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Habitat and Foraging

Prefered Habitat of the Kangaroo rat. Lidicker 1960.

Habitat

The general habitat of the kangaroo rat is semi arid and arid regions (Howard, 1994). Kangaroo rats are especially found on sandy or soft soils (Howard 1994) which allow them to make their burrows. There is a fair amount of variation between species of kangaroo rats that is due to geographic location. This is thought to be a product of the availability of varied habitats and long periods of isolation between some populations (Nader 1978).

The Merriam kangaroo rat can be found in Southern California, Utah, Southwest New Mexico, Arizona, and Upper Mexico (Texas Tech University 1997). These areas are characterized by low rainfall and humidity and high summer temperature and evaporation rates (Reynolds 1958). Altitudenly Merriam's can be found from below sea level to about 4500 feet (Reynolds 1958). They prefer stonier soil such as clays gravel and rocks, which are harder than the soils that some other species of kangaroo rat including the Banner-tail prefer (Howard 1994). Their soil preference affects both their burrow structure and caching behavior. The environment where the Merriam kangaroo rat lives is very hot and dry, thus they must have adaptations to deal with the heat. Merriam kangaroo rats conserve water and only use metabolic sources (Lidicker 1960). This means that they do not have to drink water to survive, they can survive only on the breaking down of seeds by their metabolism. They can also lower their metabolic rate which reduces respiratory loss of water and loss of water through the skin in order to conserve water (Lidicker 1960). The harsh environment is the selectional force for these physical adaptations.

Banner- tail kangaroo rats are found in Northeastern Arizona southward to Aguascalientes and San Luis Posi, Mexico and from Arizona to Western Texas. Their habitat is a desert grassland with scattered shrubs (Texas Tech University 1997). Thus it is dry much like the Merriam kangaroo rats habitat, but the Banner-tail has more water available in their habitat so they have less of the physically adaptations that Merriam kangaroo rats have.

Kangaroo rats are sensitive to extremes in temperature so they will remain underground in their burrows during inclement weather such as rain storms (Howard 1994). In the event of snow coverage, it creates a large disadvantage for the kangaroo rat because it covers their food supplies and blocks travel (Reynolds 1958).

Distribution of the Merriam Kangaroo rat (which is also very close to the distribution of the Banner-tail kangaroo rat because they live in similar environments and have overlapping home ranges). Lidiker 1960.

Foraging

Kangaroo rats utilize their habitat and home range for foraging for seeds. Morgan and Price define foraging as a combination of two behaviors, (1) locomotion to, between and from foraging sites, and (2) scratch- digging at foraging sites. Because of the habitat and the nature of their patchy food source, the individual fitness of a heteromyid such as the kangaroo rat is greatly influenced by foraging success. Natural selection has favored individuals that can harvest the maximum number of seeds in the minimal amount of time (Morgan and Price. 1992). It is important to kangaroo rats to reduce the amount of time foraging for several reasons. One reason is that decreasing time away from burrow (foraging time) is beneficial in desert environments where it is cool and dry and minimizing energy expenditure is important. Also, foraging behavior makes the kangaroo rat susceptible to predation (Morgan and Price 1992). Morgan and Price performed a study which compared the cost of foraging in Quadra pedal and bipedal species of Heteromyid rodents. They found that bipedal species, such as the kangaroo rat, had a larger energetic cost of foraging. They proposed that heteromyids can compensate for these costs in two ways, either they spend more time foraging or they specialize on rich food sources (Morgan et al. 1992). Since spending greater amount of foraging poses a threat to their survival the kangaroo rat must develop a feeding strategy that balances benefits of the time spent foraging with the cost of predation and focus foraging activities on patches that will be abundant.

Kangaroo rats will hoard the seeds that they find during foraging trips. The ability for seed hoarding has a large impact on the competition in the environment of the kangaroo rat. In his study, Schroder outlines what conditions make it so that hoarding is effective, (1) at some period of the year an animal must encounter food items at much higher rates than they are consumed, (2) an animal must defend and perhaps rediscover a food cache and (3) an animal must remain within the same area long enough to utilize these resources (Schroder 1979). Since all three of these conditions match the behavior of the kangaroo rat, then hoarding behavior is an effective option for a feeding strategy. Thus, it makes sense that the behavior of active cache management may have been selected for in the kangaroo rat (Jenkins et al. 1995). The use of different seed caching methods by different species may be an important factor in the ability of kangaroo rats to coexist despite their similar food habits. This would seem to be the case with the Merriam and Banner- tail kangaroo rats that have overlapping home ranges (Nader 1978).

The Merriam kangaroo rat have been shown to often scatterhoard small clumps of seeds in many small holes ( Jenkins et al. 1995). Merriam kangaroo rats will scatterhoard in close proximity to the burrow. The benefits of scatterhoarding for the Merriam kangaroo rat are that it maximizes the harvest rate where there are high concentrations of seeds at discrete sites such as beneath shrubs or in clumps of grass. This condition matches seed distribution of the Merriam kangaroo rat. Scatterhoarding also allows the Merriam to gather seeds much quicker because they do not have to return to the burrow . Draw backs of scatterhoarding are that when you forage away from the burrow recovery costs are high and scatterhoard caches are more likely to be discovered and stolen. This is because there are more caches and they are closer to the surface. Since Merriam's kangaroo rats scatterhoard near the burrow, Jenkins et al. propose that travel costs are minimized and harvest rates are maximized. Scatterhoarding near the burrow also provides the advantage that if new seeds are no longer being produced; individuals can redistribute caches more widely which minimizes the pilferage by other rodents (Jenkins et al. 1995). The nature of the soil of the Merriam kangaroo rat also seems to be conducive to caching. Since the soil is rockier and harder to dig in than the soil of the Banner-tail, the Merriam kangaroo rat has shallower caches and has less burrow entrances.

Banner-tail kangaroo rats occupy large mounds in which they larderhoard (Jenkins et al. 1995). Banner-tail kangaroo rats hoard more than any other species of kangaroo rat (Schroder 1979). Jenkins et al. argue that larderhaording seeds has the advantages of allowing an animal extra time and energy and it decreases the risk of predation. When kangaroo rats larderhoard instead of scatterhoard they spend less time on the surface digging small caches. The costs of larderhoarding are that the burrow and seed supply may be more likely to be attacked by microbes, fungi and insects. The burrow might also attract other rodents that would steal seeds (Jenkins et al. 1995). The hoarding behavior of Banner-tails makes burrow maintenance and defense extremely important to the Banner-tail and thus Banner-tails will defend their mound and the immediate area around it as a territory (Schroder 1979).

Kangaroo rat feeding. photograph curtesy of Ralph Rose

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