Monitoring small mammals using predator scats

Bank vole

Bank vole photographed by Maurice Flynn in Co. Waterford, Ireland.

Ireland has few species of small mammals, and only some of these are native. The pygmy shrew (Sorex minutus) and the wood mouse (Apodemus sylvaticus) are considered to be native to Ireland. However, more recently introduced species including the bank vole (Myodes glareolus) and the greater white-toothed shrew (Crocidura russula)  have been recently shown to affect the distribution of our indigenous species (Montgomery et al. 2012). This results in a situation where the invasive species out-competes the native one, similar to the red squirrel (Sciurus vulgaris) and grey squirrel (S. carolinensis) situation. Unlike the squirrel situation, which is easier to directly observe, the lives of small mammals often go unnoticed, and directly monitoring these species requires trapping. Given our tightened economic situation, it is becoming more difficult to obtain funding for this kind of direct monitoring. It led us to believe that there must in an indirect alternative…

The pine marten (Martes martes) has recently recovered from very low numbers in Ireland. The most recent survey of pine marten in Ireland was based on a survey by Declan O’Mahony and co-authors. O’Mahony surveyed  woodlands in Ireland for signs of pine marten. To do this, O’Mahony walked woodland tracks and collected scats that resembled those of the pine marten. There is a list of key characteristics used to identify pine marten scats, including smell, size and shape. However, even when using these clues, identification can still prove inaccurate, and quite often scats can be misidentified as fox, mink, stoat, or even polecat (only found in Britain). Experts now agree that DNA testing is the most accurate way of identifying scats.

Pine marten photographed by Maurice Flynn in Co. Waterford

Pine marten photographed by Maurice Flynn in Co. Waterford

In light of this, O’Mahony et al. extracted DNA from all the scats that were used to map the current Irish pine marten distribution. Many of the scats collected from O’Mahony’s study overlapped with the known range of the bank vole and greater white-toothed shrew. Consequently, it may have been possible that the pine marten had been predating on these small mammals, and analysing the scats for the presence of these small mammals could help understand their distribution in more detail.

We decided to test our idea using DNA analysis. As DNA had previously been extracted from the pine marten scats, there was a further possibility that if pine marten had been predating on small mammals, then the DNA of their prey would also be present in the scats. We used  tests specifically designed to target  small mammals including the pygmy shrew, wood mouse, bank vole and greater white toothed-shrew. We found what we were looking for, and were subsequently able to map the distribution of the species, based on where the pine marten scat had originally been collected.

This is an important finding for us, as we have now been able to map the distribution of the small mammals based on scats that were collected during the period of 2006-2007. Surprisingly, we found that the greater white-toothed shrew was more widely dispersed than  we had originally considered it to be at that time. When Tosh et al. (2008) first found the skulls of the shrew in owl pellets collected in Tipperary and Limerick, they were unable to pinpoint the source of the introduction, although it appeared to be have been clustered in that general area. Our new results have shown us that the greater white-toothed shrew was also in Co. Laois during this time, suggesting that the greater white-toothed shrew has been present for longer in Ireland than we previously thought.

Red squirrel in Co. Waterford, photographed by Maurice Flynn.

Red squirrel in Co. Waterford, photographed by Maurice Flynn.

Could we detect squirrel DNA in pine marten scats?

We also used additional pine marten scats from other sites to investigate if our DNA techniques could also identify squirrels in the diet of the pine marten. Red squirrels are known to feature at very low quantities in the diet, and we were unsure if we would even detect grey squirrel from any of our sites. We decided to head to Wales, where we could conduct a feeding trial with a pine marten that had been bred in captivity. The pine marten was fed grey squirrels that had been culled for a management programme in mid-Wales. After extracting the DNA from the scats, we found that we were successfully able to identify grey squirrel DNA from the pine marten scats. This meant that if we happened to come across a pine marten scat in the wild that had been feeding on grey squirrel, our techniques would be able to identify it.

We also joined forces with the  Irish Squirrel and Pine Marten Project, and tested pine marten scats from known red squirrel sites to see if we could detect the species in the pine marten diet. We were able to detect red squirrel DNA in a small proportion of the overall scat collection, but did not detect any grey squirrel DNA. More intensive studies focusing on sites where both pine marten and grey squirrel both occur are needed to investigate the occurrence of grey squirrel in the pine marten diet, but we have shown with this study, that this is a useful way to detect low frequency prey items in lots of pine marten scats.

Did the results tell us anything interesting about pine martens in Ireland?

Our results, while providing us with valuable information about the distribution of small mammals in Ireland, also showed us that pine marten do not eat large quantities of small mammals. In fact, just over 60% of all the scats did not contain any of the small mammal DNA we were checking for. This means that other items such as fruit, insects and birds are probably all important items in the Irish pine marten diet. This isn’t  hugely surprising, as Ireland contains fewer small mammal species than Britain and mainland Europe. The field vole is also absent in Ireland, and that has been shown to be a significant prey item in Scotland.

Pine marten photographed by Maurice Flynn

Pine marten photographed by Maurice Flynn

Overall implications

While we’re not suggesting that our technique is a replacement for traditional hard-part analysis studies, we do believe that the strategy is a useful one when looking for particular species of interest, such as small mammals and squirrels. The molecular analysis used in this study could also be applied to scats of other predators such as the fox, mink or even bird of prey pellets. Using a DNA technique to test hundreds of samples across a broad geographical area is a useful and efficient way to assess a species’ distribution, and may be especially useful for monitoring invasive and indigenous species in Ireland. This approach could be adopted in other countries too, by simply designing primers to identify the target species of interest.

The paper can be accessed online or alternatively, contact me directly.


O’Meara, D., Sheehy, E., Turner, P., O’Mahony, D., Harrington, A., Denman, H., Lawton, C., MacPherson, J., & O’Reilly, C. (2013). Non-invasive multi-species monitoring: real-time PCR detection of small mammal and squirrel prey DNA in pine marten (Martes martes) scats Acta Theriologica DOI: 10.1007/s13364-013-0155-8

O’Mahony, D., O’Reilly, C., & Turner, P. (2012). Pine marten (Martes martes) distribution and abundance in Ireland: A cross-jurisdictional analysis using non-invasive genetic survey techniques Mammalian Biology – Zeitschrift für Säugetierkunde, 77 (5), 351-357 DOI: 10.1016/j.mambio.2012.04.001

Montgomery, W., Lundy, M., & Reid, N. (2011). ‘Invasional meltdown’: evidence for unexpected consequences and cumulative impacts of multispecies invasions Biological Invasions, 14 (6), 1111-1125 DOI: 10.1007/s10530-011-0142-4

Tosh DG, Lusby J, Montgomery WI, O’Halloran J (2008). First record of greater white-toothed shrew Crocidura russula in Ireland. Mammal Review : DOI: 10.1111/j.1365-2907.2008.00130.x

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