The population dynamics of the Grass rat Arvicanthis niloticus were studied in relation to their diet and the climate in the Nakuru district of Kenya between September 1980 and May 1982. Numbers in the study area were low at the end of 1980 with groups of animals isolated in patches of grassy habitat. Local populations declined and some became extinct during the first half of 1981. Breeding began in April, one month after the start of the rains, but with a small initial population and high juvenile mortality, numbers did not increase significantly until September, when animals born earlier in the season were old enough to reproduce. This second generation reproduction was probably very important to the survival of the population over the following dry season when adult mortality was estimated at 15% per month. Reproduction was facilitated by a nutritious diet, consisting mainly of young grasses early in the season and seeds later on. Seed production was itself related to rainfall and as that declined towards the end of 1981, breeding ceased. Arvicanthis were considerably more numerous then than one year previously, with the difference being ascribed to the longer rainy season experienced in 1981 than 1980. The density of animals in favourable habitats increased with immigration from less suitable areas between January and March 1982, and then declined until the end of the study. Breeding resumed in April, one month after the start of the rains. Peak numbers of Arvicanthis were recorded in the middle of the dry season, when few crops were in the ground. They would only become a serious pest to agriculture if they were still abundant at the start of the breeding season some months later. Expansion then could result in very high numbers by harvest time, in August or September.

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Summary Reproduction and post‐natal development of Arvicanthis niloticus was studied in the laboratory. The gestation period was 22–24 days with a post‐partum oestrus. Mean litter size was 3.7 ± 1.0. Young animals developed rapidly relative to other African murids and attained sexual maturity at 36–52 days. The growth of animals under laboratory conditions was compared with that of wild Arvicanthis in Kenya. Laboratory reared young gained up to 40g month‐1 while the maximum recorded in the wild was 30g month‐1. This rate was reduced during the dry season and adults virtually ceased growing at that time. A comparison of natural and laboratory growth curves showed the limitations of applying data obtained in the laboratory to wild populations. Résumé La reproduction et le développement post‐natal de Arvicanthis niloticus fut étudiée en laboratoire. La durée de gestation est de 22–24 jours avec un oestrus de post partum. La portée moyenne est de 3.7 ± 1.0. Les jeunes animaux se développent rapidement en comparaison d'autres muridés africains et atteignent la maturité sexuelle vers 36 à 52 jours. La croissance des animaux en conditions de laboratoire fut comparée avec celle d'Arvicanthisà l'état sauvage au Kenya. Les jeunes élevés en laboratoire croissent de 40 g mois‐1 alors que le maximum observéà l'état sauvage est de 30 g mois‐1. Ce taux est ralenti en saison sèche et les adultes ont une croissance pratiquement nulle à cette époque. La comparaison des courbes de croissance naturelles et en laboratoire met en évidence les limitations de l'application de données obtenues en laboratoire à des populations sauvages.

The movement and spatial distribution of marked Nile rats, Arvicanthis niloticus, was traced through phases of a low density population from January to August 1981, a rapidly growing population from September to January 1982, and a high and declining postreproductive population from February to April 1982. Apart from late in the population cycle there was considerable movement, interchange and establishment of new ranges. As the population increased individual ranges became smaller. The rats apparently preferred field edges and did not favour the fields themselves. Females frequently established themselves at certain sites where they produced their litters; some of the offspring remained with the mother in the warren. There is also evidence that ousiders of both sexes joined these warrens. Males were generally more wide-ranging than females. Agricultural practices, e.g. mowing and trampling by cattle, influenced the rats preparedness to remain within a particular location. Exploitation of all the available habitat took several months to achieve.

The techniques of microscopic analysis of rumen contents and dung, and the use of tame animals to study the feeding ecology of wild ungulates are discussed with reference to their use on impala in the Savanna Ecosystem Project at Nylsvley in the northern Transvaal. Both techniques usually suffer from small sample sizes but if this is avoided they can provide much useful information. Observations on tame animals and the analysis of rumen contents of wild animals can supply good qualitative and quantitative data but the results obtained from faecal analysis are less reliable and only qualitative.