Sniffing out the biases in small mammal trapping

August 2020 Student of the Month - Chloe Warren

Undergraduate Research at University of Derby

Contact: chloeawarren@hotmail.co.uk     Twitter: @chloeawarren    Instagram: @ecology.chloe

The term ‘small mammals’ does not define a taxonomic group, but it is widely accepted and used to describe non-flying mammals that weigh less than 35g when fully grown. In the UK, this includes the rodents and the shrews, with one of the most abundant species being the wood mouse (Apodemus sylvaticus). Other common species include the bank vole (Myodes glareolus) and the common shrew (Sorex araneus). Each of these three species have been recorded in woodlands, heathlands, hedgerows, and even gardens! Although they are generally quite elusive creatures, small mammals are an integral component of ecosystems as they aid in seed dispersal, are top-down influencers of vegetation and invertebrate communities, and are a key food source for other wildlife such as birds of prey and terrestrial carnivores.

Typical habitat for small mammals is a matrix of interconnecting runways through grass and leaf litter that they use in both the day and night. The light conditions under the vegetation can be relatively poor, and so small mammals are highly dependent upon their nose! These animals receive information about their surrounding environment in the form of odours left behind by predators, competitors, and conspecifics in the form of urine, faeces, and other glandular secretions.

The standard method used by ecologists and mammal enthusiasts for assessing small mammal populations is live trapping and the use of the capture-mark-recapture method. This methodology relies on several assumptions, including that each individual within the population has an equal probability of capture. This implies that any odour left behind in the trap after an individual has been released from it does not affect the 'trappability' of subsequent individuals. Since small mammals use olfactory cues to assess their environment, there is a strong possibility that the residual odours left behind in traps affect the capture rates of small mammals and therefore have the potential to bias population estimates and assessments made from live-trapping surveys.

Some previous research has looked into the possible effects that odours have on the trapping success of small mammals, but the literature appears conflicted as to whether odorous traps are preferred by small mammals or not. For my independent study project on my BSc (Hons) Zoology degree at the University of Derby, which I have very recently virtually graduated from, I was keen to investigate this question further. I therefore studied the olfactory relationships of small mammals and aimed to determine whether traps that were cleaned with soapy water after containing an individual would incur a difference in capture rate than those that were not cleaned (which is the current standard method). My study site was a small private grassland field in Derby that features a number of hedgerows and is bounded by a brook to the north and a public footpath to the east. Also adjacent to the site are two fields, one of which is a regularly-used playing field and the other a fairly unimproved grassland used by local community members for dog walking.

Throughout my study, the most commonly captured species was the bank vole. Where possible, and following standard procedure, each captured individual was handled (Figure 1) to determine its gender and assess its overall health condition, and was given a small fur-clipping (Figure 2) to allow for identification should it be captured again.

It was interesting to note that the traps that were placed closest to the public footpath captured the most individuals, and these were predominantly bank voles. To me, this suggested that bank voles have a greater tolerance to human disturbance and odour. My finding on this supports earlier work by Dickman and Doncaster (1989) that highlighted how populations of wood mice did significantly better in undisturbed habitats than bank voles. My research found that significantly more small mammals preferred to enter clean traps compared to odorous ones, however there is a range of literature that has found evidence of the contrary. Much of this literature reports that where individuals preferred odorous traps, they had a strong preference for using traps that contain the odour of an individual of the same species. One explanation of the preference for clean traps is that they are less likely to contain the odour of a potential predator after they have been cleaned. 

Throughout my study, I also set up wildlife camera traps to record the activity of the animals as they approached and entered the live traps (Figure 3). This gave a novel insight into the behaviour of small mammals around traps, and documented individuals investigating the olfactory cues within the environment and in the trap. In Figure 3, a bank vole was observed investigating two traps that were placed side by side. The left trap was odorous and the right trap had been cleared with soap and water, and the bank vole entered the left, odorous trap. This highlighted that individuals do investigate their environment first and that odorous traps have the potential to bias the results of important ecological studies of these small inquisitive creatures.

I feel that the findings from my research provide evidence that the standard methods for small mammal trapping need to be further studied and possibly updated, so that where possible, clean traps are used as standard. More research is needed to ensure there is minimal bias in small mammal capture rates, and therefore more accurate population estimates for small mammals.

References

Dickman, C. & Doncaster, C. (1989) The ecology of small mammals in urban habitats. II. Demography and dispersal. Journal of Animal Ecology. 58:119-127.