Petromyscus collinus
pygmy rock mouse

Pygmy rock mice (Petromyscus collinus) are found in arid regions of southern Africa. This species has been documented to inhabit the southwestern region of Angola and the southern area of Nambia. (Dempster and Perrin, 1989; Skinner and Smithers, 1990)

Pygmy rock mice occupy rocky habitats in high desert areas up to 1000 m in elevation. (De Graaff, 1981; Withers, et al., 1980)

Pygmy rock mice weigh an average of 18 to 20 g. Body length is usually 80 mm, tail length around 82 mm, and hind foot length 15 mm.

Pygmy rock mice have darker skin at birth than do similar species, such as Gerbillurus sp., which have skin that is almost translucent at birth. Proliferation of hair begins within 4 days of birth and is first visible on the dorsum. Pelage coloration of young is dark grey, whereas adult pelage is brown-grey in coloration.

The masseter muscles appear to be well developed at birth, yielding disproportionately large cheeks that are thought to be related to nipple-clinging behavior during the nursing period. Nipple-clinging has been documented in several rodent species, although its function has not been clearly established, nor has it been clearly defined. (Brown, et al., 1998; Dempster and Perrin, 1989)

Little is known about reproduction in pygmy rock mice. The mating system of these animals has not been described.

It is thought that pygmy rock mice become reproductively active only in the presence of fog. As a result, their reproductive behavior is highly seasonal and of short duration, resulting in low reproductive potential but high annual survival (Withers, 1983).

Female pygmy rock mice produce one liter per year, with an average litter size of 2.8 young. A distinguishing behavior of young is nipple-clinging, wherein the young remain attached to the nipple of the mother until they are weaned. The length of gestation is not known, but in other murine rodents of similar size, gestation does not often last more than about one month. Weaning is reported to occur betweeen 30 and 33 days of age. These mice probably become independent and disperse around the time of weaning. (De Graaff, 1981; Dempster and Perrin, 1989; Nowak, 1999; Withers, 1983)

In one study of maternal behavior, that mothers remained in the nest when disturbed; one mother took defense by tooth-chattering and mounting a threat posture. The young of this species cling to the teat. However, females were seldom observed to leave the nest, so it is not known whether they forage with the young attached to the teats. Mothers mouth carry their young if they become detached from the teats and also retrieve older young that have left the nest; this response waned 21 days after parturition. Grooming was observed, but rare, in one litter, where a pup approached its mother and licked her mouth in the diastema region. (Dempster and Perrin, 1989)

Beyond these details on specific behaviors of P. collinus, we can infer that this species is not unlike other small murine rodents. The mother provides care to the offspring, including protection and food. It is unlikely that parental care extends much beyond the period of lactation. (Nowak, 1999)

There are no data on lifespan in the wild or captivity.

Small Namibian desert mammals avoid the extreme temperature and humidity by being surface-active at night and remaining underground during the day. (Withers, et al., 1980)

In the region where these mice occur, rocky outcroppings tend to be separated by over a kilometer, acting as habitat islands for pygmy rock mice. Rock outcroppings are very important to serve as protection from predators. (Brown, et al., 1998)

Neonates of the pygmy rock mice squeak loudly when disturbed and also make a rhythmic clicking noise. It has been observed that adult pygmy rock mice squeak loudly when disturbed. One mother who was disturbed adopted a threat-poster and chattered her teeth. (Dempster and Perrin, 1989; Withers, 1983)

In addition to using these vocal and visual means of communication, there is significant tactile communication in this species between mother and offspring, as well as between mates.

Most mice use olfactory cues to identify mates and rivals. It is likely that these mice are similar in this regard. (Nowak, 1999)

Petromyscus species are generally reported to be omnivorous, and it is likely that P. collinus is similar in this regard. Pygmy rock mice are sometimes reported to be granivorous, but this granivory may be indirect. Although the rocky outcroppings which they inhabit may offer protection, they provide little in the way of feeding opportunities. Studies have shown that heavy grazing by hyraxes leaves little vegetation for pygmy rock mice. Instead, it is thought that pygmy rock mice are either removing seeds from the feces of hyrax or eating hyrax fecal pellets. Alternatively, they may be feeding on insects and arthropods that in turn are exploiting the feces. In any case, the diet of these animals remains slightly mysterious, and merrits further investigation. (Brown, et al., 1998)

Yellow mongoose and slender mongoose inhabit the same area as pygmy rock mice and are likely predators of these small rodents. However, actual predation on pygmy rock mice by these species has not been documented. (Brown, et al., 1998)

Since pygmy rock mice seem to depend on hyrax feces as a food source, an obligate relationship might exist between the two species. They may play some role in dispersing seeds, and may act as a control on some insect populations. They may play some role as a prey species in local food webs.

There are no known human benefits from this species.

Hantavirus might be transmitted during handling procedures. (Scott, 2002)

The conservation status of these small animals is not known.