Six hundred and twelve male gerbils, consisting of 357 specimens of Tatera afra and 255 of T. brantsii, were collected in South Africa in 1946. Seasonal changes in the reproductive organs and in the structure of the field population have been compared and contrasted in the two species. Growth of the young male to puberty has been studied in animals from the field and in four litters of T. afra reared in the laboratory.
The male T. brantsii tends to be heavier than the male T. afra, but is somewhat shorter. In both species the largest males exceeded the largest females in body weight and length, but there is no significant difference between the mean values of these measurements for the two sexes.
The testes have a greater actual and relative weight in T. afra than in T. brantsii. The reproductive tract is remarkably large for the size of the animal in both species, and in T. afra can reach 20-7 per cent of the body weight.
The annual pattern of sexual activity in the males corresponds with that of the females in both species. In T. brantsii fecund males are found in all months of the year: in T. afra reproductive activity is intense from July to December, is slowing down in January and February, and ceases completely in March and April; in May and June there is preparation for the breeding season.
Male T. afra become sexually mature at six to eight weeks old if born in spring and early summer, at six to seven months old if born later in the season. In T. brantsii puberty is delayed during the end of the dry season and beginning of the rainy season. In both species the old animals (i.e. those over 150 mm. head-body length) die out during the early summer and have disappeared from the field by the end of December, for none were caught in the first quarter of the year. It is estimated that T. afra have a possible life span of seventeen months and can experience two breeding seasons. It was not possible to estimate the length of life in T. brantsii owing to the spread of births over the year.