Validity of the Center of Activity Concept

August 1973 GENERAL NOTES 747 VALIDITY OF THE CENTER OF ACTIVITY CONCEPT Hayne (1949) presented a method of calculating a center of activity from data collected by the use of live traps and the mark-release technique. The concept was further developed by Dice and Clark (1953) and Calhoun and Casby (1958). The center of activity is the geometric mean of all captures and is simple to calculate. For many theoretical arguments it is useful to assume that the mathematical center of activity is close to the nest or home of the animal. This concept has been criticized by Tanaka (1963), and it is our purpose to document that this assumption is indeed not true for the old-field mouse, Peromyscus polionotus, and to discuss the evolutionary significance of the placement of the nest peripheral to the spatial distribution of activity. Three separate studies were conducted in South Carolina (Boize, 1970; Smith, 1971) and in central Florida (Smith, 1968). Mice were captured by live traps spaced 5, 9, or 15 meters apart on grids. One grid was in a 0.8-hectare rectangular enClosure (Boize, 1970). The others were in unenclosed old fields (Smith, 1968, 1971). Trapping was conducted over a variety of schedules always less than a month in duration. Data are presented only for mice that were captured four or more times. Home ranges were calculated by the minimum area method (Stickel, 1954). Immediately after cessation of trapping, the burrows of mice were excavated and the occupants captured by hand. Home ranges were smallest in the enclosure and ranged from 4 to 93, square meters with a mean of 32 ± 8 SE. The values for the other mice varied from 18 to 342 square meters with means of 118 ± 36 for South Carolina and 87 ± 28 for Florida. These values are close to those found by Caldwell (1964) and Davenport (1964) for P. polionotus in South Carolina old-fields, but lower than those found by Blair (1951) for P. polionotus on the beaches of Florida's Gulf Coast. The most important finding was that 32 (66.7 per cent) of 48 animals were found in burrows outside the calculated home range. There were 26 animals captured eight or more times and 13 (50 per cent) of these were found outside the c3.Iculated home range. The distance between the edge of the home range and the burrow was less than the intertrap interval in 36 (75 per cent) of the cases. Clearly the burrow or home is located on the outer edge of the home range and is definitely not at the mathematical center of captures. If sufficient trapping with more closely spaced traps had been conducted the distance between the center of activity and the burrow may have approximated the radius of the true home range. Other methods of calculating home range would include the burrow within the estimated area more often, but would not alter the conclusion that the burrow is on the periphery of the area of above-ground activity. The burrow of P. polionotus consists of an entrance tube, nest cavity, and escape tube (Fig. 1). In the Florida study (Smith, 1968), the orientation of the burrows was recorded but not reported. Four quadrants were formed by drawing a straight line along the entrance tube and extending through the nest cavity and the entrance of each burrow and another line perpendicular to the first and bisecting the nest cavity. Most captures (86.3 per cent) occurred in the two quadrants that included the entrance tube. In addition, the angle between a horizontal line passing through the ends of the entrance and escape tubes and a line from the entrance to the center of activity averaged 23.20 ± 6.10 (Fig. 1). Hence, orientation of the entrance tube was directed towards the calculated center of activity and if extended would pass near to this center. We have observed on road shoulders in the Ocala National Forest and beach sand dunes in Florida that captures. appear more uniformily distributed in the four quadrants but sufficient data are not currently available for analysis. The greater relief in these areas probably affects placement of the burrow and the distribution of above ground activity more than in a relatively flat old-field. The generality of these results for other small mammals is not known but the adaptive significance of the burrowing behavior may have a bearing on this question. Peromyscus- Vol. 54, No.3 JOURNAL OF MAMMALOGY 748 '\-0123.2 .... . \ Center of Activity \ \ \ \ \ \ \ \ \ Ground level /.. ~.:?:.~:~~T~?~~~~~~~:~~~··t· Sand Plug -- .. ': . :: :'. :.:.: :'. ': . .... '. :::. ':',' . .. Nest Cavity Lateral View· Top View FIG. 1. A schemat ic represen tation of the location of the center of activity in relation to the spatial orientation of the burrow of Peromyscus polionotus. Not drawn to scale. polionotus was derived from a P. maniculatm ancesto r as isolated populat ions on islands in the Florida region during the Pleistocene (Blair, 1950; Smith, 1966). The species probably first evolved in the relatively open beach habitat and develop ed the sterotyped burrowing behavior to avoid predation (Bowen , 1968). Locating the burrow peripher al to the center of above,g round activity would make it more difficult for a predato r to use information from direct or indirect observations of the mouse to precisel y predict the location of its burrow. Additionally, mice probabl y use other unoccup ied burrows in their home range (Smith, 1966) and the runways of insectiv ores (Davenp ort, 1964) as tempora ry refuges. This behavio r makes it more difficult for the predato r to spatially associat e the mouse with its burrow and nest. This type of argume nt could be applied to other species, and perhaps negates the usefulness of the concept of a center of activity for small mammals. This study was support ed by Contract AT(38- 1 )-310 between the Univers ity of Georgia and the U.S. Atomic Energy Commission. Paul Ramsey , Robert Gardner , and J. Whitfield Gibbons critically read the manuscript. Irma Smith prepare d the illustration. LITERATURE CrrED BLAIR, W. F. 1950. Ecological factors in speciati on of Peromys cus. Evoluti on, 4:253.275. 1951. Populat ion structure, social behavio r, and environmental relation s in a natural populat ion of the beach mouse (Peromyscus polionotus leucoce phalus). Contrib. Lab. Vert. BioI., Univ. Michiga n, 48:1-47 . BOIZE, B. J. 1970. Behavio ur and home range size in the old-field mouse, Peromyscus polionotus. Unpubli shed M.S. thesis, Univ. Georgia, 51 pp. \ August 1973 749 GENERAL NOTES BOWEN, W. W. 1968. Variation .and evolution of Gulf Coast populations of beach mice, Peromyscus polionotus. Bull. Florida State Mus., 12:1-91. CALHOUN, J. B., AND J. U. CASBY. 1958. Calculation of home range and density of small mammals. PubI. Health Monogr., 55:iv 1-24. CALDWELL, L. D. 1964. An investigation of competition in natural populations of mice. J. Mamm., 45:12-30. DAVENPORT, L. B., JR. 1964. Structure of two PeromysCU8 polionotus populations in oldfield ecosystems at the (...truncated)