How Invasives Get In Your Head (And Your Poop)

Image Credit: Hedera Baltica, CC BY-SA 2.0, Image Cropped

Invasive alien species as an environmental stressor and its effects on coping style in a native competitor, the Eurasian red squirrel (2022) Santicchia et al., Hormons and Behaviour,

The Crux

We know that human activities can cause enormous stress for local species, and the introduction of invasive species is one of the most harmful stressors on a global basis. We know that new, harmful species can cause local extinctions, but how does their introduction affect the locals on a behavioural level?

Grey squirrels were introduced to Europe last century and have been spreading since, displacing the native red squirrels and wiping them out in many areas. This week’s authors wanted to know exactly how red squirrels’ behaviour changed when the grey squirrels were introduced, by looking in detail at the behaviour of red squirrles in both invaded and non-invaded areas, and seeing if they could see evidence of these changes in the expression of hormones (more on this in Did You Know).

Did You Know: Testing Behaviour Through Hormone Expression

Full disclosure – this week’s paper took me well outside of my comfort zone. Reading about chemical signals and their expression was fascinating though. Animals are often measured along a proactive-reactive axis, with more proactive animals being bolder, more social and more active. Reactive animals are generally shyer and less active.

Proactive animals are often characterised by low hypothalmic-pituitary-adrenal axis activity (HPA). Essentially, this is a measure of the activity between these three glands, with more reactive individuals showing higher activity. Higher HPA activity results in the production of more gluocorticoids (GCs), which suggests that shyer, less active individuals should show higher levels of GCs.

What They Did

The researchers found 200 red squirrels across Northern Italy in six different sites – three where only red squirrels were present, and three where the grey squirrels had invaded. The squirrels had two tests performed on them, one which measured levels of activity and exploration of their surroundings, and another that measured their level of sociability when confronted with another squirrel (their reflection). Additionally, their scat (fancy scientific word for poop) was tested for levels of GCs.

The researchers then compared the relationship between all three traits (activity, sociability and GC concentration) in populations with and without the grey squirrels present.

What They Found

In the red only sites, there was a strong relationship between how active or explorative and how social a squirrel was, with more active squirrels also being more social. However this relationship disappeared in the sites where grey squirrels were present, indicating that the presence of the greys caused a disruption in normal squirrel behaviour. Interestingly, the relationships that were hypothesised to turn up between either personality trait and GC levels didn’t show up.


If you want to be able to rely on your data, then often it needs to be repeatable. Here, repeatability meant getting similar GC reading from similar individuals. If there is low repeatability, then it means that GC may not have been a reliable variable to use in this experiment. That was the case here, with the relationship described in Did You Know not being found. I still enjoy that the paper was published though, as too many papers are overlooked simply for not having the desired results, leading to bias in our scientific knowledge base.

So What?

Knowing a bit more about exactly how the presence of an invader can disrupt local populations fascinates me. Often we get wrapped up in the large-scale effects, and forget what happens on a small scale.

On a more emotional note, building more of a story around the interaction between an invader and the species they are a problem for helps the public understand why invaders are a problem. Grey squirrel eradication programs have faced severe pushback from animal rights campaigners who don’t like to see a cute rodent harmed by humans. Showing how stressful the greys are for the native red squirrels is a useful way to educate the public on the effect of invasive species and show how important their manaegement is for the sake of the natives.

Dr. Sam Perrin is a freshwater ecologist who completed his PhD at the Norwegian University of Science and Technology and hates that one of the best examples of squirrel SciComm was performed by Hans Landa. You can read more about his research and the rest of the Ecology for the Masses writers here, see more of his work at Ecology for the Masses here, or follow him on Twitter here.

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