At first (and probably second) glance you wouldn't think that tigers and kiwis have all that much in common. Bengal tigers (Panthera tigris tigris) live in India and the surrounding countries, where the predators can weigh more than 220 kilograms. Little spotted kiwi (Apteryx owenii) live exclusively in New Zealand, where the flightless birds weigh in at less than two kilograms. But while the big cats and the little birds don't appear very much alike from the outside, new research reveals that both the tiger and the kiwi are actually suffering from the same internal problem: low genetic diversity.

The news comes from two papers published May 15 in Proceedings of the Royal Society B: "Demographic loss, genetic structure and the conservation implications for Indian tigers" by researchers from the National Centre for Biological Sciences in Bangalore and Cardiff University in Wales, and "Genetic consequences of a century of protection: serial founder events and survival of the little spotted kiwi" by researchers from Victoria University of Wellington, the Department of Conservation in New Zealand and the University of Montana–Missoula.

Let's start with the tigers. The researchers examined tiger skins and bodies stored at the National History Museum of London, which has a collection of Bengal tigers killed during the British Raj period (1858–1947) when Great Britain ruled the subcontinent. The team extracted DNA from those remains and then compared them with the genetics of present-day tigers. They found that 93 percent of the historical mitochondrial DNA variants in the museum samples were no longer present in modern populations.

Mike Bruford, a professor at Cardiff School of Biosciences and one of the authors of the study, told BBC News that overhunting during British rule took the first toll on Bengal tiger genetics, a situation worsened by modern habitat fragmentation that keeps remote tiger populations from mixing. "This is important," he told the BBC, "because tigers, like all other species, need genetic diversity to survive—especially under climate change—so what diversity remains needs to be managed properly so that the Indian tiger does not become inbred, and retains its capacity to adapt." Although Bengal tigers are the most numerous of the six remaining tiger subspecies—numbering about 2,000 out of an estimated 3,200 total wild tigers—Bruford warns that simply counting the tigers is not enough. The various, spread-out populations must also be allowed to mix and interbreed in order to maintain their genetic health.

Thousands of kilometers from India, New Zealand's little spotted kiwis have a similar problem, albeit one with a different origin. Like all kiwi species, the little spotted kiwi was almost wiped out in the 19th century when European settlers brought dogs, cats, stoats (a type of weasel) and other predators to the islands. The last five birds of this particular species were caught and transferred to predator-free Kapiti Island in 1912. All 1,200 or so little spotted kiwis living today are descended from those five birds.

The tiny founder population meant that mating a century ago could only be done with siblings. Lead researcher Kristina Ramstad, a postdoctoral fellow at Victoria University, told The Dominion Post that today's birds are so similar they are almost like clones. Although the birds remain protected and are probably the least threatened of the five kiwi species, she warned that "If the right disease came along, it could kill them all. There's no way to combat that."

Previous research had already shown that both the Bengal tiger and the little spotted kiwi have limited genetics. These new studies show that the problems for the big cat as well as the little bird are worse than previously understood and illustrate the challenges that conservationists face when keeping species alive in the face of rapidly shrinking populations.

Previously in Extinction Countdown:

Photos: Bengal tiger by Allan Hopkins. Used under Creative Commons license. Little spotted kiwi drawing by R.H. Porter circa 1876. Public domain