Social Spacing

Group living and cooperation characterize the social behavior of the least shrew. Members of a nest group or colony share one nest and cooperate in burrow digging, huddle together when resting, and share a common home range. A home range is not defended and is just the geographical area that an animal uses. As noted in the habitat section, a single nest of least shrews has been found to contain up to 31 individuals, which contrasts with the antisocial behavior of other species of shrews. Other species of shrews are rarely found together in large numbers in the same nest and are more territorial and spaced out. However, in some other species such as in Crocidura russula, communal nesting is exhibited in the winter, most likely to reduce energy spent and conserve heat (Genoud and Hasser 1979 ; Vogel and Genoud 1981). In the least shrew, researchers state that the communal living likely occurs in order to conserve heat as well. One researcher discovered 31 individuals of Cryptotis parva in Nacogdoches county in eastern Texas (McCarley 1959). He discovered the nest in January and is is interesting to see that temperatures were cooler than normal for Texas at this time of year. For about 2 weeks before the authors found the nest, temperatures were significaly and consistently below average. The temperatures at night dropped down to about 32 degrees Fahrenheit. Therefore, “It seems probable that these 31 individuals had formed this large colony in order to share body heat, thus counteracting the effect of the prevailing cool temperatures" (McCarley 1959). As temperatures drop down lower and lower, it may be that communal nests of least shrews may increase in size in order to conserve more body heat. An interesting study could be conducted based on this premise in which least shrew nest size was analyzed in comparison to the ambient temperature.

In a study of the American shrew-mole, Neurotrichus gibbsii, researchers noted that as ambient temperatures decreased, the shrew mole reduced its body temperature and its overall thermal activity (Campbell and Hochachka 2000). On a few occassions, the shrews' metabolisms became hypometabolic. The authors speculate that the shrew mole decreases its body temperature and metabolic rate when food resources are scarce or when ambient temperatures are low for an extended period of time (Campbell and Hochachka 2000). This lowering of metabolic rate and body temperature thus allows the shrew mole, a species sharing a good number of characteristics with the least shrew, to conserve energy in times of food shortage or low temperatures. As a similar species, least shrews may be able to engage in similar behavior by lowering their metabolisms and also may conserve energy during periods of cold weather via their communal nesting behavior.

The least shrew is not territorial. Most species of shrews are territorial, this territoriality stemming from fairly scarce and evenly distributed food resources, but there exists no territoriality in the least shrews. Some information suggests that the lack of territoriality in the least shrew can be attributed to the fact that they live in mostly warm regions with poor food resources. It also may be that the least shrew does not need to be territorial because it can make up for sharing of food resources via communal nesting. Lowering its metabolic rate and communally nesting to conserve heat may offset the need to maintain a territory in order to acquire sufficient food resources. Splitting up the resources in the least shrews' home ranges may be feasible because of this energy-saving nesting. Even though prey items may be scarce and territoriality can begin to be seen more readily in other shrew species, the least shrew simply creates these nest communities to conserve energy. By conserving this energy through communal nesting, they do not need to hold down a territory in order to get sufficient prey to survive. Another benefit may be that they do not need to expend energy in order to fend off other shrews and protect a territory. Protecting a territory can be highly energetically demanding. Territoriality due to population density may be limited or not seen in the least shrew because as the number of individuals taking part in communal nesting increase, so do the heat and energy saving benefits increase. With more individuals inside the nest, heat can be more easily conserved, and thus population density may not play a significant role in this species. Naturally, there must be a threshold or upper limit on number of individuals where the negative effects of population density begin to take effect. At this upper threshold, the cost of food sharing within the home range cannot be offset by the benefit of energy-saving communal nesting. However, we have seen that this number of individuals may be quite high, with as many as 31 individuals being found in one communal nest.

Little information is available on the population dynamics of the least shrew because it is so hard to trap (Whitaker 1974). In a study of the home range of the least shrew, it was estimated that the home range of a female is 0.57 acres and the home range of the male is equal to approximately 0.41 acres (Howell 1954). In this study, Howell attempted to determine the population sizes and home ranges of six small mammals, one being the the least shrew, in a field near Knoxville, Tennessee. As expected, the population density was hard to determine because not many leasts shrews were able to be trapped and marked. It was noted that home range data for the least shrew was hard to determine as well due to difficulties in trapping and high mortality rates throughout the study period. During the study period, one female was trapped nine times and one male was trapped five times. Howell does note that it is almost certain that the recorded home range of the male would have been larger if it had been trapped more times (Howell 1954). After examining the ecology and mating behavior of the least shrew, I believe this to likely be the case. The male probably does possess a larger home range than the female in order to overlap the home ranges of several females and engage in promiscuous copulatory behavior. Howell also notes that the measured home ranges may be smaller than the shrews' actual home ranges or the results may have been minimized because of the techniques used to measure home ranges in the study and also because of the low number of captures. In the research community, home ranges determined by less than ten total captures are considered likely to be inaccurate.

Although the least shrew does not engage in territorial behavior, many other species of shrew do in fact maintain territories. This information on other species and their territoriality is simply provided to present a contrast to the least shrew's behavior. Analyzing the territoriality in other species of shrew can help to shed light on why exactly the least shrew is not territorial itself. The majority of other shrew species who maintain territories do not exhibit communal nesting like the least shrew. These species need to be solitary, aggressive, and must viciously defend their territories in order to acquire sufficient food resources. On the other hand, the least shrew does not require as much energy from food resources due to the heat and energy conserving benefits of communal nesting.

In the species Sorex araneus and Sorex minutus, aggression and fights are very important for maintaining territoriality (Michielsen 1966). Territorial behavior has been documented in the shrew species Sorex vagrans and Sorex obscurus as well (Zuri and Rado 2000). This territorial behavior ceased when the breeding season began (Hawes 1977). Hawes analyzed the spatial and temporal home ranges of the sympatric species Sorex vagrans and Sorex obscurus. Intraspecific and interspecific competition occured when defending territories and in both of the shrew species, when the breeding and reproduction season began, the home ranges of individuals got larger. During this period, the males’ ranges get bigger than do the females’ ranges, but both do in fact expand in size. Hawes discovered that at the onset of the breeding season, the home range of male multiplies in size by a factor of five. The fact that the males' home ranges grow larger than the females during the breeding season lends support to the notion that the least shrew exhibits overlap promiscuity in its breeding behavior and social spacing. Throughout the lifespan, these shrew species tend to have the same home range. This is particularly true of the female members of a community. Individual home ranges do not overlap between species because of the presence of interspecific territoriality and because of the ability of each particular species to compete better than another species in its own preferred habitat (Hawes 1977).

Territorial behavior comes to a halt when the shrews become reproductively active. Gaining a territory at a young age ensures successful individuals enough food for them to tide over the tough times of the winter. Establishing territories also acts as a density regulation mechanism for the populations of shrews. Hawes further suggests that these species are territorial primarily because of population density issues in times when sufficient prey is available (Hawes 1977). So in most cases, it seems that a shrew does not maintain a territory due to the need for food resources, but may exhibit territoriality when food is scarce.

This website was completed in partial fulfillment of the requirements for Biology 323, Animal Behavior, at Davidson College in the Spring Semester 2008.

Please send any comments, questions, or suggestions regarding this website to Nate Geigle or Professor Verna Case.