could lead to a more complex social system (Linn 1984).
Thus a comparison of behavior of closely related solitary
species could provide new insights into the processes that
lead to the evolution of more gregarious species (Brockman
1984; Waser 1988).
Spacing of kangaroo rats
Because individuals within a distribution are influenced by
the proximity or absence of other individuals, spatial orga-
nization can provide clues to the social structure of a pop-
ulation (Lott 1991). In kangaroo rats, spacing varies with
body size and methods of storing food and ranges from in-
dividual, nonsexually dimorphic territories to overlapping
home ranges (tables 31.2 and 31.3). Species with larger
body masses defend individual, dispersed territories against
members of both sexes, in which they store larders of seeds,
their main food source. Smaller-sized species inhabit over-
lapping home ranges in which they scatter-hoard seeds.
Males have home ranges that overlap those of one or more
females and each other, whereas females maintain area ex-
clusive of other females. Although a large sex difference
in home range size might be expected in kangaroo rats,
sex differences in spacing behavior are usually small, and
home range sizes of males are similar to those of females in
most species outside the breeding season (table 31.3). Dur-
ing breeding, in all species investigated thus far, male home
ranges expand to overlap those of females, because males
visit female territories or core areas for mating (Maza et al.
1973; Behrends et al. 1986; Randall 1984, 1989b, 1991a;
Perri and Randall 1999; Randall et al. 2002; Cooper 2002;
Shier and Randall 2004).Social behavior and food
The more social species of kangaroo rat are smaller in
body size, have overlapping home ranges, and hoard seeds
in widely spaced caches throughout their home ranges
(table 31.2). The chances of an individual recovering seeds
are enhanced by being able to remember where the seeds
are cached (Jacobs 1992). Although the kangaroo rats meet
conspecifics in the process of gathering and caching seeds,
they do not defend their caches, and D. merriamionly re-
sponds to another rat’s presence if they observe caching be-
havior (Preston and Jacobs 2001). Because seeds are usu-
ally abundant enough to support the solitary kangaroo rats
gathering them, and pilfering of each other’s seed caches
is reciprocal (Daly et al. 1992; Vander Wall and Jenkins
2003), it is probably adaptive for the kangaroo rats with
overlapping home ranges to tolerate the presence of conspe-
cifics that are also gathering and caching seeds. Aggression
would be costly in time, energy, and water conservation
(Ganem and Nevo 1996). Because D. merriamiis relatively
long-lived and individuals remain in the same area for most374 Chapter Thirty-One
Table 31.2 Traits of Dipodomyson a social continuum from the least to the most social
D. deserti (1) D. spectabilis (2) D. ingens (3) D. heermanni (4 ) D. ordii (5) D. merriami (6)Body size Large, 83 –148 g Large, 98 –132 g Large, 93 –195 g Medium, 70 – 80 g Small, 50 – 96 g Smallest, 33 –54 g
Spacing
Territorial
Males Yes Yes Yes No No No
Females Yes Yes Yes Yes No No
Food hoarding Larder Larder Larder Larder? Scatter Scatter
Tolerance of conspecifics Low Moderate Moderate Moderate Moderate High
Males visit? Yes Yes Yes Yes Yes, females
to mate
Philopatric? Both sexes Probably Maybe Probably Yes
Neighbors related? Yes Probably?? Probably
Neighbor recognition
Olfactory? Yes Yes? Yes Yes
In encounters No Yes Yes No Yes Yes
Scent gland No No No No Seasonal Yes, dimorphic
Footdrum
To conspecifics Yes Yes Yes Yes No No
Individual signatures No Yes Maybe No No No
Drum back Seldom Yes Yes Yes No No
Predators Yes Yes Yes No No No defense
SOURCE: See Randall 1993, 1994 for references. For column (1): Randall 1997, 2001; Sullivan 2000; Randall and Boltas King 2001. For column (2): Jones 1993; Randall 1997;
2001; Randall and Matocq 1997; Winters and Waser 2003. For column (3): Randall 1997, 2001; Murdock and Randall 2001; Randall et al. 2002; Cooper 2002; Busch 2003. For
column (4): Fitch 1948b; Yoerg 1999; Yoerg and Shier 1997; Shier and Yoerg 1999; Randall 2001; Shier 2003; Shier and Randall 2004. For column (5): Perri and Randall 1999.
For column (6): Randall 1989b; Perri and Randall 1999. For body size: Kays and Wilson 2002.