Sunday 5 March 2017

By the Light of the Silvery Moon

Why bother being nocturnal? Being active only at night makes it much harder to see what you're doing, and while it's possible to develop good night vision to minimise the problem, this is evolutionary costly. Well, if you're a herbivore, the advantage of being active at night is that it's much easier to hide from predators, since, they, too, would have to evolve good night vision to find you. And, if you're a predator, the advantage is that at least some of your competitors won't be around at night, allowing you to snack on nocturnal herbivores and not have to share your food supply.

But this, of course, cuts both ways. For example, while darkness hides you from predators, it also makes it more difficult to spot predators coming if they have seen you. As so often, this leads to a balance, and different species taking advantage of different points on the continuum of possible behaviours.

We can see some of the effects of this in how animals respond to different levels of darkness. Not all nights are equal, after all. The most predictable change is in the amount of moonlight, with the night of a full moon being considerably brighter than a night without a visible moon. Somewhat less predictably, of course, there's the weather, unless, perhaps, you live in a desert where overcast skies are fairly unlikely. So, if you're a nocturnal herbivore, should you be more active on the night of a full moon, or less?

There are at least two possible answers to this, depending on what factors are most important, and we should not be surprised that different animal species might use either one of them. Firstly, there is the "visual acuity hypothesis", in which bright moonlight helps to offset the disadvantages of being nocturnal, so that animals should be more active on moonlit nights when they can at least see what they're doing, and whether or not anything nasty is coming for them. This has been observed, for example, in tarsiers.

Secondly, there is the "predator avoidance hypothesis", under which you should be more active on the darkest nights, such as when the moon is new, because then it's much harder for predators to find you. This seems to be the more common approach, and has been observed in a wide range of nocturnal herbivores. Sticking with primates, for instance, it has been noted in lorises.

There are many reasons why a particular species might favour one possibility over the other, or, indeed, pick an approach somewhere in between. These might include the quality of their own night vision - clearly it has to be pretty good if you're going to most active when it's darkest - and the nature and behaviours of the predators that are out to get them. With this in mind, a recent study looked at the various prey species of one specific predator, the ocelot (Leopardus pardalis).

Ocelots are medium-sized wild cats, about two to three times the size of a typical domestic cat, that live in forests and open woodland across almost the whole of central and northern South America. They are mostly nocturnal, or at least crepuscular (active mainly around twilight), and feed on a range of small mammals, along with a few birds, lizards, and fish. Like most cats, they have excellent night vision, although this isn't necessarily the only sense they use while hunting.

The reason that ocelots have such good night vision is that they have a highly developed tapetum lucidum. This is a structure at the back of the eye that reflects light within the eyeball, increasing the amount of it available to any given light receptor. It's the structure responsible for the very bright "eyeshine" produced by a camera flash when taking a photograph of animals such as dogs and cats. Humans, along with similarly diurnal primates, don't have one, which is why the worst you'll get in a photo of a human is "red-eye", not some spooky looking glow.

However, many herbivorous mammals do have a tapetum lucidum, and therefore night vision that we can reasonably assume is better than a human's. However, the mere presence of the structure doesn't necessarily mean that all the animals concerned have equally good night vision, since the tapetum may be of variable quality between species, and other factors, such as the size of the animal's eyes, may also play a role.

The study was conducted in northern Argentina, near the southern end of the ocelot's range, but where it is the main terrestrial woodland predator active at night. Of course, it's not quite the only one, and prey animals are presumably pretty eager to escape from, say, owls, as well as ocelots, so it isn't as if their activity is entirely dictated by the latter.

Using camera traps to count the number of animals of each species active at night during different moon phases, the researchers concluded that the majority were more active when the moon was full. This was particularly true for rabbits and deer. The local rabbits (Sylvilagus brasiliensis) are closely related to the cottontails of North America, and, while they are nocturnal, they do not possess a tapetum lucidum, and so (presumably) have relatively poor night vision. It's unsurprising that, while they forage when local predators such as tayra and jaguaroundi are asleep, they still prefer to do so when they can actually see what they're doing.

The deer are a slightly different matter, since they do have a tapetum. However, this has a very different structure to the one found in ocelots. In deer, as with other hoofed mammals, the tapetum consists of arrays of collagen fibres arranged around the cells that lie behind the retina. In ocelots, and other carnivores, the cells themselves contain tiny crystals that reflect light, and the external fibres just aren't there. It's unclear as to how much of a difference this really makes, but it has at least been argued that the latter arrangement is probably more efficient, so that the night vision of deer is not quite as good as one might suppose.

I'd also note that deer probably have less to fear from ocelots than animals the size of rabbits do. The deer in question are red brockets (Mazama americana), which are not particularly large as deer go, but still stand about 75 cm (2' 6") at the shoulder. This is considerably larger than an ocelot, and while it's certainly true that ocelots have been known to attack and kill them, especially when the deer are young, they aren't really a regular part of their diet. Since they'd be even less scared of the local owls, that only leaves the relatively rare local pumas as a significant risk, and there may just not be enough of them to be worth the extra effort of coming out only when it's darkest.

Opossums, on the other hand, are quite a common prey animal for ocelots, and at least two species lived in the study area. The larger are white-eared opossums (Didelphis albiventris), and these, too, showed a preference for brighter nights, but the smaller species (Philander opossum) did not. Both do have a tapetum, although the structure is different again than in the deer or the ocelots, with the reflective cells being located inside the retina itself. It may be that, regardless of the night vision capabilities of the respective species, the smaller ones find the risk of being eaten - likely by owls, as well as ocelots - sufficient not to chance the moonlit nights, and end up striking a different balance of risk v. foraging opportunity than their larger cousins.

They weren't alone in doing so. Another common prey species for the local ocelots was the nine-banded armadillo (Dasypus novemcinctus), the same species that is so well known in Texas and other parts of the south-eastern US. These really didn't care what the moon was doing, possibly because their eyesight is pretty rubbish anyway, so bright light didn't really help them all that much.

It's likely significant here that the ocelots themselves had only a slight preference for moonlit nights, meaning that there was almost as much chance of encountering one another on dark nights, leaving little incentive for prey animals to prefer such times to be about. In general, though, the prey animals didn't seem to be trying to avoid ocelots by sticking to nights when they thought they wouldn't be active, as many animals in other similar situations are known to do. They doubtless had other avoidance strategies, but, for most of them at least, foraging in bright moonlight, when both food and potential foes were more readily visible, was evidently well worth the risk.

[Photo by "Danleo", from Wikimedia Commons.]

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