By Kieren Mitchell
Animals that have evolved on islands become adapted to very specific and stable environments, leaving them extremely vulnerable to outside disturbances. For example, many island birds became flightless in the absence of predators (most famously the dodo). Consequently, these species frequently become extinct soon after the arrival of humans, through either direct (e.g. hunting) or indirect actions (e.g. habitat destruction, introduced species). This gives us very little opportunity to study these creatures, and means that their origins and evolutionary relationships are often unknown (e.g. Sylviornis from New Caledonia). Ancient DNA analysis is a valuable tool in unravelling the history of these mysterious species. The extinct and enigmatic Chatham Island duck was the subject of a recent study I undertook with Bastien Llamas and Alan Cooper at ACAD, along with our collaborators: Jamie Wood (Landcare Research, NZ) and Paul Scofield (Canterbury Museum, NZ).
The Chatham Island duck (formerly Pachyanas chathamica) lived on a single island in a small archipelago around 850 km east of New Zealand: the Chatham Islands. It was a relatively large (~1.5 kg) and stout duck, which probably made it quite an attractive meal for early settlers of the island. In any event, it was extinct by the time European naturalists visited the island in the 19th century. The mystery is that no one could be sure exactly what type of duck it was, and so it was posthumously given its own genus: Pachyanas (“pachy-“ Greek for thick + “anas” the Latin for duck; a rather unflattering moniker). It seemed to have some similarities to the shelducks: namely its size and robust skeleton (hence the thick). However, other researchers tentatively concluded that it might be more closely related to the dabbling ducks. This is a non-trivial disagreement as the two groups separated around 17 million years ago. In order to put the issue to rest, we extracted DNA from a Chatham Island duck bone and compared it to the DNA of a wide range of other duck species.
What we found was that not only was the Chatham Island duck definitely a dabbling duck but its closest relatives appear to be the teal species living in New Zealand and the nearby Campbell and Auckland Islands. This means that the Chatham Island duck in fact falls within the genus Anas. While it had been suggested that the Chatham Island duck was possibly a dabbling duck no one had previously suggested it was actually part of Anas (All Anas are dabbling ducks but not all dabbling ducks are Anas). Consequently, we can dispense with the unique genus Pachyanas and instead more correctly place the Chatham Island duck in Anas. This result was completely unexpected given that the Chatham Island duck was up to three times heavier than its close cousins. What appears to have happened is that the ancestor of the Chatham Island duck arrived (possibly from New Zealand) relatively soon after the Chatham Islands rose out of the ocean two to three million years ago, liked what it found, and then never left again: eventually becoming a new species. The most curious thing is that its cousins in the Campbell and Auckland islands became smaller than the New Zealand teal, while the Chatham Island duck became much larger. The next most curious thing is that despite being able to fly (unlike the teals from the Campbell and Auckland islands) the Chatham Island duck appears to have only lived on one of the six main islands in the Chatham Islands archipelago. These observations suggest that the Chatham Island duck evolved to fill a completely different ecological niche to its cousins. It is perhaps no coincidence that the island where it lived was the only one in the archipelago to have Sporodanthus bogs. Adoption of a more herbivorous diet in association with this unique habitat may have driven an increase in the size of the Chatham Island duck: large body size is thought to be advantageous in efficiently digesting and processing plant material.
By confidently determining the relationships of the Chatham Island duck (now more properly Anas chathamica) ancient DNA has allowed us to solve yet another evolutionary mystery. Importantly, our discovery also helps us to understand how different species adapt to island conditions and how animal communities on islands are established.