Cheek pouch capacities and loading rates of heteromyid rodents

Abstract

Rodents of the family Heteromyidae are proficient gatherers and hoarders of seeds. A major component of their adaptive specialization for harvesting and transporting seeds is their spacious, fur-lined cheek pouches. Precise measurements of cheek pouch capacities are essential if ecologists are to understand the foraging ecology, possible constraints on locomotion patterns, and competitive relationships of heteromyid rodents. To measure the size of these cheek pouches and the rate at which animals load seeds into their pouches during seed harvest, we attracted 56 individuals representing ten species of heteromyid rodents to bait stations in the field and allowed them to fill their cheek pouches with seeds several times while we observed and timed the events with the aid of night-vision equipment. The largest load taken by each individual was used as an estimate of its cheek pouch capacity. At the end of observations, each subject was captured and its mass and other data gathered. The allometric relationship between cheek pouch capacity and body mass for ten species of heteromyids was significant [pouch capacity (ml) = 0.148 body mass (g), r =0.91, P<0.0001]. The regression coefficient is ≈1.0, which indicates that the volume of the cheek pouches scales in direct proportion to body size. When the data were subdivided into quadrupeds (Perognathus and Chaetodipus) and bipeds (Dipodomys) (n=5 for each), the relationships between pouch capacity and body mass were significant, but the two regressions were not significantly different from each other. When all loads (full and partial) were considered, subjects filled their cheek pouches an average of 93 ± 10% of pouch capacity (n=185). Cheek pouch capacities from published studies of artificially filled pouches of heteromyids in the laboratory averaged about 40% below the field measurements obtained here. The allometric relationship between mean loading rate and body mass was also significant [seeds/s=1.067 bodymass (g), r =0.85,P=0.0011), but when quadrupeds and bipeds were considered separately, the relationships were not significant. Seed densities and bulk densities were used to calculate packing coefficients for seed species, which, when used in conjunction with the allometric relationship between cheek pouch capacity and body size, can be used to estimate the maximum load carried by a heteromyid. Except for the very largest kangaroo rat species, a full pouch load of Indian ricegrass seeds represents less than the daily energy requirements of an active heteromyid.

Morphology of the oral cavity of the African giant rat (Cricetomys gambianus, Waterhouse)

“…The mean pouch capacity and the mean pouch capacity to mean body weight ratio (MPC/MBW) of the AGR -27.32±0.30 mL and 0.033 mL/g body weight were significantly higher as compared with those of the deer mice: 0.45± 0.11 mL and 0.019 mL/g body weight respectively (Vander Wall & Longland, 1999). The two most important factors governing cheek pouch capacity -body mass and the density of the preferred food -have been described (Morton et al, 1980).…”

“…The cheek pouches play an essential role in the economy of these rodents in that the pouches permit individuals to exploit seed resources efficiently throughout their home range and move those seeds to sites where the hoarder can exert a greater degree of control over them (Vander Wall et al, 1998). Interestingly, AGRs find almost as much value in the act of carrying as much as the act of hoarding food (Ajayi, 1977a;Ajayi et al, 1978).…”

“…This presumably increases efficiency of deer mice in exploiting small, particulate food items that are patchily distributed (Vander Wall & Longland, 1999).…”

“…Thus knowledge of the cheek pouch capacity is essential to addressing general questions of AGR biology. The pouch capacity of the deer mice was reported as 0.45±0.11 mL, about 12-27% of that carried by similar-sized heteromyid rodents (Vander Wall et al, 1998) Salivary glands are soft spongy tissues that secrete saliva and amylase. There are three pairs of salivary glands viz: parotid, mandibular and sublingual (Hebel & Stromberg, 1976).…”

“…Cheek pouches are sac-like folds of skin on either side of the mouth playing an essential role in that they permit individuals to exploit seed resources efficiently throughout their home range and move those seeds to sites where the hoarder can exert a greater degree of control over them (Vander Wall et al, 1998). Thus knowledge of the cheek pouch capacity is essential to addressing general questions of AGR biology.…”