[Vertebrate Pests] Grocery shopping in rodent communities

When people go food shopping, it is generally of three types. First, we tend to stock up on groceries on a weekly or bi-weekly basis. This is supplemented by additional, occasional trips to convenience stores for milk, bread, snacks, etc. And third, we supplement our food gathering by eating out in any number of eateries.
Well, without any anthropomorphic exaggerations (giving an animal human characteristics or behaviors), we can state that there is a similar food gathering and distribution system within domestic rodent communities. Most times, rodents find and consume food outside of their nests. But they also select and collect found foods and transport this food back to their burrows and/or nest zones and may actually store such food. Incredibly, they also may distribute found foods throughout the general rodent colony’s community area, which enables rodents beyond their own family unit to benefit.
In textbooks, this rodent behavior is often broadly discussed as “hoarding.” The most common example of hoarding in rodents is what we learn in grade school about tree squirrels gathering acorns and burying them all around a yard (i.e., saving the acorns for a “rainy day”). Of course, it is not that simple. In fact, it is far from it. Food “hoarding” in rodents, whether mice, squirrels (or some other mammal), is indeed a complex behavior pattern and one that is not very well understood. Moreover, hoarding is better defined as food resource utilization pattern behaviors. These behaviors can have dramatic impacts on professional rodent management programs.
Some of the best insights on domestic rodent food finding, feeding, collecting, transport, storage and distribution (i.e., food resource utilization) thus far has been provided by John Calhoun in his classic book The Sociology of the Norway Rat (1963) and in S. A. Barnett’s book, The Rat: A Study in Behavior (1977) as well as in some other research published in various journals.

RESOURCE UTILIZATION THOUGHTS. Taken altogether, the research indicates that food resource utilization by rodents and rodent colonies is complex and varies dramatically because it is influenced by several biological and non-biological factors. Nevertheless, here are some facts about food resource utilization we have learned from the research and, not any less important, from the many real-world observations from pest professionals like yourself, in the field.
• Under average conditions, most domestic rats and mice are opportunistic foragers. That is, they “seize the opportunity” and frequently eat all, or parts of, the food they encounter while foraging about at the point of encounter.
• When not consumed at point of encounter, the food item is often carried off in the mouth to a spot that provides shelter. This may be the rodent’s burrow or nest area, or simply some nearby area offering protection (e.g., the nearest corner, a nearby rock, an overhanging branch, etc.). Here the food may be consumed or simply dropped and left. This may happen repeatedly, and thus piles of food may accumulate at these locations.
• Rodents may transport food (and non-food) items and disperse them in different locales throughout their colony’s foraging areas. This is referred to as the “social dispersal of food.”
• Transported food that is deposited and left by one rat may never be retrieved by the same rat. It may be consumed by other rats or it may simply go to waste.
• While rodents are transporting food to another location, they often drop the food item as a result of a noise, a strange movement nearby or an encounter with another rodent.
• Rodents of lower social rank engage in more storage activity than do higher ranking rodents.
• Females hoard more food than males.
• Individuals subjected to periods of food deprivation hoard more than those rodents that weren’t.
• Storage of food into harborages occurs during all seasons of the year.
For those of us experienced in rodent control, we all have rodent hoarding stories. In a large racing horse barn, where I was trying to control a large Norway rat infestation, I had placed about 20 pounds of loose pellet bait into my bait stations. A week later, every bait station was completely empty. Two weeks later, I returned expecting good news from the client. “No improvement,” he groaned, and I suspected him of false complaining, or that perhaps someone was stealing my baits and taking them home for their own rodent problem. I rebaited each and every bait station. After two weeks of claiming no improvement, the client threatened to cancel.
Frustrated, I started digging up burrows and opening up stall-dividing walls. Sure enough, I retrieved more than 25 pounds of my rodent bait. The rats were taking the bait all right. They just weren’t eating it. It was like they were trying to set up their own drive-thru fast food store in the barn.
And just recently, I was working on a Norway rat infestation in a big mall environment. In one restaurant, the rats were “stealing” mayonnaise sachets from a container in the storeroom and carrying them back into their nest area in a cabinet base void about 10 feet away. Here, in the privacy and protection of their void burrow, they chewed into the sachets and ate the mayo (or spread it on their sandwiches).
I could go on with several more real-world cases. And so could you.

HOW RODENTS BENEFIT. How do rodents benefit by collecting, transporting, storing and possibly distributing the foods they find while foraging about instead of simply eating them there and then? Actually, this behavior may benefit an individual rodent/offspring and help to maximize the survivability rate of the general colony.
An individual rodent carrying the food off to a protected zone, for example, decreases the length of time the rodent remains stationary, which then decreases their exposure time to predators. Or, if food is not abundant and the rodent is hungry, carrying off the food enables the rodent to be able to consume the food without the threat of the food being taken from them (from perhaps a stronger rodent).
From a colony perspective, some researchers have hypothesized that rodents dispersing food throughout an area in which many colony members might encounter the food at a later time facilitates less conflict in the colony. In addition, such behavior increase the chances of rodents of different social ranks to have a chance to get food (such as newly weaned rodents, subordinate individuals and peripherally existing rodents). Thus, this helps the colony survive in case some of the dominant animals succumb to predation (including human extermination campaigns) or disease.
Additionally, should the colony’s primary food source suddenly disappear due to some environmental event (a massive human clean-up campaign, or the food is washed or blown away, covered over with snow or ice), food hoarded away in different locations now can provide an essential secondary source of food at least for some individuals to survive until the normal food supply returns.

EFFECT ON CONTROL PROGRAMS. Relative to pest management professionals’ everyday baiting programs, food resource utilization in rodent colonies is among the most important of all natural behaviors. Here’s why:
1. First and foremost, food resource utilization patterns by rodents only underscores the importance of ongoing sanitation in urban environments as part of IPM programs. If mice and rats inside restaurants, office buildings, alleyways, parks, school yards and classrooms, etc., have weeks, maybe months, of opportunity to collect and possibly “squirrel away” food items prior to any control programs, this has the potential of affecting how long it will take to eliminate the infestation with baits and traps.
In other words, poor sanitation practices may have a ripple effect on subsequent control programs. But certainly research is needed to measure the factors that may impact control programs. It is likely to vary from none to highly significant. Nevertheless, clean-up campaigns usually facilitate much better control results overall.
2. Choice of rodenticide bait formulation is critical. It is no surprise to any pest professional that rodents commonly carry off loose pellet-style baits as well as packet-style baits containing either pellets or ground meal. Just because the rodents take these bait formulations out of a bait station doesn’t mean the bait will be eaten by the rodents, or that the bait will be maximally consumed within a rodent community as we would hope. In other words: bait taken does not necessarily mean bait consumed.
3. From a safety aspect, we must be on alert of bait being translocated by rodents from one area to another where a child, a pet or wildlife might encounter the bait.
4. Because of the previous reasons, the block formulation of bait has grown in popularity. Blocks either can be secured into a station on a rod, or used within a station that due to its design, confines the block due to the block’s size or shape. Either way, rodents lose a great deal of their “option” to be able transport the bait away from the point of encounter.
The bottom line: we need to be appreciative of food resource utilization patterns among rodent colonies and employ the right formulations for maximum effect and safety. We also need to continue the good fight, namely, constantly informing our clients as to the importance of ongoing sanitation efforts.

The author is president of RMC Pest Management Consulting and can be reached at rcorrigan@giemedia.com or 765/939-2829.