Thursday 19 January 2017

Mink – Evil or Interesting?

A Talk by Dr Joanna Bagniewska to the Berkshire Mammal Group

6th October 2016

write-up courtesy of Edwin AR Trout, Berkshire Mammal Group

Dr Joanna Bagniewska, University of Reading

Opening the meeting Amanda Lloyd introduced Joanna as a zoologist specialising in behavioural ecology and invasive species research, gaining her MSc and PhD from Oxford University. Over the years she has undertaken research in five countries, having studied foxes and jackals in South Africa, wombats and wallabies in Australia, mole-rats in the USA, and both bees and mink in Britain. (Joanna observed that in view of mink’s past fortunes, she was one of few researchers able to wear her study species!) Currently Joanna works as a teaching fellow for the School of Biological Sciences at the University of Reading and has won awards as a science communicator.

She started by suggesting she was one of a minority in this country to admire the otherwise widely denigrated American mink, perhaps because she approached the subject from the point of ecology rather than conservation and control (where mink are considered pests). She defined them as mustelids and took a few moments to outline their ecology.

Mink Ecology

The American mink is native to North America, at least north of the south-western arid zone up to and including Alaska, but south of the high Arctic. Mink are to be found in areas of high water quality. They are small bodied – at 15-18” the males are twice the length of the females – and sexually dimorphic. In some areas males have shown a preference for eating rabbits (for which they have proportionately bigger jaws and teeth) and females – water voles. They are opportunistic feeders and both sexes will eat insects, fish and birds.

While native to North America, they have been introduced into Europe, Asia (especially Siberia) and South America (Patagonia in particular),

largely as a result of the former demand for their fur. In the 1920s and ‘30s they were farmed, but later, in the 1950s, many escaped or were released. They are now regarded an invasive species.

They have, in many ways, become too successful for their own good. Why? They are generalist foragers, with a broad diet: birds, amphibians, reptiles, rodents, insects, and crustaceans. They can hunt like foxes, climb like cats and swim like an otter. They are semi aquatic and very adaptable.

An Invasive Species

As an invasive species, what has been their impact? Traditionally, successful incoming species upset the balance of nature by:

  • Increased predation
  • Resource competition
  • Hybridisation
  • Disease transmission

Of these the first two are of most relevance, though there have been some recorded cases of interbreeding between wild and released mink in North America.

The problem of increased predation is exacerbated by the higher proportion of carnivores among invasive species. Carnivores make up 5% of mammalian species, but when invasive mammals are examined, that proportion rises to19%.

Direct predation by mink is most notable in the UK in the case of water voles. Female mink are the right size to enter voles’ underwater holes and the water vole population is declining fast. This is, admittedly, for various reasons, but the presence of mink is not helping. There is no problem with water voles in Europe, incidentally, but rather the Pyrenean Desman instead is vulnerable and declining.

The Landscape of Fear”
Other vulnerable species are ground-nesting sea birds, for which the security of islands and rocky

outcrops are no protection against the semi-aquatic mink. On arrival in a colony mink will behave like ‘a kid in a candy store’ and take as many as 100 chicks per night, with no sense of forward planning or restraint. Indirect consequences follow, as birds leave their nests – in fear or to protect their young by attacking predators – and the chicks or eggs succumb to the cold or neglect. The very presence of fear can have further negative effects on the predated population.

Over-sized, Over-sexed and Over here”
A notable impact of resource competition has been on the European mink, which is now limited to Estonia and the Pyrenees, as the American mink is sufficiently well equipped to out-compete.

American European
1310.2 g v. 976.6 g
size of female = size of male
litter of 6 v. litter of 2.5

The American mink is voracious and adaptable, and its European cousin is unable to keep up.

Another measure of its success lies in the competition with polecats and otters. When these species were declining in the 1920s-50s, mink expanded into their vacated niches, on both land and water.

Horrible – but Interesting

American mink are semi-aquatic BUT …

  • They are small, with a large surface area, and so loose heat quickly
  • Their fur is neither buoyant nor insulative
  • Their propulsion is inefficient – they swim like dogs rather than otters
  • Their paws have little webbing (more than polecats, but less than otters)
  • They have poor eyesight, especially underwater.

Is swimming innate and instinctive? It seems the young have to be taught, as indicated by trials on captive mink. Other tests suggest mink will work harder to get to water than even to obtain food! And mink spend much of their time in the water, diving for food.

Diving Behaviour

How do we study diving animals? It has been difficult until now. Research has focussed onterrestrial behaviour, which is more readily observable and can be indicated by footprints, scat and camera traps. For wild mink only ‘surface events’ have been traceable in the water, and underwater behaviour monitored only in captivity. Joanna’s research was the first monitoring of mink diving in the wild. Indeed, the only previous study of mammals was one of the platypus in Australia. Since then there have been studies of water rats and beavers as new data-logging technology becomes available. Devices small enough to be attached to mink can measure time spans, temperatures, pressure and acceleration.

Such data logging was first developed in the 1950s, weighing as much as 1.5 kg, based on ‘egg timer’ technology and recording output on paper. The first devices were used on Weddell seals. They were accurate to about 20 m and therefore only useful for extensive depths at sea. Now devices are digital, weigh 1g and are accurate to 3 cm, so may be used effectively in the shallow water of British rivers.

Monitoring Programme
Joanna’s project monitored 16 distinct animals on the rivers Thames and Cherwell in Oxfordshire, generating 20 datasets, with a mix of males (six) and females. The programme was conducted over three seasons, spring being excepted on animal welfare grounds.

The method made use of mink rafts, by which an enticing ‘home’ or tunnel was placed on a floating platform away from the bank. Pliable clay would be spread on the base of the raft to receive the impression of foot prints should a mink investigate the tunnel. With the presence of mink thus established, the tunnel would be fitted with a trap and bait, and, if caught, the mink fitted with a collar and data-logger.

Quantitative Results
The collective results of the study indicated great variety in depth and duration, with a total range of 1 to 189 dives per day, with the greatest individual range between 5 and 143. It became notable that the smaller females were the more energetic divers, performing over 100 dives per day (as explained below). The time spent diving wasn’t long: a total of just 38.4 minutes per day on average, with 57.9 seconds the maximum individual dive-length recorded. Median depth times varied between 7.4 and 18.0 seconds.

(Previous studies had suggested even shorter times.) Diving was undertaken throughout the year, not just in summer as had been postulated, and 83% of dives took place in daylight – presumably in consequence of minks’ poor eyesight. The maximum depth of around 3 m was more an indication of river depth than diving ability.

Dive Clustering
The pattern of surfacing suggested that dives were clustered in bouts of activity and so was analysed according to the Hidden Markov Model (HMM). Three types of diving frequency were identified:

  • A “cluster” of multiple dives
  • Loosely aggregated: an “activity session”
  • Single or terminal dives

Temperature and Persistence
Mink experience a temperature range of 5-35 oC, in which temperature falls during activity. The body cools in water, and warms through when basking on the bank; it is typically 35 oC when sleeping, in the 20s when on land, and drops when diving. It is noticeable that dive effort is higher in females; female ‘persistence’ may extend to performing 80 dives in a row. Is this because they are excellent divers, or ineffective hunters who have to repeat dives to eat?

Either way, the smaller the mink the greater the persistence and this may be expressed as a negative relationship between temperature and bodyweight. Persistence is greater during the day, when temperatures are higher, and become more concentrated the shorter the day. That said, mink tend to be inactive 80% of the time, spending only 50 minutes or so in the water per day. Males are more nocturnal – perhaps, because of their size, they are less afraid of meeting otters?

Competition with Otters

Behaviour has altered over time, with the evidence of scat declining since the otter’s resurgence in the 1990. However, if other evidence such as footprints is taken into account, it appears that mink remain, but are no longer deliberately marking their territory. Otters are bigger, and may weigh 8 kg, so while the feisty mink might be prepared to fight, it wouldn’t,

wish to make a habit of it. So what seems to be happening is that when the nocturnal otter returns, mink behaviour adapts to the new circumstances and becomes diurnal, thus avoiding confrontation.


The survey testifies to the success of new TDR (Time Depth Recorder) technology and HMM techniques in monitoring the behaviour of otherwise difficult to observe species.


Have there been studies of the American mink in its natural habitat, North America?
Yes, in the 1940s and 50s, but mainly of terrestrial behaviour. There have been diving studies elsewhere, in Tierra Del Fuego for instance, where mink are diving in the sea, and of feeding in Sweden.

How long did the mink wear collars?
Joanna aimed to collect collars after three weeks, but if missed, they were designed to be readily biogradeable, rather than made of more durable leather.

Where do mink live?
They don’t excavate, but occupy existing holes. Sometimes these are in trees, but mink are particularly fond of rabbit warrens.

Are males and females mutually antagonisic?
Yes, males and females join only to mate. Mink of each sex maintain their own, linear, territories. The males occupy about 5 km, overlapping two of the smaller females’ 3km territories. Males will not tolerate each other. Mink are not social; apart from mothers and kits they do not share.

How related are they to the European mink?
Less so than previously thought; they are of a different genera. Indeed there is no knowledge of hybridisation. European mink used to be found in the Baltic states, Poland, Belarus and Russia south to northern Ukraine. No population is known to have existed in the British Isles.

Has the return of the otters in recent times deterred the mink?
It appears not, though it has affected the mink’s behaviour.