What makes kiwi a night owl: it’s in the DNA
Are you a night owl? Owls are undoubtedly the iconic nocturnal bird. What about a night kiwi? Albeit less known, kiwi, the iconic bird of New Zealand, is also a night creature.
When and how did kiwi adapt to a nocturnal lifestyle? Scientists set out to answer this question by looking into the kiwi genome, or its complete set of genetic blueprint.
“We’ve 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,” says Diana Le Duc of University of Leipzig and Max Planck Institute for Evolutionary Anthropology, Germany, who is the lead author of the study. “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.”
Kiwi is a ground-dwelling, flightless bird of New Zealand. It is the smallest (about the size of a chicken) and the only nocturnal member of the ratites, a group of flightless birds that include ostrich and emu.
At a glance, kiwi already has a one-of-a-kind appearance, with its round body covered with brown fuzz, beady eyes, and long, tapered beak. It has several more unusual traits, however, that set it further apart from other birds. First, kiwi is nocturnal; less than 3% of all bird species, including owls, are nocturnal. Kiwi also has a highly developed sense of smell and low metabolic rate. All these traits help kiwi to survive in its ground-dwelling, nocturnal niche. Scientists have been puzzling over what genetic changes undermine these unique traits.
For the first time, a team of scientists decoded almost an entire genetic blueprint of North Island brown kiwi to understand how the genes changed over time to allow kiwi to survive a night life. This is the largest bird genome sequenced to date, according to the study.
The scientists identified evolutionary changes in the kiwi genome that enable unique nocturnal capabilities: vision, smell, and metabolic rate.
Several genes involved in color vision are inactivated in the kiwi genome, according to the study, indicating that kiwi lacks color vision. This could explain why the kiwi retina has a higher proportion of black-and-white receptors than color receptors, a feature which improves night vision. Limited color vision is also typical in nocturnal animals for the same reason.
Befitting a bird that needs to sniff out food hidden on the ground, kiwi has a keen sense of smell. The scientists found a significantly higher diversity in the kiwi genes involved in sensing smell than that of other ratites. These genetic changes likely allow kiwi to distinguish a wider range of smells, indicating an increased reliance on smell rather than sight during foraging.
In fact, kiwi is more similar to mammals than birds in its foraging behavior. Kiwi relies on the touch and smell when searching for food. To help with this, kiwi’s nostrils are at the lower end of its long beak, contrary to most other birds, which have nostrils on the upper end of their beaks, as shown below (my budgie being the representative in this comparison 🙂 ).
Kiwi has the lowest metabolic rate among all birds. This makes sense because without any heat from the sun, kiwi would need to spend more energy producing and regulating its body temperature. For the same reason, nocturnal animals tend to have low metabolism for conserving energy. The scientists identified multiple changes in the kiwi genes related to energy expenditure and reserves and also other metabolic processes, likely contributing to the low metabolic rate.
The study, thus, reveals that kiwi evolution parallels that of nocturnal mammals: limited color vision, strong sense of smell, and low metabolic rate.
The scientists date these gene modifications to 30-38 million years ago, during the Oligocene epoch, likely after the ancestor of the modern kiwi arrived in New Zealand.
Here is a scenario proposed by the scientists: At the time the ancestor of kiwi arrived to New Zealand, a much bigger bird moa dominated food sources at daytime. The moa, now extinct, reached up to 3 meters (close to 10 feet) in height in some species. Being the smallest of ratites, the kiwi likely had to content itself to foraging at night. Over time, offspring with genetic changes that bestowed traits suited for nightlife outperformed the predecessor, resulting in the modern-day kiwi.
As fascinating as the findings are, the study also points to a sad fact: the kiwi species is facing extinction.
“Despite conservation efforts, North Island brown kiwi are still at high risk of extinction,” says Le Duc. “We made a first estimate of the diversity of the kiwi genome by comparing the sequence of two individuals, and it appears to be as low as that of inbred birds. This is an important indication of the level of the threat, and we expect further insights from the genome to help in developing conservation management strategies.”
The study is published in this month’s issue of Genome Biology and is open access (no subscription is required).
Here’s a fun thought to conclude the story: which one is named after the other, kiwi bird or kiwi fruit?
The answer: kiwifruit is named after kiwi bird.
We might imagine that kiwifruit is native to New Zealand but not so. Kiwifruit, or Chinese gooseberry, originates from northern and eastern China. The plants were introduced into New Zealand in the early 20th century, where the fruits became popular among Americans stationed there during World War II. Henceforth, New Zealand has become the major distributor of kiwifruits to various parts of the world. In 1960’s, New Zealand growers came up with the name kiwifruit to make the fruit more appealing. (Well, kiwifruit is so much catchier than Chinese gooseberry, product of New Zealand, don’t you agree?) And the name has stuck till this day. (You can find out more about kiwifruits here.)