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Social Spacing

Banner-tail mound. curtesy of Kerry S. Kilburn Ph.D: link to picture

Social spacing and territorial defense

Kangaroo rats live in overlapping home ranges. As the population size in a given area increases the home range size of the individual kangaroo rat decreases (UFWS 1997). Their home ranges are not that large, however, and their radius of activity is commonly 200 to 300 feet and rarely exceeds 600 feet (Howard 1994). In one study, Reynolds found that Merriam kangaroo rats have an average home range size of .466 acres which is larger than the Banner-tail average home range size .373 acres (Reynolds 1958). Since Merriam kangaroo rats utilize caches for foraging where as the Banner- tails return to their burrows to larderhoard it is logical that the Merriam kangaroo rat would have a larger home range. Male kangaroo rats have larger home ranges than female kangaroo rats with more activity centers in their home range. While male home range size remains relatively constant year round, female home range size varies seasonally. Females will have smaller home ranges when they are in lactation (USWF 1997). During this time period, females have to invest energy into their young providing both food and protection. This means that the female will not travel as far to forage and will stay closer to her young in the nest. After weaning, immature animals will tend to occupy new areas which are not occupied by adults (Reynolds 1958). Home range defense is also less in the winter months because it is less profitable due to the low plant productivity, higher energy cost and continued risk of predation. During these months the kangaroo rat will defend a smaller area, usually only their burrow and any food caches (Schroder 1979). Distribution of the kangaroo rat within the home range depends on many factors including soil, kind and amount of precipitation and winter temperatures. The availability of water has very little effect on the distribution of kangaroo rats, especially in Merriam’s, because of their ability to rely of metabolic sources of water ( Reynolds 1958).

Why home ranges?

The Kangaroo rat inhabits a home range for multiple reasons. The main reason why they live in home ranges is their food sources which is patchy and unevenly distributed (both seasonally and spatially). Thus, they need a larger area in order to gather the food that they need. Their food source is not defendable because if a kangaroo rat fed in one place all of the time the resources would be depleted quickly. The nature of their food (seeds and small vegetation) makes it so that they need to be constantly gathering and eating because they do not get a large amount of nutrition out of any given foraging period.

Within their home ranges, kangaroo rats will have territories, which generally consist of the burrow system. Based on class discussion, I would classify the territories of the Banner-tail kangaroo rat as mating and nesting territory and I would classify the Merriam kangaroo rats territory as a nesting only territory. The territories can not be described as a classical territory because Kangaroo rats must leave the burrow to forage for food.

Forms of territorial defense for the kangaroo rats are chasing, sun bathing, olfactory signals (urine and feces) and foot drumming. While both Merriam and Banner-tail kangaroo rats have territories, the Banner-tail has stronger territorial defense due to the higher investment level into the burrow. One method Banner-tails employ is “foot drumming”. “Foot drumming” or ground thumping can be used as a territorial signal by varying its rate and frequency (California EPA Department of Pesticide Protection 2002). Kangaroo rats will start foot drumming to repel intruders (Ward 1986) Active burrows can often be identified by the presence of the drumming signal California EPA Department of Pesticide Protection 2002). Research has shown that kangaroo rats will respond differently to different foot drumming intensities and may be able to distinguish foot drumming from familiar rats such as neighbors (Ward 1986) When kangaroo rats experience a stimulus that may be threatening they respond by becoming alert and posturing. Next they approach the stimulus to determine its threat level so that they do not waste energy on territorial defense where it is not necessary. They also determine the level of defense at this stage. Finally they retreat to the mound if the stimulus is determined to be threatening(Ward 1986). This is gainful to the kangaroo rat because in an agonistic encounter the resident kangaroo rat has an advantage to the invading kangaroo rat (Ward 1986). The ability to interpret and respond to threatening signals is important in territorial defense (Ward 1986).

Burrows

Above ground picture of a burrow. Lidicker 1960

Kangaroo rats produce fairly elaborate burrow systems with several sleeping, living and food storage chambers (Howard 1994). There are usually many more burrow openings than there are kangaroo rats, and each active burrow will usually contain one adult rat (or an adult female and its young) . Burrows are spaced according to both the number of kangaroo rats in an area and the abundance of food (Howard 1994). Kangaroo rats can live in colonies ranging from 6 to several hundred dens (Reynolds 1958) The number of individuals per acre and the number of burrows will vary as well, depending on location and time of year ( Howard 1994)

The burrow of the kangaroo rat not only provides important protection from predation, it provides protection from the harsh desert environment, with generally high temperatures, on the surface. Kangaroo rats often plug their burrow entrances with soil during the day to maintain a more constant temperature and relative humidity (Howard 1994) When there are high external temperatures, kangaroo rats will stay in their cool humid burrows and only leave at night (Lidicker 1960) When kangaroo rats sleep in their burrow, they bury their nose in their fur so that a small pocket of moist air accumulates and thus a very small amount of moisture is lost through respiration (Lidicker 1960)

Merriam kangaroo rats dig shallow burrows which open near the base of shrubs (Texas Tech University 1997). Mesquites are a favored location of the Merriam kangaroo rat for their burrows, in observations by Reynolds 95% of mesquites had a burrow beneath it. Merriam kangaroo rats are also more present in ranges grazed by livestock because grazing lowers density of perennial grasses which seems to deter kangaroo rats Reynolds 1958). The burrows of the Merriam kangaroo rat are shallow because the hard nature of the soil and their weak forelegs in their habitat and there will be a greater number burrows in the areas with softer and sandier soils. (Reynolds 1958). The burrows of the Merriam kangaroo rat are solely for the purpose of daytime refuge from predators and rearing of young (Texas Tech University 1997), thus they protect it as a nesting only territory. Because predation is so high and their habitat, Merriam kangaroo rats they spend 75% of their time underground in their burrows ( Reynolds 1958). In comparison to the Banner-tail the burrow of the Merriam kangaroo rat is simpler and less conspicuous and elaborate and consists of a labyrinth of storage tunnels (Reynolds 1958). Merriam kangaroo rats will also not mate in their burrows like Banner-tails.

Banner-tail kangaroo rats burrow in more open locations than the Merriam kangaroo rat. On the ground surface, the burrows create mounds that are 4 meters in diameter and 30 cm high (Texas Tech University 1997). The burrow of the Banner-tail kangaroo rat requires lots of investment because they will collapse easily if they are not constantly maintained (Texas Tech University 1997). The burrows of the kangaroo rat are reused and rebuilt by successive rats. Banner-tail burrows typically consist of many chambers up to 25cm in diameter and contain layered plant material (Schroder 1979) Banner-tail kangaroo rats can store up to 5 g of seeds in their mounds through their foraging strategy, larderhoarding ( Waser and Jones 1991). The Banner-tail kangaroo rat then depends on these caches through periods of low seed availability in winter and spring (Waser and Jones 1991).The burrow is the center of many activities, grooming, sleeping, digging and defense of food, but it is no the site of food gathering activity ( Schroder 1979). Banner-tails’ burrows are extremely important to them and they will defend their mound and the immediate area around it (Schrolder 1979) Banner-tails will spend on average 25 minutes of every hour away from their mound and only 13 minutes out of every hour over 6 meters away (Schroder 1979)

Locomotion

Locomotion in kangaroo rats is mainly bipedal. The Merriam kangaroo rat wan reach tremendous speeds and leap up to 7 or 8 feet. They also have the ability to change their direction of jumping without hesitation upon landing. These are adaptations that kangaroo rats have developed in order to avoid predation above ground (Lidicker 1960) Banner-tail kangaroo rats also have a rapid mode of travel to minimize energy cost and exposure to predators (Schroder 1979). Well defined runways are one way they do this, they are established coming from the burrow to help speed travel (Schroder 1979). Banner-tails also show a “move- freeze” mode of activity which may help reduce predation from nocturnal predators. When they perform this activity they will move very rapidly short distances then stop abruptly if they feel a predation threat.

Banner-tail runway extending from its burrow. Lidicker 1960.

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