Sunday 7 March 2021

How Rodents Can See Ultraviolet Light

Primates, including our own species, have unusually good colour vision. This is thought to be due to the need to identify when fruit is ripe and ready to eat. Most other mammals don't see the world in the same way that we do, having colour vision that's at best limited and often, so far as we can tell, absent altogether. 

But at least some animals do have an advantage that we don't: in addition to visual light, they can also see in ultraviolet.

In terms of physics, there's no real difference between visual light and ultraviolet; the latter simply has a higher wavelength. In a sense, as its name suggests, it's just a colour (or range of colours) that are more violet than violet. Our own inability to see it naturally gives us something of a bias here, but, in principle, there's no real reason why an animal shouldn't be able to see UV light if it would be useful to do so.

That some animals can, in fact, do so was first demonstrated all the way back in 1876 by John Lubbock (later Lord Avebury) a naturalist and politician who was an early supporter of the theory of evolution. He was studying ants, and it was almost a hundred years before anyone seems to have seriously considered whether the same might be true of vertebrates. Hummingbirds were among the first such animals confirmed to see in the ultraviolet, but mammals were added to the list not much later. We now know that at least some reptiles, amphibians, and fish have similar abilities.

In fact, one might even wonder why it is that we can't also see ultraviolet. It turns out that this is because the lens and cornea of our eyes are not transparent to this form of light, rendering us entirely blind to it. This is common in diurnal mammals with acute eyesight, especially primates, and may help protect our eyes from excess damaging radiation in bright sunlight. It seems that if you want your vision to be really good, especially in daylight, giving up the ability to see in UV is a sacrifice worth paying.In order to be able to see in UV, a the number of mammal species known to be able to do this isn't very high. Most of the ones where it has been confirmed are nocturnal rodents, including house mice, brown rats, and the like, although there are also a number of bat species with the same ability. The chances are that many other nocturnal mammals can do so too, but we just haven't done the studies to prove it yet.

One animal that can (probably) be added to the list now is Ord's kangaroo rat (Dipodomys ordii). This is a long-tailed, leaping rodent native to most of the western US away from the Pacific coast, and to north-eastern Mexico. It's a burrowing, nocturnal, animal and one that has already been thoroughly studied for its ability to survive in harsh, dry, environments. A new study extends to that examining its visual system.

The reason I say "probably" here is that the researchers did not perform experiments to see how, or if, the rats reacted to UV light. There's no direct evidence here, just a study on the eyes that shows that they really should be able to see ultraviolet and that there's no obvious reason why they wouldn't. Which, in fact, is how a fair amount of the studies on mammalian UV vision have been conducted, and is regarded as a solid technique. But, nonetheless, it's certainly worth pointing out.

In order to be able to see in UV, a vertebrate needs two things to be true. Firstly, the lens and cornea need to allow UV light to be able to pass into the eye or, as with humans, nothing is going to happen. In the case of Ord's kangaroo rat, both structures allowed 68% of near UV light to pass through, although this drops off as the light heads further into the UV spectrum. Furthermore, the UV light is focussed by the lens (as one would expect) so it should be possible for the rat to see images in the spectrum, not just a general glow.

Secondly, however, the retina needs to be able to actually detect light of the relevant wavelength. This is, perhaps, less of a difficulty than one might think. 

Light is detected via special proteins in the retinal cells called opsins. In the case of mammals, there are usually three types of opsin involved in vision. Rhodopsin is found in the rod cells of the retina and is responsible for black-and-white night vision. It's possible that some variants of rhodopsin can at least vaguely detect UV light, although, if so, they wouldn't be able to tell it apart from other colours of light. (Not that they'd necessarily need to, if seeing where you're going is the only goal).

The other two common mammalian opsins are known as LWS and SWS1 (mammals generally don't have SWS2, although it does exist). The first of these mostly detects green and yellow light, while the other detects blue. Together, these provide 2-colour vision, although they don't work so well in low light conditions. Primates, incidentally, are unusual in that they have two variants of LWS - one for green and one for red. This is why red-green colour blindness is the most common kind in humans, since it's basically normal for most other mammals that have any colour vision at all.

The one that's key here is SWS1. This is usually described as having a maximum sensitivity in the blue part of the visual light spectrum. Which is true enough, although there it doesn't take much in the way of genetic change to tweak the protein, so the actual peak varies quite a bit between different mammal species. What's more important, though, is that the proteins can still detect light of wavelengths outside the peak, having quite a broad absorption spectrum compared with (say) the detectors in a digital camera.

Which means that, while there is some variation between species, SWS1 can usually detect some near-UV light, rather than cutting off as soon as we head beyond what are, to humans, visual wavelengths. Indeed, the "blue" cells in human eyes are perfectly capable of seeing UV light should any somehow get past the lens and cornea.

With the aid of immunohistochemistry (also called 'immunocytochemistry' because we like to confuse you) the researchers were able to show that all three kinds of opsin are found in Ord's kangaroo rat. The rhodopsin-containing rod cells for night vision are evenly distributed across its eyes, as we'd expect for an animal that's basically nocturnal. The colour-sensitive cone cells are more concentrated, but there's a remarkably high number of them for a nocturnal animal, suggesting that they may have better colour vision than we'd expect. True, the blue cones are less common than the yellow/green ones, but there's enough to suggest that the rats should not only be able to detect, but to form at least a dark-tinted image from, UV light.

The main advantage to this for the rats would be that it would help them move about around dusk and dawn, when UV light is still present in the sky but "visible" light is in short supply. This would be particularly useful when trying to see flying predators outlined against a glowing sky that looks dark to we mere humans. There are other possibilities, such as detecting UV-reflective urine traces that might carry interesting scent marks, or seeing animals that may have coats that fluoresce in UV. But whether any of these things apply to kangaroo rats isn't something a mere 'evidence of capability' study can tell us.

Ord's kangaroo rat is one of over 60 species in its family, with the others spread widely across the Americas. Unsurprisingly, we don't have similar studies on any of the others, although one would reasonably expect many of them to be similar. The closest relative that has been studied in this manner is Botta's gopher (Thomomys bottae), which lives from northern California and north-west Mexico to western Texas. The results for that were very similar and include the added information that a nerve signal to the brain is generated by exposure of the relevant cells to UV light.

Gophers, of course, spend most of their lives underground, where UV light doesn't penetrate any better than visible light does. Perhaps this form of vision passed down to them from their surface-dwelling ancestors, never being lost because it's still handy for them when they do come above ground to mate. Gophers and kangaroo rats probably last shared a common ancestor around 33 million years ago. While it's possible that there's some parallel evolution going on here, which we'd need more detailed molecular studies of the relevant proteins to unravel, it could also be that the trait has been inherited in some form all the way from that early ancestor... in which case, it's likely quite widespread elsewhere, too.

[Photo by the US Fish and Wildlife Service, in the public domain.]

No comments:

Post a Comment