Behavioral Interactions between Microtus and Sigmodon: A Model for Competitive Exclusion

Abstract

An alleged negative interaction between Microtus ochrogaster and Sigmodon hispidus was examined in the laboratory. Sigmodon proved to be the dominant animal, and excluded Microtus from confined areas except when dense cover was provided. Movements of Microtus were reduced by a free-ranging Sigmodon but not by a confined one. Frequency of interspecific contact apparently was responsible for negative interactions. A model that emphasizes frequency of interspecific contact (FIe) explains the probable relationship between Microtus ochrogaster and Sigmodon hispidus; a variation of this model might explain the phenomenon of competitive exclusion in all myomorph rodents.The prairie vole, Microtus ochrogaster, and the hispid cotton rat, Sigmodon hispidus, occur sympatrically near the peripheries of their respective ranges in central Kansas. Microtus ochrogaster has long inhabited the central Great Plains (Hibbard, 1970), whereas Sigmodon hispidus is an invader from the south that first appeared in central Kansas about 40 years ago (Cockrum, 1948). Northward dispersal of S. hispidus onto the Great Plains apparently has coincided with periodic declines in population density of M. ochrogaster; Martin (1956), Frydendall (1969), and Terman (1973 gave examples of this, and suggested that there is a negative interaction between the two species in the field. However, their studies were conducted under natural conditions and lacked precise information derived from systematically manipulated variables. The following hypotheses were suggested by those field results and by the observations and comments of other investigators.If interspecific aggression between Microtus and Sigmodon is the mechanism favoring cotton rats over voles (Baker, 1971;Martin, 1956), then Sigmodon should be aggressive toward Microtus and should be dominant in most interspecific encounters. Therefore, I hypothesized that Sigmodon would actively attack Microtus and would be aggressively dominant in interspecific pairings.If Sigmodon limits the spatial distribution of Microtus (Baker, 1969;Fleharty and Olson, 1969;Terman and Johnson, 1971), this phenomenon should be observable in a laboratory arena. Therefore, I hypothesized that voles would reduce their use of areas occupied by Sigmodon. However, it is possible that the amount of habitat cover (complexity) might affect this relationship (Crombie, 1946; Krebs et ai., 1971;MacArthur, 1972); thus, I hypothesized that dense cover would enable Microtus and Sigmodon to occupy the same area.If the Microtus-Sigmodon interaction lacks interspecific recognition and avoidance (Baker, 1971;Brown, 1966; Calhoun, 1963;Jameson, 1947), cotton rats should affect the· movement of voles only when actual physical contact is involved. Therefore, I hypothesized that movements of voles would not be 705

The Effect of Sigmodon Hispidus on Spatial and Temporal Activity of Microtus Ochrogaster: Evidence for Competition

“…The time lag between the onset of cotton rat reproductive activity and allotopy (Fig. 2) is expected under Terman's (1974) frequency of interspecific con tact exclusion hypothesis. According to his hypothesis the rate of interspecific contact is an increasing func tion of the numbers of cotton rats.…”

“…communication is used and was not observed. Or, as Terman (1974) suggested, expe rienced voles (field caught adults, in this case) have learned to avoid cotton rats. Regardless of the cause for avoidance, the tendency remains in small mam mals, for the larger member of interspecific pairs to be dominant (Grant 1969, Terman and Johnson 1971, Ter man 1974, Ambrose and Meehan 1977, Petersen and Helland 1978, Randall 1978 and for the general inter action to be one of avoidance by subordinate individ uals (Eisenberg 1963, Murie 1971, Turner and Iverson 1973, Ambrose and Meehan 1977, Petersen and Hel land 1978, Randall 1978.…”

“…During daylight hours animals were ob served unaided. After dark, observations were made using a 25-watt red light (Terman 1974) suspended 30 cm below the arena, and a Startron nightscope.…”

“…Cockrum (1948) studied the northward dispersal of Sigmodon hispidus Say and Ord, the cotton rat, in Kansas fro m 1933until 1947. Martin (1956 and Terman (1974) noted that this expansion coincided with declines in the abundance of Microtus ochrogaster (Wagner), t he prairie vole, and concluded that inter specific competition with S. hispidus was responsible for the M. ochrogaster population declines. This hy pothesis was supported by Frydendall's (1969) study in central Kansas in which he demonstrated that, in the presence of S. hispidus, M. ochrogaster failed to occupy re mnant prairie grassland but with the decline of S. hispidus, M. ochrogaster from peripheral habitat migrated i nto the grassland.…”

“…Martin (1956) suggested that S. hispidus excluded M. ochrogaster from suit able ha bitat through aggressive interference. Terman (1974) stu died behavioral interactions of S. hispidus and M. ochrogaster and reported that S. hispidus were aggressive towards M. ochrogaster in laboratory enclosures b ut that the frequency of interspecific con tact determined the degree of exclusion between species in the field. This paper reports the results of a laboratory study of spatial and temporal activity patterns of mixed and single spe cies groups of M. ochrogaster and S. his pidus.…”