Ord’s kangaroo rat (Dipodomys ordii) COSEWIC assessment and status report: chapter 8
- Life Cycle and Reproduction
- Territoriality, Home Ranges and Dispersal
- Interspecific Interactions
Our knowledge of Ord’s kangaroo rats in Canada is primarily based on research conducted in the Middle Sand Hills by Gummer and colleagues. Reference to other studies in more southern localities and other Dipodomys species is used where detailed data on the Canadian population of kangaroo rats are not available.
Desert rodents tend to be relatively long lived and slow to reproduce for their body size (Stearns and Crandall 1981, Kenagy and Bartholomew 1985, Brown and Harney 1993) but Canadian kangaroo rats do not fit this trend (Gummer 1997a). Most individuals survive less than one year. Based on mark-recapture studies involving > 3150 individuals over 11 years in the Middle Sand Hills, over-winter survival is frequently ≤10%, (range 5 to 34%) depending on winter severity (Kenny 1989, Gummer 1997a, Gummer and Robertson 2003b, Gummer and Bender unpubl. data) meaning that populations reach low levels during early spring. Factors contributing to mortality include predators (Gummer and Robertson 2003c), macroparasites (botfly larvae; Gummer et al. 1997), vehicle traffic, agricultural and industrial equipment crushing animals in their underground burrows, trampling by large grazers such as cattle and elk, and starvation (Gummer 1997a). Two individuals are known to have survived at least 4 years in the wild (Gummer and Robertson 2003b).
Kangaroo rats breed whenever conditions are favourable (Beatley 1969, Beatley 1976, Hoditschek and Best 1983, Best and Hoditschek 1986, Kenagy and Bartholomew 1985, Gummer 1997a). Mating occurs aboveground (Engstrom and Dowler 1981). For Canadian kangaroo rats, reproduction only occurs during the snow-free period, generally from early spring (March or April) to early autumn (September; Kenny 1989, Gummer 1997a). However, breeding attempts were observed during mild mid-winter conditions in one year (Gummer 2005).
Average litter size according to counts of embryos and placental scars from museum specimens is 3 (range 1 to 6, n = 16; Royal Alberta Museum) and is similar across the species’ geographic range (Gummer 1997a). Gestation is approximately 29 d (Duke 1944, Day et al. 1956, Smith and Jorgensen 1975, Jones 1993) and lactation is 14 to 21 d (McCulloch and Inglis 1961). Lactating females appear capable of conceiving before their previous litter is weaned (Nielson 1941, Gummer 1997a). Number of juveniles weaned in 98 successful litters in the Middle Sand Hills ranged from 1 to 4 (Gummer 1997a).
Adult females in Canada may raise up to 4 litters per year (Gummer 1997a), considerably more than for other populations (1 to 2 litters per year; Gummer 1997a). Juvenile females attain sexual maturity when they reach approximately 73% of adult body mass, corresponding to an average age at first reproduction of approximately 47 d (Gummer 1997a). This is considerably earlier than elsewhere (60 to 90 d; Best and Hoditschek 1986, McCulloch and Inglis 1961, Smith and Jorgensen 1975, Jones 1985, Eisenberg 1993). Males become reproductive at approximately 79% of adult body mass and 61 d of age (Gummer 1997a). Given that most northern kangaroo rats fail to survive 1 year but produce up to 2 litters during their first year (Gummer 1997a), generation length for this population is <1 year.
Known predators of Ord’s kangaroo rats in Canada include Great Horned Owls (Bubo virginianus; Schowalter et al. 2002), Burrowing Owls (Athene cunicularia; Gummer unpubl. data, Poulin et al. pers. comm. 2005), prairie rattlesnakes (Crotalus viridis; Gummer and Robertson 2003c), and badgers (Taxidea taxus; Gummer unpubl. data). Domestic cats have been observed hunting in kangaroo rat habitat in the Middle Sand Hills (Gummer unpubl. data); both cats and dogs have brought dead kangaroo rats to their owners (Royal Saskatchewan Museum, Gummer unpubl. data). Although diurnal raptors are not generally considered predators of kangaroo rats, Gummer (unpubl. data) reported landowner accounts of (unidentified) diurnal raptors occasionally taking kangaroo rats in the daylight when burrows are disturbed by cultivation. Other likely predators of kangaroo rats in Canada are Short-eared Owls (Asio flammeus), Long-eared Owls (A. otus), red foxes (Vulpes vulpes), swift foxes (V. velox), coyotes (Canis latrans), bobcats (Lynx rufus), least weasels (Mustela nivalis), long-tailed weasels (M. frenata), striped skunks (Mephitis mephitis), raccoons (Procyon lotor), and bull snakes (Pituophis melanoleucus).
Kangaroo rats have many anti-predator strategies. Their erratic, bipedal locomotion likely evolved in response to selection for predator avoidance, primarily because this style of movement provides fast, energy efficient movements across open surfaces to the safety of burrows (Bartholomew and Caswell 1951, Yousef et al. 1970). The middle ear of Ord’s kangaroo rat is sensitive to the sounds of owl wing beats and the movements of striking snakes, presumably allowing them to better avoid these predators (Webster and Webster 1971, Webster and Webster 1975). Banner-tailed kangaroo rats (Dipodomys spectabilis) perform foot-drumming as an alarm signal (Randall and Stevens 1987). Ord’s kangaroo rats may also foot-drum (Brown 1989), which may alert predatory snakes to the fact that they have been detected and cause them to leave rather than investigate (Randall and Stevens 1987). Kangaroo rats are less active in open habitats under bright moonlight or northern lights (aurora borealis), presumably to minimize detection by visually orienting predators (O’Farrell 1974, Rosenzweig 1974, Kaufman and Kaufman 1982, Gummer unpubl. data).
Kangaroo rats are adapted to hot and dry desert environments (MacMillen 1983, French 1993, Tracy and Walsberg 2002). Their nocturnal and fossorial nature facilitates heat avoidance and water conservation (Mullen 1971). Kangaroo rats can survive without exogenous water: their metabolic requirements met by eating seeds (Schmidt-Nielsen 1964, MacMillen and Hinds 1983). They select seeds with the highest water content in feeding tests (Frank 1988), and seeds cached in burrows undergo hygroscopic uptake of water (Reichman et al. 1986, Nagy and Gruchacz 1994). Kangaroo rat nasal passages are structured so that moisture condenses by counter-current heat exchange, minimizing water loss (Jackson and Schmidt-Nielsen 1964, Schmidt-Nielsen et al. 1970, Collins et al. 1971). On average, kangaroo rats have lower metabolism and water loss than other mammals of comparable size (Schmidt-Nielsen 1951, Fairbanks et al. 1983, Hinds and MacMillen 1985). Kenny (1989) considered these adaptations as evidence that northern Ord’s kangaroo rats may be sensitive to drought, although this hypothesis may be inconsistent given that the species is adapted to dry desert conditions elsewhere in its range. Rather, cold and snow are likely to be limiting factors for northern Ord’s kangaroo rats because these increase energetic requirements and restrict foraging (Gummer 1997a, Gummer 2005).
Canadian Ord’s kangaroo rats use daily torpor to conserve energy during winter (Gummer 1997a, Gummer and Robertson 2003c, Gummer 2005). Individual kangaroo rats carrying or implanted with temperature data-loggers used torpor exclusively during the winter when the ground was snow covered (Gummer 1997a, Gummer and Robertson 2003c, Gummer 2005). Torpor was used primarily during daylight hours, with bouts extending up to 17 h and body temperatures falling to 13.5 ºC. Animals aroused from torpor during early evening and presumably fed from underground food caches during the night. Individuals generally did not emerge from burrows if there was snow on the ground. Kangaroo rats entered torpor on up to 70 d per winter (Gummer 2005), though some individuals did not exhibit torpor during mild winters (Gummer 2005).
The Canadian population of Ord’s kangaroo rats is the only population of the genus known to use torpor in the wild. Laboratory studies of congeners reveal a drastic mass loss and death within several days if they are forced into torpor through starvation and exposure to low temperatures (Dawson 1955, Carpenter 1966, Yousef and Dill 1971, Breyen et al. 1973, MacMillen 1983). Likewise, there are reports of captures and observations of Ord’s kangaroo rats aboveground in southern localities throughout the year (Reynolds 1958, Kenagy 1973, O’Farrell 1974, Nagy and Gruchacz 1994), even when air temperature approaches –19 ºC (Kenagy 1973, O’Farrell 1974) and there is up to 40% snow cover (Mullen 1971, Kenagy 1973, O’Farrell 1974).
Kangaroo rats are territorial and defend burrows and underground food caches (Eisenberg 1963). They are not colonial: individuals are solitary and exhibit little tolerance for conspecifics within the territory (Bartholomew and Caswell 1951, Garner 1974, Daly et al. 1984). Some species use foot-drumming as a territorial signal (Ward and Randall 1987).
Core home ranges of radio-collared kangaroo rats average 1750 ± 620 m2 (± 1 standard error, n = 28; Gummer and Robertson 2003c). However, Ord’s kangaroo rats occasionally move beyond this range at night, with overall home range size averaging 7830 ± 2930 m2 (n = 38). The average maximum home range width is 130 ± 35 m (n = 38).
Juveniles are more likely to disperse than adults. Juvenile dispersal distances do not differ among the sexes, and are highly skewed with a median of 100 m; 75% of juveniles disperse < 500 m (Gummer 1997a). One juvenile male travelled approximately 10 km along a sandy fireguard (Gummer unpubl. data). The high connectivity of anthropogenic habitats could facilitate greater dispersal and colonization of additional anthropogenic habitats. Kangaroo rats are unlikely to successfully disperse across densely vegetated sand hills because they cannot move inconspicuously through these areas.
Ord’s kangaroo rats are primarily granivorous. They collect food items in cheek pouches and store them in underground burrows. In addition to seeds, they collect green vegetation (Best and Hoditschek 1982) and other plant parts (silicles, pods, stems), insects (Johnson 1961, Alcoze and Zimmerman 1973, Flake 1973), bone fragments, and dry grasses for nest material. Canadian kangaroo rats collect seeds from at least 55 species of native plants (Beaudoin and Gummer unpubl. data) such as annual sunflower (Helianthus couplandii), cactus (Opuntia spp.), and scurph pea (Psoralea lanceolata). The diet also includes a high proportion of non-native, weedy plant species, such as common knotweed (Polygonum aviculare) and Russian thistle (Salsola kali), particularly in anthropogenic (disturbed) habitats.
Kangaroo rats are important in grassland and desert communities because they remove and eat seeds and grasses, and disturb soil (Brown and Heske 1990, Heske et al. 1993, Kerley et al. 1997, Curtin et al. 2000, Brock and Kelt 2004a). The low survival rate of northern Ord’s kangaroo rats (Gummer 1997a), combined with their seed collection and caching behaviour, likely leads to a large number of abandoned seed caches for germination or for other granivores to exploit.
Other small rodents that are sympatric with Ord’s kangaroo rats in Canada include the olive-backed pocket mouse (Perognathus fasciatus), bushy-tailed woodrat (Neotoma cinereus), deer mouse (Peromyscus maniculatus), northern grasshopper mouse, western harvest mouse (Reithrodontomys megalotis), house mouse (Mus musculus), meadow vole (Microtus pennsylvanicus), southern red-backed vole (Clethrionomys gapperi), sagebrush vole (Lagurus curtatus), northern pocket gopher (Thomomys talpoides), thirteen-lined ground squirrel (Spermophilus tridecemlineatus), and Richardson’s ground squirrel (S. richardsonii; Epp and Waker 1980, Smith 1993, Reynolds et al. 1999). In a study of interspecific competition, Munger and Brown (1981) found that exclusion of Dipodomys resulted in increased abundance of smaller granivorous rodents and did not affect omnivorous species. In some cases, omnivorous northern grasshopper mice (Onychomys leucogaster) appear to displace Ord’s kangaroo rats into microhabitats where there may be higher risk of predation (Rebar and Conley 1983).
Canadian Ord’s kangaroo rats are the only population of the genus known to be parasitized by botfly larvae, Cuterebra polita (Gummer et al. 1997). The primary host of C. polita is likely the northern pocket gopher (Capelle 1970), but none of 86 northern pocket gophers caught in the Middle Sand Hills from 1994 to 2004 were parasitized (Reynolds et al. 1999, Gummer unpubl. data). C. polita could have shifted to kangaroo rats as the primary host, or alternatively the botfly may be a species not previously recognized. Parasitism by botfly larvae compromises survival, reproduction, and growth of some small mammals (Boonstra et al. 1980, Burns et al. 2005). For Canadian Ord’s kangaroo rats, botfly parasitism significantly reduces rates of reproduction and over-winter survival (Gummer unpubl. data). Kangaroo rats that inhabit anthropogenic habitats exhibit higher prevalence of botfly parasites (46%, n = 263) compared to those that occupy natural habitats (15%, n = 252; Bender et al. 2005).
The adaptability of Canadian kangaroo rats is highlighted by their opportunistic use of sandy areas produced by human activities (Nero and Fyfe 1956, Smith and Hampson 1969, Gummer 1999, Bender et al. 2005), similar to that reported for other kangaroo rats (Stangl et al. 1992, Price et al. 1994, Brock and Kelt 2004b). Although anthropogenic habitats superficially appear to represent additional habitat for kangaroo rats, a direct evaluation of the value of these habitats for the long-term persistence of the Canadian population is underway. Anthropogenic habitats are hypothesized to serve as population sinks (Gummer 1999, Bender et al. 2005).
Canadian kangaroo rats exhibit high rates of reproduction (Gummer 1997a) and breed opportunistically whenever favourable conditions arise (Gummer 1997a, Gummer 2005). This increases the potential for colonization of new habitats. Similarly, seasonal activity patterns and expression of torpor appear to vary according to the status of underground food caches and weather conditions (Gummer 2005). Cumulatively these strategies should facilitate rapid population responses to favourable weather, shifts in climate, or habitat management.
Distinct life history (e.g., age at first reproduction; Gummer 1997a) and physiological traits (e.g., torpor; Gummer 2005) of northern Ord’s kangaroo rats may represent phenotypic plasticity and/or genetic differentiation (Gummer 1997a, Gummer 2005).
Kangaroo rats temporarily reduce home range size by as much as 50% when daytime industrial activities occur in the immediate vicinity (Gummer and Robertson 2003c). Contraction of home ranges likely decreases foraging opportunities, leading to depletion of underground food caches, and fewer reproductive opportunities, all of which could potentially have significant negative effects on the local population.
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