Ecology and life history of the Vlei Rat, Otomys Irroratus (Brants, 1827), on the Van Riebeeck Nature Reserve, Pretoria

Abstract

The ecology and life history of the vlei rat, Otomys irroratus (Brants, 1827), were studied in a population on the Van Riebeeck Nature Reserve near Pretoria. Two main procedures were used to collect the basic information and material for the project. First, a permanent live-trapping grid was established where 333 animals were captured, marked, released, and recaptured over a period of 26 months, Second, snap-trapping and live-trapping were conducted elsewhere in the study area for supplementary material. Because Otomys angoniensis Wroughton, 1906, a sibling species of 0. irroratus, also occurred in the study area and grid, it was necessary to define the differences in morphology and ecological distribution between the two species. The ecology and life history of 0. irroratus are described and discussed in regard to the following major topics: taxonomy and morphology, distribution, population dynamics, reproduction, postnatal growth and development, activity, behavior, and economic importance. In addition to the presently used means of distinguishing 0. irroratus and 0. angoniensis, it was found that the hind foot length of the two species differed, being 29-34 mm in 0. irroratus and 25-28 mm in O. angoniensis. The baculum of the two species was also found to be distinct in both shape and size. In Southern Africa 0. irroratus occurs primarily in the grassland subregion of the Southern Savanna biotic zone, while O. angoniensis occurs primarily in the woodland subregion of the same biotic zone. The preferred habitats of the two species on the study grid were determined. There was little overlap of niches and each species had a high degree of association with a different veld type, O. irroratus being associated with the more mesic habitat. The population size of O. irroratus reached a peak in May and a low in September. The population also fluctuated considerably from one year to the next, with rainfall possibly being the main contributing cause. The ratio of juveniles, subadults, and adults fluctuated primarily in respect to the season when the young were born. The sex ratio did not depart significantly from 1:1. Males comprised 49,5 per cent of the total and females 50,5 per cent. The rate of disappearance after one month from first capture was 41 per cent, indicating that a large number of transients and individuals dispersing from their birth site were involved. The overall rate of disappearance, excluding the high first month rate of disappearance, was 3,1 per cent per month. The mortality rate appeared to be at its highest between August and September when the food and cover were of very poor quality and quantity. This was also the period when males demonstrated a significant loss in body weight. The major factors causing mortality appeared to be flooding of the habitat and owl predation. Floods not only reduced the population but significantly interrupted breeding. Otomys irroratus was one of the most prominent prey species of both barn and grass owls. The major factors causing mortality appeared to be flooding of the habitat and owl predation. Floods not only reduced the population but significantly interrupted breeding. Otomys irroratus was one of the most prominent prey species of both barn and grass owls. Recapture data revealed that the mean home range size was 1 443m2, while that of males (1 730 m2) was highly significantly different (P<O,Ol) from that of females (1 252m2). Home range size decreased significantly in winter but only slightly in relation to a doubling of the population size. Of 18 individuals recaptured after a flood, 12 (66,7 per cent) had returned to their previously determined home range. Despite a small mean litter size (2,33) and relatively long gestation period (about 40 days), the long breeding season (nine months) and precocial birth, rapid development, and nipple-clinging behavior of the young together give O. irroratus a good reproductive potential. Several litters of young were reared. They are precocial at birth and development proceeds rapidly. At birth the incisors are erupted, enabling the young to cling firmly to the nipples of the mother, thereby reducing mortality losses. Most adult behavior patterns are developed before weaning at 13 days of age. Sexual maturity is reached at approximately three months of age. Otomys irroratus is primarily crepuscular, but activity tests conducted in the field and laboratory indicated some activitv throughout the day and night. Individual and social behavior are described and discussed. The vlei rat is shy and retiring in captivity and in nature is not easily captured by the use of snap-traps. Interactions of adults were tested and they were found to be antisocial, with intrasexual aggression occurring when caged. Complex threat and communication patterns exist, a feature of asocial behavior. Mating failed to occur in captivity, probably as a result of their antisocial nature. Marking behavior is very distinctive and, combined with urination and possibly defecation, would appear to be useful in delineating territories. It would also appear from their social habits, marking behavior, and considerable overlap of home ranges that their interactions with conspecifics in nature are represented by a dominance hierarchy. The influence of O. irroratus upon the environment was found to be slight. The species serves as a common prey item for a number of predators and was found to harbor several endo- and ectoparasites of public health importance.