Identifying the killers’ next of kin: stoats, ferrets and weasels

Natural born killer, the stoat!

The title of number 1 killer in New Zealand is a fairly well fought over position. There are numerous introduced species that could enter this competition. Possums, even though not especially big predators, have, through sheer weight of number, a huge impact. Cats, wild and domesticated, are super effective killers but don’t have the numbers in the wild areas. Wasps are lethal on invertebrates in a habitat, with flow on effects to others. The humble hedgehog is increasingly being unmasked as a public enemy, has increasing numbers and is widespread. Rats are certainly widespread and will eat most small species. And then there are the mustelids, the weasels, ferrets and stoats. Especially the stoats. These guys are killing machines, clever and insatiable.

All these species are introduced and all are out there slaughtering native diversity. My money is on the stoats. They are efficient killers and their numbers can quickly respond to food surpluses, producing more killers.

Humans are pretty effective killers to. Hunting was never my strong point. As a young fellow I angled for fish with my grandad, whitebaited with my dad, and roamed the local swamps with a shotgun looking for ducks (who were mostly safe even when I did find them!). The highlight of my hunting career came one day at a beach on Otago Peninsula. I was an honours student at that point and had picked up a summer job monitoring Yellow-eyed Penguins. We had been losing some chicks to predation and one of my jobs was to set out kill traps to give the penguins some protection.

One day while out checking the penguin nest sites I reached a nest that was out in the open (most yellow-eyed penguin nests are in light scrub). The chick was missing (the parents were out at sea finding food for it). I looked around and two paces away was the chick. Dead. Incredibly, a stoat was standing on the chick. Normally this nocturnal species is never seen other than as a quick blur at the edge of your vision. Here was one on a penguin chick (which was at least 10 times its size) and eating around the head region (stoats can grab penguin chicks around the neck and kill them). It hadn’t sensed me. It was very windy and I had approached unobserved by the stoat.

YEP chick nest
Yellow-eyed penguin chicks, despite being much larger than mustelids, are vulnerable to these predators.

I was carrying a large stick (for putting trapped predators down), and very quietly and slowly moved to within striking distance. The stoat remained intent on its meal. Whack! I lunged out with the stick fully expecting this marvel of evolution to demonstrate its reflexes and leap away. No, I clipped it on the head and it was all over. I felt a sense of justice for the penguin chick and sense of wellbeing for doing my bit for the success of the colony.

Over the years I have had students that have worked with mustelids in one way or another. Sometimes as the bad guys who are eating their study animal, sometimes as the study animal as we seek better methods to understand mustelid behaviour. Recently, we had the opportunity to examine the DNA of stoats, weasels (the Least Weasel) and ferrets. PhD student Arsalan Emami-Khoyi was able to make use of samples at the Department of Ecology, Lincoln University to sequence the whole mitochondrial genome for these three species.

Mitochondria are effectively little cells that live inside each of our main cells. Packed inside each mitochondrion is a loop of DNA, quite separate to our main nuclear DNA. We often use this DNA, with its 37 genes, to examine the family history of species.

Although there has been some work on how mustelids are related, stoats (Mustela ermine) had never been sequenced. Seven other mustelid species (Pine and Beech Martens, Mountain, Black-footed, Siberian, Yellow-bellied, Long-tailed Weasels and European Otter), whose genomes were available were put into our analysis. Another question about mustelid relationships, given that otters are in this group, is whether they are closely related to seals. We added in two seal genomes (the New Zealand fur seal which Arsalan also sequenced and the Weddells seal) as well as two bear genomes (polar and brown bears).  One advantage of using DNA is that, in combination with known fossil ages, we can estimate how old mustelid lineages actually are. The results of this study have been published in the journal Mitochondrial DNA.

Stoat_IN TUBE3
Stoats are studied at Lincoln to understand their behavior.

The living mustelids were found to be around 13 million years old with martens the oldest distinct lineage. Stoats originated around 5 million years ago, the Least weasel and ferret evolved around 1-2 million years ago. Most of the mustelids evolved over the last 6 million years. At the bigger scale seals were more closely related to bears than to the mustelids but very distantly (nearly 50 million years ago). The two main families of seals diverged from each other around 30 million years ago.

So mustelids are a relatively modern group. Their particular brand of predation has been very successful and has led to speciation into particular niches in different parts of the world in response to environmental changes over the last ten million years. New Zealand has three species that are drawn from throughout this radiation of species and are likely specialists in different niches. These different roles mean that they are unlikely to directly compete with one another and that we have a suite of predators to deal with. This is one family of killers that will take a lot of effort to bring to account.

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