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Onychomys torridus ramona

(Southern Grasshopper Mouse)

Interesting Facts

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Common Names

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Common Names in English:

Southern Grasshopper Mouse

Description

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Habitat

The southern grasshopper mouse rangewide is found in low arid scrub and semi-scrub vegetation (Frank and Heske 1992; McCarty 1975) and the subspecies O. t. ramona known from the MSHCP planning area is found in grasslands and sparse coastal sage scrub habitats. They nest in burrows, and while they may dig their own burrows in sandy or other friable substrates, they often use burrows dug by other rodents such as pocket gophers (Thomomys spp. ) and kangaroo rats (Dipodomys spp.) (Baily and Sperry 1929; Stapp 1997). Specific habitat requirements of the southern grasshopper mouse generally are unknown, but Stapp (1997) found that grasshopper mice use open areas and microhabitats dominated by gopher mounds and burrows. Stapp suggests that grasshopper mice selected these microhabitats because of greater prey availability (i.e. , arthropods using burrows for refuge), greater mobility in open areas, and dust bathing sites.

Two of the three precision level 1 data points for the grasshopper mouse in the MSHCP planning area are located in non-native grassland habitat between Lake Skinner and Diamond Valley Lake, and the other is located in Riversidean sage scrub in the same area. Likewise, in the San Felipe Valley of northern San Diego County, the grasshopper mouse was trapped in non-native grassland/ruderal habitat (P. Behrends, pers. obs. 1992). Ten of the other less precise data points for the MSHCP data base occur in natural habitat areas mapped as chaparral (3 points), Riversidean sage scrub (2 points) and non-native grassland (5 points).

Typically found at an altitude of 0 to 964 meters (0 to 3,163 feet).[1]

Ecology: Coastal chaparral communities (Hafner et al. 1998). (Ref. 81135)


List of Habitats : 4.5 Grassland - Subtropical/Tropical Dry

Biology

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Diet

Studies of diet agree that the majority of the southern grasshopper’s diet consists of arthropods (the phylum Arthropoda includes crustaceans, insects, centipedes, millipedes, and arachnids) (Baily and Sperry 1929; Horner et al. 1965; McCarty 1975; Stapp 1997). For example, Stapp (1997) found that 85% of the mouse’s diet was comprised of arthropods. Vertebrate remains were found in only 7% of fecal samples . The species account prepared by McCarty (1975) reported that the grasshopper mouse’s diet is composed almost entirely of arthropods, with beetles, scorpions, and grasshoppers as the primary prey , but also little pocket mice (Perognathus longimembris) and harvest mice (Reithrodontomys megalotis), two very small rodent species of 7-10 g body weight . Baily and Sperry (1929) reported a somewhat broader diet: insects; rodents, including kangaroo rats (Dipodomys spp), pocket mice (Chaetodipus spp. and Perognathus spp.), deer mice and white-footed mice (Peromyscus spp,); and seeds and herbaceous materials . The grasshopper mouse is an active predator , and most of its prey are terrestrial and nocturnal (Horner et al. 1965). Prey are stalked , rushed and seized and typically killed with a bite in the head .

As described above, Stapp (1997) examined habitat selection in relation to prey availability and suggested that grasshopper mice select microhabitats with gopher mounds and other rodent burrows that appear to attract insect prey. According to the species account prepared by McCarty (1975), prey items may be consumed in the burrow and other foods may be cached for later consumption .

Reproduction

The timing of breeding probably varies geographically and in relation to environmental conditions , but the peak breeding season is May through July (McCarty 1975). Frank and Heske (1992) documented breeding from early spring to late autumn in southeastern Arizona. McCarty (1975) reported litter sizes of 1-6 pups , with a mean litter size of 2.6 and Baily and Sperry (1929) reported litter sizes of 3-5 pups. Gestation is 27-30 days and the typical interval between litters is 27-35 days. Grasshopper mice exhibit post-partum estrus and can produce up to 12 litters in a year (McCarty 1975). Grasshopper mice are sexually mature at an early age and females born in April can produce two or three litters their first year. Females born late in the year may have as many as six litters the following breeding season . Ovulation is physiologically possible by 42 days of age, with females’ first litters appearing at 4-5 months of age. Males begin producing sperm by 40 days of age. Fecundity (the measure of reproductive output) declines rapidly after the first year, and females are seldom reproductive after their second year (McCarty 1975). Their mating system appears to be polygynous (males mating with more than one female ) or promiscuous (both males and females having several mates), and there is no apparent sexual dimorphism in the species (Frank and Heske 1992). Interestingly, there appears to be both some male-female pairing in the breeding season, and both sexes care for offspring (McCarty 1975; McCarty and Southwick 1977), a behavioral trait usually associated with monogamous species. (It is possible to have both polygynous or promiscuous mating and male-female pairing and biparental care. For example, recent studies have shown that many supposedly monogamous bird species exhibit extrapair matings and broods and litters [in mammals] may have multiple paternity.)

Dispersal : There is very little information about dispersal in grasshopper mice. Stapp (1997) reported that most juveniles had disappeared from the study site by autumn, but no distinction was made between mortality and dispersal.

Behavior

Daily Activity: Grasshopper mice primarily are nocturnal (Baily and Sperry 1929; Frank and Heske 1992; McCarty 1975) and peak nocturnal activity is between 2000 and 2400 hours (8-12 p.m. ) (Frank and Heske 1992). Grasshopper mice appear to be active on the surface all year round and there is no evidence of hibernation (McCarty 1975).

Survival: Year-to-year survival appears to be low in the grasshopper mouse and juvenile mortality and/or dispersal appears to be very high. Thus, overall population turnover in the species appears to be very rapid For example, Stapp (1997) found 100% population turnover both years in a two-year study. In addition, most juveniles appearing after mid-spring had died or dispersed by autumn. McCarty (1975) concluded that while adults may live at least three years in captivity, they probably live less than 12 months in the field . This apparent high level of mortality and population turnover is consistent with their relatively precocious sexual maturity, rapid decline in fecundity , and polygynous or promiscuous mating system .

Socio-Spatial Behavior: Frank and Heske (1992) used radiotelemetry to investigate the space use patterns of the southern grasshopper mouse in the Chihuahuan Desert of southeastern Arizona. They found that the average home ranges of breeding males were about twice as large as female home ranges (3.7 hectares [ha] versus 1.7 ha); non-breeding males had smaller home ranges than breeding males (2.6 ha versus 3.7 ha); and no difference between the home ranges of non-breeding and breeding females. During the breeding season , there was extensive home range overlap between males and between males and females, but little overlap in the home ranges of females. McCarty (1975) suggests that territories are advertised by the grasshopper mouse’s characteristic high-pitched vocalization. During the non-breeding season , grasshopper mouse socio-spatial behavior is markedly different from the breeding season. Female territoriality breaks down in the non-breeding season and Frank and Heske (1992) observed sharing of burrows by individuals of both sexes. They suggest that males compete among themselves for females during the breeding season and that the observed spatial variation reflects their sex-specific reproductive strategy. During the breeding season, spacing patterns probably reflect male-male competition . During the non-breeding season, sharing of burrows may provide thermoregulatory advantages and/or may serve social functions. Frank and Heske (1992) provide an anecdotal report that burrow sharing was prominent during a cold front on their study site in the Chihuahuan Desert. Population densities of the southern grasshopper mouse are relatively low for a rodent species. McCarty (1975) reported a density of 1.8 mice/ha in a Mojave Desert creosote scrub community and others also have reported low population densities (e.g. , Baily and Sperry 1929; Frank and Heske 1992). Such low population densities are consistent with the species’ carnivorous habits and the distribution and availability of prey items.

Taxonomy

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Similar Species

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Genetics: Relatively little genetic information of value to conservation planning in the MSHCP study area is available for the southern grasshopper mouse. It has a diploid chromosome number of 48, and according to Hsu and Bernischke (1968; cited in McCarty 1975), the species exhibits considerable subspecific variation in basic chromosomal morphology. Riddle and Honeycutt (1990) investigated restriction-endonuclease-site variation of mitochondrial DNA (mtDNA) to study geographic and phylogenetic variation in the genus Onychomys. They found little mtDNA variation in O. torridus within the hot desert Mojavean-Sonoran-Sinaloan regions, but no samples were taken from the southern California coastal region. Whether the coastal region represents a distinct area of genetic endemism separate from the hot desert regions is unknown.

Members of the genus Onychomys

ZipcodeZoo has pages for 8 species and subspecies in this genus:

O. arenicola (Chihuahuan Grasshopper Mouse) · O. arenicola arenicola (Chihuahuan Grasshopper Mouse) · O. leucogaster (Grasshopper Mouse) · O. leucogaster durranti (Durrant's Northern Grasshopper Mouse) · O. torridus (Southern Grasshopper Mouse) · O. torridus ramona (Southern Grasshopper Mouse) · O. torridus torridus (Southern Grasshopper Mouse) · O. torridus tularensis (Tulare Grasshopper Mouse)

More Info

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Further Reading

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Notes

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Contributors

Identifiers

Footnotes

  1. Mean = 527.150 meters (1,729.495 feet), Standard Deviation = 413.740 based on 41 observations. Altitude information for each observation from British Oceanographic Data Centre. [back]
Last Revised: 7/16/2012