Hoarding (animal behavior)

This Western Scrub Jay will store this peanut in a cache for later retrieval

Hoarding or caching is a type of animal behavior where an animal will store its food in times of surplus for times when food is less plentiful. The term hoarding is most typically used for rodents, whereas caching is more commonly used in reference to birds, but the behaviors in both animals groups are quite similar.

The place where the food is stored is called a cache or hoard. They are usually hidden from the sight of competing individuals of the same species as well as others.

Hoarding is done either on a long-term basis – cached on a seasonal cycle, with food to be consumed months down the line – or on a short term basis, in which case the food will be consumed over a period of one or several days.

Some common animals that cache their food are rodents such as hamsters and squirrels, and many different bird species, such as rooks and woodpeckers. The Western Scrub Jay is noted for its particular skill at caching.

There are two types of caching behaviour: larder-hoarding, where a species creates a few large caches which it often defends, and scatter-hoarding, where a species will create multiple caches, often with each individual food item stored in a unique place. Both types of caching have their advantage.

Hoard distribution and size

Scatter hoarding is the formation of a large number of small hoards or caches of nuts and other seeds. Many species of squirrel, including the eastern gray squirrel and the fox squirrel, are well known for scatter hoarding. This behavior plays an important part in seed dispersal, as those seeds that are left uneaten will have a chance to germinate, thus enabling plants to spread their populations effectively.

Cache spacing is the primary technique that scatter hoarders use to protect food from pilferers. By spreading the food supply around geographically, hoarders discourage competitors who happen upon a cache from conducting area-restricted searching for more of the supply. Despite cache spacing, however, hoarders are still unable to completely eliminate the threat of pilferage. However, having multiply cache sites is costly because it requires a good memory. Scatter-hoarders generally have a large hippocampus.

Many animals are scatter hoarders, including squirrels, chipmunks, and the wood mouse.

In larder hoarding, the hoard is large and is found in a single place termed (a larder), which usually also serves as the nest where the animal lives. Hamsters are famous larder hoarders. Indeed, the word "hamster" is derived from the German verb "hamstern" which means "to hoard"; similar verbs are found in various related languages (Dutch hamsteren, and Swedish hamstra). Other languages also draw a clear connection between hamsters and hoarding: Polish chomikować, from chomik – hamster; Hebrew hamster; oger (אוגר) comes from to hoarde; le'egor (לאגור).

A disadvantage of larder hoarding is that if a cache is raided, this is far more problematic for the animal than if it were a scatter hoarder. While the hoard is much easier to remember the location of, these larger hoards must also be more staunchly defended.

Behaviors related to hoarding

Guarding

Most species are particularly wary of onlooking individuals during caching and ensure that the cache locations are secret. Not all caches are concealed however, for example shrikes store prey items on thorns on branches in the open.

Shared or individual hoarding

Although a small handful of species share food stores, food hoarding is a solo endeavor for most species, including almost all rodents and birds. For example, a number of jays live in large family groups, but they don't demonstrate sharing of cached food. Rather, they hoard their food supply selfishly, caching and retrieving the supply in secret. (Waite, 1992) There are only two species in which kin selection has resulted in a shared food store: Beavers (Castor canadensis) and Acorn Woodpeckers (Melanerpes formicivorous); the former live in family groups and construct winter larders of submerged branches, while the latter are unusual in that they construct a conspicuous communal larder (Koenig and Mumme, 1987).

Pilferage occurs when one animal takes food from another animal's larder. Some species experience high levels of cache pilferage, up to 30% of the supply per day. Previous models of scatter hoarding developed by Stapanian and Smith (1978, 1984), Clarkson et al. (1986), and others, suggested the value of cached food is equal to the hoarders ability to retrieve it. (cited from Vander Wall, 2003)

Reciprocal Pilferage

It has been observed that members of certain species, such as rodents and chickadees, act as both hoarder and pilferer. In other words, pilfering can be reciprocal and, thus, tolerable. Although this kind of food caching system seems cooperative, it has been suggested that it is actually driven by the selfish interests of the individual. (Vander Wall and Jenkins, 2003)

Recaching

Animals recache the food that they've pilfered from other animal's caches. For example, Vander Wall and Joyner (1998) found that 75% percent of radioactive Jeffery pine seeds cached by yellow pine chipmunks were found in two cache sites, 29% of the seeds were found in three sites, 9.4% were found in four sites and 1.3% were found in five sites over a 3-month period. These results, and those from other studies, demonstrate the dynamic nature of the food supplies of scatter hoarding animals.

References

• Clarkson K, Eden SF, Sutherland WJ, Houston AI, (1986) Density, dependence and magpie hoarding. J Anim Ecol 55:111-121.
• Jenkins, Stewart H. and Breck, Stewart W. (1998) Differences in food hoarding among six species of heteromyid rodents. J Mammal. 79:1221-1233.
• Koenig WD, Mumme RL. (1987) Population ecology of the cooperatively breeding acorn woodpecker. Princeton, New Jersey. Princeton University Press. 0691084645
• Stapanian, MA, Smith CC. (1978) A model for seed scatterhoarding: coevolution of fox squirrels and black walnuts. Ecology 59:884-896.
• Stapanian MA, Smith CC. (1984) Density-dependent survival of scatterhoarded nuts: an experimental approach. Ecology 65:1387-1396.
• Vander Wall SB, Joyner JW. (1998) Recaching of Jeffery pine (pinus jefferyi) seeds by tallow pine chipmunks (Tamias amoenus): potential effects on plant reproductive success. Can J Zool 76:154-162.
• Vander Wall, Stephen B. and Jenkins, Stephen H.(2003) Reciprocal pilferage and the evolution of food-hoarding behavior. Behavioral Ecology. Vol. 14, No. 5.
• Waite TA. (1992) Gray jay scatterhoarding behavior, rate maximization and the effect of local cache density. Ornis Scand 23:175-182.