Kiwi and night vision January 29, 2008Posted by David Corney in Uncategorized.
Tags: nocturnal vision
A (fairly) new paper by Graham Martin et al. in the wonderful PLoS ONE discusses kiwi and their eyes. Coming from (nearly) the opposite side of the world, I am (was) embarrassingly ignorant about kiwi. I knew they were large and flightless birds from New Zealand, but that was about it. (I even thought the plural was “kiwis” .) I now know that they’re nocturnal, like quite a few birds, but they’ve evolved surprising eyes.
To see in the dark, many nocturnal animals have evolved relatively large eyes, such as owls, lemurs and some monkeys, to gather what little light there is. Against this however, eyes are heavy, being balls of mostly-water, and weight is always a concern if you’re flying. So at first glance, you might expect (as the authors mention) that a bird that stops flying might evolve to have larger and larger eyes, as weight becomes less of an issue. Especially if it’s nocturnal. However, kiwi have small eyes for their bodies, and what’s more, they have small optic nerves and small visual cortices. They’re not blind like cave fish, although given a few million years more, who knows?
Moving forward a few inches, all birds have nostrils, usually at the base of the bill or even inside the mouth. Kiwi, uniquely, have their nostrils at the tip of their bills, coupled with fine touch sensors all over the bill tips. They feed by pecking at surface-living insects or by probing the soil with their long bill and sensing underground insects, suggesting a convergent evolution to the same ecological niche filled by mammals in many parts of the world. And if you’re finding grubs underground, you don’t need vision, of course.
According to the “wiki-kiwi” page, in areas where people are absent, kiwi are active during the day. Which makes me wonder if they have ended up with reduced visual processing simply because they can’t see what they’re eating anyway, whether it’s day or night, so why waste the effort? It seems to me that there are two evolutionary stories that fit this data:
- In version one, kiwi evolved to find food in the topsoil with their beaks and so didn’t need good vision; in turn, they spent less energy growing and using eyes; and then finally they tended towards nocturnal behaviour because there was no extra cost to them.
- In version two, kiwi became nocturnal to avoid predators (not that there were any mammals to compete with until recently) or to find nocturnal insects perhaps; and then they developed poorer eyesight because good eyesight was no longer required.
It could of course be some mixture of the two, as evolutionary histories needn’t have a nice clear narrative. In either case, I guess they still need at least rudimentary vision for mate selection, not walking into trees, that kind of thing. Anyway, I’ve learned a lot about kiwi, for which I am grateful!
Martin, G.R., Wilson, K., Wild, J.M., Parsons, S., Kubke, M.F., Corfield, J., Iwaniuk, A. (2007). Kiwi Forego Vision in the Guidance of Their Nocturnal Activities. PLoS ONE, 2(2), e198. DOI: 10.1371/journal.pone.0000198
See also: “The allometry and scaling of the size of vertebrate eyes” doi:10.1016/j.visres.2004.03.023
“Some nocturnal animals rely on senses other than vision, which is reflected in their small eye size. Others take the strategy of increasing eye size as much as possible to compensate for the low light conditions.”
 I checked the OED to see what it said about the word “kiwi”. It gives the etymology as “Maori” which isn’t terribly informative, but does have a quote from a Walter Lawry Buller and his 1873 text, A history of the birds of New Zealand: “Last Sunday I dined on stewed Kiwi, at the hut of a lonely gold-digger.” So I’ve learned something already…