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Reproduction in the Nyala (Tragetaphus angasi) (Mammalia: Ungulata)

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Summary

In the male, sexual maturity was determined by examining the seminiferous tubules for the presence of spermatogenesis. At three months of age, mean tubule diameter was 44.6μm and no lumen was present. Spermatogenesis was first recorded in an animal of about 12 months old where the mean seminiferous tubule diameter was 158μm. All animals over 14 months old showed active spermatogenesis. Once mean paired testis weight exceeds 23.5g, the mean seminiferous tubule diameter remains constant at about 200μm. This occurs when animals are between 18 and 24 months old and when testis weight, expressed as a percentage of body weight, also remains a constant (0–03%). Most of the changes in pelage colour and length occur over this period and, as these do not occur in sterile males, it is assumed that they are linked to androgen production. The males show no sign of any seasonal sexual cycle.

The reproductive status of females was investigated from serial sections of the ovaries, foetal data, and the mass and condition of the mammary gland. The age at which ovulation and first conception occurred in Ndumu Game Reserve was between 14 and 18 months. By about 20 months, 50% of the female population had attained sexual maturity. Anoestrus was not recorded between sexual maturity and senescence (at about 14 years).

Prior to oestrus, there is an increase in the number and size of visible Graafian follicles. These reach a diameter of at least 6.7 mm before ovulation. From others' observations of tame Nyala, the interval between overt oestrous cycles is between 10 and 34 days (= 19.4) and oestrus recurs until conception takes place. Copulation is only permitted towards the end of overt oestrus which lasts for between two and three days. Overt oestrus occurs where mating takes place without conception and is therefore different from the “silent ovulation” described in other bovids. Ovulation takes place ipsilaterally and implantation is usually homolateral. There is a significant increase in the size of the corpus luteum, which degenerates rapidly after parturition, during the last third of pregnancy. The only accessory corpora lutea recorded were found in animals which were in the last week of pregnancy. Multiple ovulations are rare (0–46%) and fertility in females between three and approximately 14 years old was 100%.

No precise gestation period has been documented but it is known to be less than 231 days and a post-partum oestrus follows between two and seven days after parturition. During the study, the mean parturition interval was calculated at 297 days. Foetal growth velocity falls within the range of those calculated for other Tragelaphines, and there was an even foetal sex ratio.

In the collection of data, I was assisted in various ways by members of the staff of Ndumu Game Reserve. These included V. Shongwe, G. Schütte, T. P. Dutton, T. Tomkin and R. Physick. Professor F. Eloff and Professor J. D. Skinner generously allowed me to us laboratory facilities at the University of Pretoria for the processing of the testis material. Professor J. Hanks, Professor P. A. Jewell and Dr C. A. M. Attwell kindly commented on earlier drafts of this paper.

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