Do rats mind inky feet?

Rats are fastidious groomers. So how does an animal that grooms frequently react to the prospect of running through viscous tracking ink? Are rats reluctant to get their feet dirty – and if so, what are the implications for tracking tunnel monitoring?

Two rats in hay
Black rats (Rattus rattus). Image credit: Kilessan. (via Wikimedia Commons)

Prof. Carolyn King and fellow researchers at the School of Science, University of Waikato checked out the reactions of ship rats to tracking tunnel ink pads in a series of all-night rat-watching vigils. The results of their study have just been published in the New Zealand Journal of Zoology (only the abstract is freely available to non-subscribers).

Image of labelled rat tracks
Rat tracks (Rattus rattus) on a tracking tunnel. Image credit: Warren Agnew

“Monitoring rat populations by standard tracking tunnels assumes that rats do not avoid treading on the tracking ink. We observed the behaviour of 20 rats in four groups of five, spending 2 nights each (40 trials) in large observation pens.”

As perhaps the researchers expected, rats did indeed hesitate, either from natural caution or perhaps reluctance to step onto the ink.

“In only 10 of 40 trials did the rat run through a tunnel without hesitation on first encounter, while in four trials a rat approached the tunnel up to 35 times but refused to run though it all night. Most rats approached and turned away between two and 34 times before running through.”

That’s a lot of hesitation in some cases. For some rats at least, curiosity and caution were clearly at odds. Was it the ink that was bothering them?

“Observations through transparent tunnels of rats pausing for a mean of 3.2 seconds before stepping on the ink suggest that they probably could detect it, but they usually chose to ignore it.”

Having once stepped on ink and got their feet dirty, did they go on to avoid the tunnels in future? If so, monitoring rat abundance using tracking tunnels could be compromised.

“Experience of encountering the ink pad inside a black tunnel did not deter rats from running across another one, and would not affect indices of rat abundance reaching levels high enough to trigger conservation concern, but variation in behavioural reactions to tracking tunnels could possibly affect the detection rate of rats at low density.”

The full report can be read (by those with institutional access) via the New Zealand Journal of Zoology. The abstract is freely available online.

Do rats mind getting their feet dirty? Observing the behaviour of ship rats (Rattus rattus) towards footprint tracking tunnels (2017)