A search for age-related changes in bite force and diet in shrews.

INTRODUCTION

Tooth attrition in soricine shrews (Insectivora: Soricidae) is rapid. Many invertebrates that form a major component of the diet of shrews (Yates, 1984; Nowak, 1991) have hard chitinous parts or contain particles of soil in their digestive tracts. This material and the requirement of shrews to masticate relatively large amounts of food (Broadbooks, 1939; Churchfield, 1993) form a highly erosive combination. Consequently, the teeth abrade quickly, often wearing flat and to the gumline despite the short life-spans of shrews (Pearson, 1945; Pucek, 1965; Hawes, 1975; Newman, 1976). Nevertheless, shrews with extensively worn teeth that we examined seemingly were in good physical condition and, in the appropriate season, usually were reproductively active. The possibility of older individuals exhibiting evidence of deterioration of condition or reduced productivity is unlikely as soricine shrews, irrespective of age, must consume relatively large quantities of food at regular and frequent intervals simply to remain alive (Churchfield, 1993).

Seemingly, to compensate for tooth abrasion, older shrews must adapt morphologically to increase bite force or they must alter their diets to use softer food items. Dramatic morphological changes that occur with age, in skulls of some shrews (Dehnel, 1949; Crowcroft and Ingles, 1959) suggest that bite force might be enhanced by increasing the mechanical advantage of the jaw lever or by providing relatively larger masticatory musculature. If bite force in shrews does not increase with age and older shrews shift their diets to softer items, such shifts might cause an increasingly greater overlap between trophic niches of older shrews of species with relatively high bite force and those of younger individuals of species with relatively low bite force. Older individuals of species with relatively high bite force might be hampered if they were forced to seek softer food items usually eaten by the more numerous younger age classes of a syntopic species. Interspecific competition is difficult to demonstrate in nature (Wiens, 1989), and we know of no report of a species of soricine shrew being limited in abundance or distribution by the actions of another, even though several species live in syntopy in North America (Fox and Kirkland, 1992), Europe (Pernetta, 1977) and Asia (Sheftel, 1994). Conversely, if older shrews shift to a softer diet, they might avoid trophic overlap with younger conspecifics.

Although changes in diet with age were noted in a wide variety of small mammals (Luo et al., 1994), we know of no empirical support for the premise that shrews or other species of wild mammals in older cohorts select softer foods because of tooth attrition. Some workers (e.g., Broadbooks, 1939; Terry, 1978) indicated that shrews they observed did not eat some relatively hard food items available, but these investigators did not comment on the age or condition of the teeth of the shrews.

Bite force is the result of the efficiency of jaw mechanics and the force exerted by the masticatory musculature (principally the temporalis muscle in shrews; Dotsch, 1983, 1985). The force that muscles can exert is directly proportional to their mass (Turnbull, 1970). In a previous study of bite force in 12 taxa of shrews from western North America, mass of the masticatory muscles was related directly to an index to body size. However, the mechanical efficiency of jaw morphology was correlated positively with the presence …