The kiwi bird’s status as an honorary mammal is confirmed

The kiwi belongs to a diverse group of flightless birds called ratites, which includes emus, ostriches and rheas. There are five recognized species, all of which are endemic to New Zealand. The kiwi – which is endangered – is about the size of a chicken, and is the smallest of the ratites. It belongs to the genus Apteryx, which means “wingless”, but it does have wings – albeit vestigial ones of about 5 cm in length.

Because of its geographical isolation, New Zealand has unique flora and fauna. The kiwi is quite unique among birds – it is so different from other birds that the late, great palaeontologist Stephen Jay Gould called it an “honorary mammal”. New Zealand was devoid of terrestrial mammals until European settlers introduced them in the 19th century. The kiwi’s ancestor therefore evolved, over a period of 80 million years, in the absence of terrestrial mammals, and the kiwi is adapted to the ecological niche that would normally be inhabited by small land mammals such as anteaters and hedgehogs. Consequently, it has features that are more characteristic of small mammals than of birds: well-developed senses of smell and hearing; a face covered with whiskers; bones containing marrow instead of air sacs; shaggy, hair-like plumage; and a body temperature of 37-38^oC. The kiwi also digs burrows in which it roosts and nests.

Most birds rely on vision for navigation during flight. Birds that are strictly nocturnal have very sensitive eyes which enable them to see clearly in the dark. Owls, for example, have large large eyes which maximise the amount of light captured at low light levels. But vertebrate eyes are heavy, fluid-filled structures. The ability to fly therefore places constraints on the size of a bird’s eyes, and large eyes are compensated for by a reduction in body mass and the distribution of body mass towards the core of the body. Because the kiwi is flightless, the usual constraints on eye size do not apply to it. Furthermore, it is largely nocturnal, and the emu, a close relative, has very large eyes. One would, therefore, also expect the kiwi to have large, sensitive eyes. But in fact, this is not the case: the kiwi has small, poorly-developed eyes, and previous studies have shown that it is far-sighted, so that light entering the eye is focused at a point behind, rather than on, the retina.

Now, researchers from the University of Birmingham in the U. K. and the University of Auckland in New Zealand have examined the structure of the kiwi’s eyes and brain, and analyzed the topography of its visual field. They show that the kiwi has only a minimal reliance on vision, and instead has highly developed senses of smell and touch. In the course of its evolution, the kiwi evolved these sensory adaptations to exploit the forest floor habitat, which would normally be the domain occupied by small mammals. Their findings are published today in the open access journal PLoS One.

Graham Martin, of the University of Birmingham’s Centre of Ornithology, and his colleagues obtained kiwi skins from London’s Natural history Museum and skeletal materials from Canterbury Museum in Christcurch, New Zealand. They were also given permits by the New Zealand Department of Conservation to collect post mortem kiwi and emu specimens from Keri Keri, the largest town in the Bay of Islands on New Zealand’s North Island. They also worked on live kiwi birds, under strict animal ethics guidelines.

The kiwi’ birds cornea – the transparent front of the eye -was found to have an average diameter of just 4 mm in diameter. The kiwi’s eyes are therefore similar in size to those of birds with a smaller body mass, but markedly smaller than those of birds of an equal body mass. Ophthalmological tests showed that the maximum image brightness in the kiwi’s eyes is similar to that of other nocturnal birds. However, because of their small size, the kiwi’s eyes have a reduced ability to gather light, and it is probable that they can only glean large details at night. Furthermore, the kiwi was found to have the smallest visual field yet reported in any bird. The frontal binocular field – where the visual fields of both eyes overlap – is virtually non-existent: the kiwi’s vision is so poor that it cannot even see the tip of its own bill.

Most birds have little or no sense of smell. The kiwi, however, compensates for its poor vision with highly developed senses of smell and touch. The kiwi’s bill appears to be unique, as Martin and his colleagues now have evidence that it is used to gather both olfactory and tactile information. In all other birds, the openings of the nostrils are located either externally near the base of the bill, or internally on the roof of the mouth. In contrast, the kiwi’s nostril openings are found at the end of its long and slender bill. The tip of the bill has a high concentration of sensory pits, each of which contains a cluster of mechanoreceptors (pressure-sensitive receptors). The pits cover the entire tip of the bill, and almost completely encircle the 3 mm-wide openings of the nostrils.

Side view of a bone from the bill of the North Island brown kiwi Apteryx mantelli, showing the densely-packed sensory pits. The arrowhead indicates the approximate position of the nostril opening; scale bar 5 mm.

The structure of the kiwi’s brain reflects its reliance on smell and touch. Upon examination, the optic tectum – that part of the non-mammalian vertebrate brain which processes visual information – was found to be very small, and much thinner, than in birds of with similar body mass. The diameter of the kiwi’s optic nerve was also found to be much smaller than that of the other birds examined: 0.77 mm, compared to 4.59 mm in the emu, 1.6 mm in the barn owl, and 1.6 mm in the pigeon. In contrast, the olfactory regions of the brain were found to be large and complex, and the region of the brain which receives inputs from the beak – the principle sensory trigeminal nucleus – is well-defined and pronounced, in comparison to the other species.

The visual system was almost useless in the habitat in which the kiwi evolved. Thus, during the course of evolution, natural selection favoured regression of the kiwi’s eyes. As it adapted to the niche normally occupied by small mammals, the kiwi gradually became less and less reliant on its sense of vision, until its eyes were made redundant. By convergent evolution, two unrelated groups of animals independently evolved similar sensory capabilities to perform the same task – foraging for food on the dark forest floor.

References:

Martin, G. R. et al. (2007). Kiwi forego vision in the guidance of their nocturnal activities. PLoS ONE 2: e198. doi:10.1371/journal.pone.0000198.

Sales, J. (2005). The endangered kiwi: A review. Folia Zool. 54: 1-20.