. Biology of the Heteromyidae. Heteromyidae. < 0 ^ 1972 1973 1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 Q. 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 YEAR Fig. 2. —Patterns of long-term fluctuation of heteromyid populations at two sites approximately 200 km apart: above, D. merriami and P. amplus in the Sonoran Desert approximately 40 km northwest of Tucson, Arizona (data from Petryszyn, 1982, and unpubl.); below, D. merriami and P. flavus in the Chihuahuan Desert approximately 5 km east of Portal, Arizona (data from Zeng and Brown, 1987a, and Brown and co-workers, u
. Biology of the Heteromyidae. Heteromyidae. < 0 ^ 1972 1973 1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 Q. 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 YEAR Fig. 2. —Patterns of long-term fluctuation of heteromyid populations at two sites approximately 200 km apart: above, D. merriami and P. amplus in the Sonoran Desert approximately 40 km northwest of Tucson, Arizona (data from Petryszyn, 1982, and unpubl.); below, D. merriami and P. flavus in the Chihuahuan Desert approximately 5 km east of Portal, Arizona (data from Zeng and Brown, 1987a, and Brown and co-workers, unpubl.)- Note that D. merriami shows substantial fluc- tuations at one site but not the other, whereas the Perognathus populations are highly variable at both sites. Both species o^Perognathus become torpid, accounting especially for the midwinter lows at the Chihuahuan Desert site. cline during the subsequent droughts. The timing of reproduction appears to be cued, at least in part, by substances ingested along with new plant growth, so that the first lit- ters are weaned at about the time that abun- dant seed crops are produced (Bradley and Mauer, 1971; Chew and Butterworth, 1964; Reichman and Van De Graaff, 1975; Reyn- olds, 1958; Van DeGraaflf and Balda, 1973). Despite facultative timing of reproduction to coincide with high food availability, the rate of population increase is limited by the relatively small size of litters and the long intervals between litters. Even during fa- vorable times, adults appear to devote much of their energy to activities, such as seed collection and caching, that tend to promote their own survival at the expense of repro- duction. The ability to utilize stored food, torpor (in all genera except Dipodomys), and effective predator avoidance strategies fa- cilitates survival and tends to prevent high rates of population decrease during the un- favorable dry periods after most of the available food has been harvested. Nevertheless, despite life
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