An international group of scientists led by the University of Leipzig, Germany, has sequenced the genome of the North Island brown kiwi (Apteryx mantelli) and identified several sequence changes that underlie the bird’s adaptation to a nocturnal lifestyle.

Kiwi, comprising five species from the genus Apteryx, are endangered, ground-dwelling bird species endemic to New Zealand.

These birds are the smallest and only nocturnal representatives of the ratites, a group of flightless birds that includes the ostrich and emu.

They are also unusual in that they have a highly developed sense of smell, low metabolic rate, and enormous eggs in relation to body size. However, the genetic adaptations that underpin their unique traits have so far not been well understood.

“We have seen for the first time that kiwi lack color vision, and that their olfactory receptors can probably detect a larger range of odors which may be essential for their night-time foraging,” said Dr Diana Le Duc from the University of Leipzig and the Max Planck Institute for Evolutionary Anthropology, lead author on the study published in the journal Genome Biology.

“These adaptations seem to have happened around 35 million years ago, soon after their arrival in New Zealand, probably as a consequence of their nocturnal lifestyle.”

The gene responsible for black and white vision, rhodopsin, was found to be similar to other vertebrates. However, the scientists identified mutations in the green and blue vision receptor genes, which could render blue and green color vision absent in the kiwi.

The genomic changes in kiwi vision and smell are consistent with changes that are thought to occur during adaptation to nocturnal lifestyle in mammals.

Dr Le Duc and co-authors estimated the onset time of these changes to around 35 million years ago, suggesting that the kiwi adopted its nocturnal lifestyle shortly after the arrival of its ancestor in New Zealand.

At the time the kiwi arrived other ratites, the moa, already inhabited New Zealand. These now extinct birds are thought to have monopolized food sources during the day, forcing the kiwi to adopt an alternative nocturnal lifestyle.

“Already French botanist and zoologist Jean Baptiste de Lamarck, who lived in the 18th century, hypothesized that evolution works in accordance with a ‘use it or lose it’ principle. It’s therefore very likely that the kiwi lost its color vision since this was no longer needed for its new nocturnal lifestyle,” Dr Le Duc said.

“The kiwi’s sense of smell – which was required for foraging in the dark of the night – became more acute and the repertoire of odorant receptors increased adapting to a wider diversity of smells.”

Kiwi are unique among birds in having nostrils present at the end of their long beaks and it has long been thought they are more similar to mammals than birds in their reliance on tactile and smell senses for foraging. The kiwi genome showed significantly higher diversity in smell receptors than other investigated birds, suggesting that they may be able to distinguish a larger range of odors.

Nocturnal animals tend to have low energy metabolism, and kiwi have the lowest metabolic rate among all birds.

In the kiwi genome, Dr Le Duc’s team found enriched changes in genes related to energy expenditure, reserves and metabolic processes, which may also be linked to this nocturnal lifestyle.

“The genome of the kiwi is an important resource for future comparative analyses with other extinct and living flightless birds,” said study co-author Dr Janet Kelso of the Max Planck Institute for Evolutionary Anthropology.

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Diana Le Duc et al. 2015. Kiwi genome provides insights into evolution of a nocturnal lifestyle. Genome Biology 16: 147; doi: 10.1186/s13059-015-0711-4