Metabolism

Least shrews have extremely fast metabolisms and because of their small body size, must forage and eat almost constantly. Digestion also occurs quite rapidly. As a result of the need for large quantities of food and a high energy lifestyle, least shrews usually only live about eight to twelve months. Shrews generally have high metabolisms and the smallest species of shrews are the smallest endotherms possible, considering their high surface to volume ratio and the excessive heat loss which comes along with this. This fact is largely agreed upon, with many researchers believing that the smallest species of shrews “lie close to the theoretical lowest limit of adult mammalian size" (Redmond and Layne 1958).

It has been recorded that a 4.7 gram least shrew eats approximately 3.6 grams of food a day (Hamilton 1944). Another researcher that a 5 gram Cyrptotis P. floridana, a subspecies of Cryptotis parva, ate an average of 5.5 grams of food per day for over 33 days (Springer 1937). Hamilton suggests that captive least shrews eat larger quantites of food than do feral least shrews. He hypothesizes that this is probably because wild shrews have a wider variety of food to choose from and also have more natural foods. In another study, researchers measured the food intake and assimilation efficiency of an adult female least shrew over a period of 21 days (Barret 1969). The shrew was fed newborn baby mice, which were readily consumed by the female. The results show that the least shrew ate more than its own body weight each day over the 3 week period that the research was done, producing a 1.1 ratio of live weight food per gram live weight animal per day. Springer and Hamilton's ratios are similar, with their research results producing a 1.1 ratio and an 0.8 ratio, respectively. Food requirements could be the limiting factor in the natural environment for the least shrew, as the least shrew is a secondary and tertiary consumer species, feeding mostly on insects (Barret 1969).


	Image courtesy of F.A.Cervantes Reza and the Mammal Image Library of the American Society of Mammalogists. http://www.mammalogy.org/mil_images/MSLInsectivora.htm

Lowering of Metabolic Rate

In an interesting study of metabolism, researchers examined the metabolic rates of various types of tissues of small mammals (Redmond and Layne 1958). They examined the metabolic rates of kidney, lung, diaphragm, and liver tissue. The researchers found that the metabolic rate is not correlated with body size in some body tissues. All least shrew tissue types, excepting the kidney tissue, showed lower metabolisms than the tissues of the other small mammals involved in the study, despite the fact that the least shrew's overall metabolism was the highest among the small mammals studied. Additionally, the kidney was the only tissue that had a high metabolic rate in both the least shrew and the harvest mouse. This was not understood by the researchers and cannot be explained in terms of diet, because the mouse is herbivorous and the shrew is carnivorous. So essentially, compared to the metabolisms of other small mammals such as the white mouse, white rat, and rabbit, the shrew’s had the lowest tissue metabolism. This finding suggests that the high metabolisms of shrews stem mainly from “extrinsic factors such as nervous stimulation, hormone levels, or concentrations of metabolites in blood or tissue fluids rather than to generally higher ‘inherent’ rates of tissue metabolism” (Redmond and Layne 1958).

Adaptive Significance of Lowered Metabolism

A similar trend of lowered tissue metabolism was shown in the rodent Reithrodontomys, a mammal of similar size to the least shrew. This demonstrates that the depression in metabolism of certain tissues may just be an adaptation for small mammals in general and is not specific to the shrew and its phylogenetic line. Redmond and Layne hypothesized that the least shrews' lowered metabolism could be due to the relatively primitive status of the insectivores among other mammals. In other studies, the metabolism of the mole has also been measured and it has been speculated that insectivores in general have inherently low metabolic rates which could by a physiological indication of their “primitive nature”. Researchers further hypothesize that the high metabolic rate of shrews may function as an adaptation, compensating for heat loss (Redmond and Layne 1958). The lowered metabolism of certain tissues that were observed in these very small mammals may aid them in conserving energy during periods of inactivity. The shrews may lower their metabolism when not active, thus conserving a good deal of energy for later periods when it is necessary.

It also is interesting to note that shrews of the species Blarina and Cryptotis sleep very soundly and when they awaken, they make feeble and slow movements. These shrews take awhile to get going, akin to bats emerging from a torpid state. These observations may indicate a lowering of the metabolic rate while the shrew is sleeping. However, measurements show that the shrew’s body temperature, unlike the bat's body temperature, is not lowered during periods of sleep. Therefore, further research and measurement of respiratory rates while the shrew is sleeping may help in shedding light on this situation.

References Page

Least Shrew Home Page

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.