Can mink be friendly?

Can the invasive American mink, often considered an aggressive species, appear to be friendly? It might surprise some people, but the answer is yes. At any rate, this was the conclusion of a study conducted by the University of Iceland, the West Iceland Nature Research Centre, the University of Oxford and Stockholm University. These findings raise important questions about mammalian aggression in general and the factors that influence aggression.
 
“This research has a rather unusual background, you might say. Many years ago, I was studying the habitat use and activity of wild mink at Lónakot on Reykjanes. I trapped live mink and fitted them with radio transmitters, so that I could find them again and monitor their behaviour. While collecting data, I observed behaviour that contradicted everything we thought we knew about mink. The American mink is a solitary species, generally believed to be highly aggressive towards other mink and related species. However, on many occasions I saw unrelated mink interacting in ways which could be interpreted as friendly,” says Menja von Schmalensee, doctoral student at UI and department head at the West Iceland Nature Research Centre.  

Menja was intrigued by this unexpected behaviour and had always wanted to explore it in greater depth. Her latest research, looking at this very topic together with Róbert A. Stefánsson, director of the Nature Research Centre, and Snæbjörn Pálsson, professor at UI, has now been published in the journal Animal Behaviour. Snæbjörn is Menja’s PhD supervisor. The other members of the team are David W. Macdonald from the University of Oxford and Anders Angerbjörn from Stockholm University.

A solitary and aggressive animal becomes friendly 

To better understand why mink are not always aggressive toward each other, Menja and Róbert reviewed the datasets from their behavioural research involving radio tagged mink. This data was collected in three different areas, Lónakot and the Sog river area in South Iceland and Kolgrafafjörður on the Snæfellsnes peninsula, over a period from 1996 to 2007. The data includes information about 66 tagged individuals, as well as recorded observations about the behaviour of untagged mink seen in the study areas.

Since mink are not a social species and live solitary lives defending a territory, the vast majority of Menja and Róbert’s observations were of single animals. However, there were 50 occasions on which two or more unrelated mink were seen together. 

“Although 50 might not seem like a high number,” says Menja, “this is incredible data for this elusive and solitary species. We analysed these cases, looking at the specific individuals involved and how they interacted. We saw very clearly that friendly and neutral interactions were more likely to occur between tagged individuals with territories within the study area, i.e. neighbours. Aggression was more commonly observed if one or both mink were untagged strangers.”

This behaviour, when a territorial species distinguishes between neighbours and strangers and treats them differently, is a recognised phenomenon which has previously been described in several species, although never in mink. 

“Often animals that make this distinction show less aggression towards neighbours, as we saw in the mink, which is known as the ‘dear enemy’ strategy. But in some cases, species, populations or individuals might exhibit the opposite behaviour, i.e. higher aggression towards neighbours compared to strangers. This has been called the ‘nasty neighbour’ strategy,” explains Menja.   

A short, fun video about the study, created by Charlotte Bird at the University of Oxford.

Important for our understanding of evolution

Neighbour-stranger discrimination has mainly been researched in birds, but for this study the team wanted to take a closer look at mammals, particularly whether social species were more likely than non-social or solitary species to discriminate in this way. 

“This could help us understand the evolutionary drivers behind the behaviour. Possibly, social species would be more likely to discriminate between neighbours and strangers if they had a better overall ability to distinguish between individuals, as a consequence of their sociality. To test this hypothesis, we conducted a detailed literature review looking at neighbour-stranger discrimination in mammals, considering the social systems of the species involved and putting the findings into context with what we uncovered regarding mink in Iceland.”

What species were included in the study?

The literature review identified 63 published studies specifically looking at neighbour-stranger discrimination in mammals. These studies involved 48 different species with varying degrees of sociality, across 23 families and 5 orders. Rodentia, Carnivora, and Primates were the most common orders.

The literature review also showed that several species used both dear enemy and nasty neighbour strategies under different circumstances. 

“It has sometimes been theorized that the dear enemy strategy is simply a consequence of animals getting used to each other, but our review indicates that a more likely explanation is that the animals adapt their behaviour to suit the circumstances. The same category of individuals may pose different threats at different times. Generally, strangers suddenly appearing in your territory pose a greater threat, since they could be aspiring to take over your territory, rather than neighbours who already have their own territories. But in certain circumstances, such as during the mating season or relating to seasonal changes in resources, neighbours may pose more of a threat than strangers.”

We need to consider more factors to understand the behaviour 

Our results and the conclusions drawn are highly significant. “We humans tend to underestimate the cognitive abilities of other animals, especially if they are very different to us. There has been a certain bias in animal research when it comes to understanding animal cognition, since scientists have previously tended to concentrate on social species rather than solitary species. This means that many people are aware that social species like elephants, dolphins and chimpanzees can exhibit complex behaviour and make complex decisions, but fewer people realise that solitary mammals also need all kinds of cognitive skills to survive and reproduce in a complex world.”

The findings underline the importance of considering many different environmental factors and evolutionary drivers in order to understand the cognitive abilities of animals. 

“Perhaps being territorial, in and of itself, is a factor we need to explore in more detail when it comes to cognitive evolution in animals, since finding, procuring and maintaining a territory is very challenging.”

This widespread ability in mammals to distinguish between individuals, regardless of sociality, also highlights the need to reassess the methodology that has previously been used to study self-recognition in animals.

How important is the publication of this paper? 

“Hopefully, our paper will raise awareness and curiosity amongst both other scientists and the general public about cognition in solitary mammals and its evolution,” says Menja. 

“If we ever want to understand the internal lives of animals in a broad context, it is not enough to focus on animals that we can identify with because they behave somewhat like us.”

In recent years, there has been some progress in this regard, for example some exciting research looking at cognition in octopuses and related species, “but there is always room for improvement!” says Menja.

As we are all aware, we are living through an age of catastrophic human-driven environmental negative changes which threaten the survival of many species and populations, including humans. We are not isolated from the rest of the biosphere.

“By better understanding the other animals we share the planet with, we will hopefully develop more respect for them and become more willing to intervene to prevent population decline or extinction.”

Menja is delighted with the paper. “For me personally, this publication is also quite important. This is a topic that I have been thinking about for a very long time and it’s amazing to see it all finally down in black and white. The paper is also an important part of my PhD thesis at UI, but I have been working on my Doctorate these past few years alongside my work on other subjects at the West Iceland Nature Research Centre in Stykkishólmur.”

You can read the paper here: