Animals do the most amazing things. Read about them here in this series by Janaki Lenin.
The kiwi bird. Credit: New Zealand Birds Online.
The dumpy bird may look odd, but it is built to make the most of its nocturnal lifestyle. It scrounges along the forest floor, while tapping its long beak, much like a visually impaired person taps a cane. It probes the ground up to 12 cm. in depth, like an ibis would in water, and sniffs loudly. Unlike any other bird, its nostrils are at the tip of its bill. But the beak-end does much more than smell for fallen fruits. It also has numerous sensory pits called mechanoreceptors that feel earthworms and grubs wriggling underground. This sensitive bill-tip is the kiwi’s primary sense organ.
Unlike other night-time birds such as owls, it has tiny eyes. Owls fly and they have to see where they are going. The kiwi can’t fly since it has a pair of nubby, useless wings. It compromised sharp vision for acute hearing, touch, and smell. These unique adaptations became possible because of alterations to its genes. Little was known of these genetic changes until now.
A group of 15 molecular biologists from six institutions in four countries collaborated on this four-year project to sequence the kiwi’s genome, the largest bird genome to date.
They found the green and blue colour receptors in the bird’s eye were non-functional. So the kiwi has no colour vision. But it has more smell receptors than any other bird, enabling it to detect a wide range of odours. The biologists are silent on the genetic origin of the mechanoreceptors on the bill tip.
Diana LeDuc, the lead author of the study, told The Wire, “It would be impossible to look at the mechanoreceptors in the bill with the data we have. For that we would need to sequence the transcriptome [the set of all RNA molecules] of the bill mucosa, for example, to see exactly what genes are being expressed there.”
Other adaptations to life in the darkness include low energy metabolism. The researchers suggest that since large eyes require high metabolism, the kiwi’s low metabolism may be one reason for its small eyes.
Professor Emeritus Graham Martin, University of Birmingham, U.K., who specialises in sensory perception of birds, told the Wire, “I do not think it will hold up, since many birds with much higher metabolic rates than kiwi have small eye sizes. It seems much more likely that small eye size is a case of regressive evolution, as we have argued previously based upon evidence from brain and eye structure. Small eyes can never be as sensitive as large ones. It is always necessary to distinguish between sensitivity and resolution at night.”
The kiwi wasn’t always a denizen of the night. Millions of years ago, when its ancestor arrived in New Zealand, it hunted by day. But by then, the now-extinct, 3-metre-tall moa had already established itself, and hogged food during the day. The researchers say the chicken-sized kiwi was forced to become a creature of the night to survive. They peg these adaptations to have occurred around 35 million years ago, after the kiwi ancestor’s arrival in New Zealand and the kiwi adopting a night life.
Professor Martin says he thinks this is the most important insight of the paper. “However, whether this was a response to predatory pressure from the moa, or simply because the nocturnal habitat of the forest floor was an available foraging niche with many opportunities for food exploitation is not clear. It could just be that no other animals were exploiting this rich food source.”
Despite the uniqueness of the kiwi’s beak, it has one drawback. While the bird pokes its beak into the ground, the nostrils get plugged up, and the kiwi snorts loudly. Since it evolved on the predator-free islands of New Zealand, there was no need to be silent. However, these earthbound birds, with two nubby, useless wings, now have to contend with legions of dogs, cats, stoats, weasels, and ferrets brought by colonising Europeans. Although there are about 25,000 brown kiwis in North Island, the numbers have been falling for decades.
Since only 5% of the chicks make it to adulthood, New Zealand’s Department of Conservation runs an artificial incubation program called ‘Operation Nest Egg.’ It takes the 450-gram-eggs from wild nests and incubates them artificially. Chicks are reared until they reach 850 grams in weight before being released in the wild. At that size, at least the stoats won’t get them. DNA required for the study was extracted from the embryos of two such eggs of the brown kiwi.
Diana LeDuc said in a press release that her team found the diversity of the kiwi genome was as low as inbred birds, and further insights from the genome could aid conservation management strategies.
“Low genetic diversity and inbreeding leads to further population decline, because adaptation happens at a much lower pace,” she told The Wire. “Hence, making use of the genome, the diversity of that population can be determined, and an informed decision can be made in regard to how the different kiwi populations should be mixed.”
The world would be a poorer place if such an enigmatic unique bird were to disappear.
The paper, ‘Kiwi genome provides insights into evolution of a nocturnal lifestyle,’ was published in Genome Biology.
Janaki Lenin is the author of My Husband and Other Animals. She lives in a forest with snake-man Rom Whitaker and tweets at @janakilenin.
