Photo by Walter Siegmund licensed under CC BY-SA 3.0
Any organism that lives in one place for a long enough time is going to have to deal with pests. For mammals, this often means fleas and ticks. Nests, dens, and other roosting spots tend to accumulate high numbers of these blood suckers the longer they are in use. As such, anything that can cut down on pest loads in and around the home has the potential to confer great advantages. Evidence from California suggests that wood rats may be using the leaves of a shrub to do just that.
Dusky-footed wood rats (A.K.A. packrats) build giant nests out of twigs and other plant debris. These nests serve to protect packrats from both the elements and hungry predators. Packrat nests can last for quite a long time and reach monumental proportions considering the size of the rat itself. Because they use these stick nests for long periods of time, it should come as no surprise that they can build up quite a pest load. Fleas are especially problematic for these rodents.
Photo by Peterson B Moose, U.S. Fish and Wildlife Service
A dusky-footed woodrat (Neotoma fuscipes) and its den. Photo by Martin Jambon licensed under CC BY 2.0
When researchers took a closer look at what packrats were bringing into their nests, they realized that not all plant material was treated equally. Whereas packrats actively collect and feed on leaves from various oaks (Quercus spp.), conifers (Pinus spp., Juniperus spp., etc.), and toyon (Heteromeles arbutifolia), the packrats seemed to have a special affinity for the leaves of the California bay (Umbellularia californica). However, instead of taking huge bites out of bay leaves, the rats appear to nibble them along the margin and spread them throughout their nest. What’s more, fresh bay leaves are brought in every few days.
This led some researchers to suggest that, instead of packrats using bay leaves as food, they may be using them to fumigate their homes. Indeed, California bay is rather chemically active. It is an aromatic shrub noted for its resistance to insect infestation. Of special interest to the research team were a group of chemical compounds called monoterpenoids. They noted that bay leaves were especially high in two types of of these compounds - 1,8-cineole (which gives the shrub its characteristic odor), and umbellulone (which has shown to be quite toxic to rodents). Why else would packrats bring something potentially deadly into their home other than to drive off pests?
Closer observation revealed that the packrats were in fact treating bay leaves differently than other leaves. For starters, bay leaves were disproportionately used to line the sleeping chambers within the stick nests. What’s more, the bay leaves were cycled out every 2 to 3 days. Even the nibbling patterns were significantly different. As mentioned above, bay leaves were merely nibbled along the leaf margins, which is an ideal place to nibble if releasing volatile compounds is the desired effect.
When researchers tested the effectiveness of a variety of leaves in the lab, their results added further evidence to the fumigation hypothesis. More than any other leaf found in packrat nests, bay leaves had clear negative effects on flea numbers. Flea survival in the lab was reduced by upwards of 75% when California bay leaves were present whereas flea survival was only reduced by less than 10% with all other leaves. It goes without saying that, whether they are conscious decisions or not, packrats definitely stand to benefit by decorating their homes with California bay leaves.
Further Reading: [1]