-nosed bats evolved this practice twice, hitting on a subtly different method each time.

The end of the tongue of glossophagine bats is covered with thread-like 'papillae', making it look a bit like a brush. When the bat feeds, it uses its tongue muscles to squeeze the tongue out, making it longer, and simultaneously pushing blood into the papillae, making them stiff and erect. This traps nectar between the structures, ready to be pulled back into the mouth. Which basically makes their tongue a mop.

The lonchophylline bats, on the other hand, have no such structure. In their case, they have grooves down the side of their tongues that suck the nectar up, presumably via capillary action aided by muscular pumping. (If you're wondering, hummingbirds are different again, although what they're doing is closer to the glossophagine method). 

The two subfamilies are restricted to the tropical parts of the Americas and the Caribbean, but species of each manage to live alongside one another without difficulty. Which means that there has to be some difference in what they're eating, or one would outcompete the other. A logical assumption is that the two different styles of nectar-feeding have advantages and disadvantages depending on the exact shape and size of the flower being used. That way, even in the same area, different species can feed on different flowers, and nobody runs out of food.

To see whether or not this is true, researchers recently published the results of a test in which they took nectar-drinking bats and offered them a choice of different flowers to feed from, to see which they preferred. They used bats from three species native to the eastern slopes of the Andes. Representing the lonchophyllines, they used orange nectar bats (Lonchophylla robusta) while, for the glossophagines, they used two closely related species: Handley's tailless bat (Anoura cultrata) and the brilliantly-named tailed tailless bat (Anoura caudifer).

And, yes, I refer you back to common names not having to make sense. The tailed tailless bat's tail is very short, at a maximum length of 7mm (¼ inch) but it's definitely there. For that matter, the tailed tailless bat isn't even the only tailless bat to have a tail. Handley's tailless bat has one, too, although it's even shorter... and so do most (but not all) other tailless bats. So now you know.

It turned out, however, that the bats didn't really have any preference for the sorts of flowers they picked. Nor did how they were drinking make any difference to how quickly they were able to obtain nectar from the different flower shapes they were presented with. The one thing that did make a difference was how long their tongues happened to be, with the tailed tailless bats, which have the shortest tongues, being consistently at a disadvantage relative to the other two species. Longer tongues, it seems, are more efficient, allowing the bat to gulp down more nectar in the same period of time, regardless of how they are getting the nectar into their mouths.

Evolution, it seems, has come with two equally efficient and effective methods of drinking nectar from deep flowers, with nothing much to choose between them. That multiple species can live alongside each other must be due to other factors; tongue length, perhaps, but maybe also the size of the bats, whether they are optimised for long-distance or for aerobatics, and so on. The researchers also speculated that, since they had had to use fake nectar so that they could make precise measurements of how much was drunk, the real stuff might be different between flowers; thicker, more viscous nectar might favour the mop-drinking bats, for instance.

But, so far as we can tell, bats can drink their nectar by mopping or pumping, and it makes no difference. Nature has provided two equally good ways of doing the exact same thing.

[Photo by © Hans Hillewaert / CC BY-SA 4.0, from Wikimedia Commons, cladogram adapted from Botero-Castro et al. 2013 and Davalos et al. 2012.]