Reproductive biology of the southern African ice ratKen WILL AN Willan K. 1990. Reproductive biology of the southern African ice rat. Acta theriol. 35: 39-51.Reproduction and the postnatal development of nine litters of the southern African ice rat Otomys sloggetti robertsi Hewitt, 1927 were studied in the laboratory. Parturition and lactation occurred in the presence of up to seven cage-mates, reflecting the colonial social organization of O. s. robertsi. Mean litter size was 1.44, reproductive effort 0.53, gestation period 38 days and fecundity approximately six young p.a. Newborn young weighed 10.6 --12.2 g and were semi-precocial. Early behaviour was dominated by nipple-clinging. Weaning occurred at 16 days old, and sexual maturity at 16 weeks in males and 11 weeks in females. Compared to O. s. robertsi, young of the vlei rat O. irroratus (Brants, 1827) are born more precocial and develop more rapidly. The microthermal environment of O. s. robertsi is harsh but predictable, and the subspecies is K-selected in terms of most reproductive parameters. Reduced fecundity and AT-selection is further attributable to the microhabitat stabilizing effects of nesting, social huddling and food hoarding in an underground burrow.

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Post-zygotic reproductive isolating mechanisms between two chromosomally distinct, allopatric Otomys irroratus (Brants, 1827) populations (Kamberg and Karkloof) were studied in the laboratory by means of breeding trials, and by comparing the postnatal development of young of intrapopulation and interpopulation pairs. In contrast to the 100% breeding success of intrapopulation pairs, fewer than half of the interpopulation pairs produced young, and overall reproductive performance of these pairs was reduced. Furthermore, fitness of hybrids was dramatically impaired, as indicated by increased pre-weaning mortality, and inhibited growth in respect of body mass (postnatal development), as well as almost complete sterility (backcross and hybrid-cross matings) of surviving young. The results indicate the existence of chromosomal and/or genetic incompatibility between the two populations. The presence of a tandem fusion in the Kamberg karyotype is thought to be particularly significant in causing hybrid sterility, and the Kamberg population may be regarded as an incipient sibling species.

Reproduction and the postnatal development of 48 litters (148 young) of the Angoni vlei rat Otomys angoniensis Wroughton, 1906 were studied in the laboratory. The gestation period was 37 days, mean litter size 3.1, mean mass of the newborn young 11.4g, and reproductive effort 1.07. Young were born semi-precocial, and both physical and behavioural postnatal development were rapid : locomotion was coordinated by day 5, the eyes were fully open by day 8, and young were weaned around day 16. Early postnatal development was characterised by nipple-clinging. The minimum age at sexual maturity was five weeks in females and eight weeks in males. Otomys angoniensis young are less precocial than those of the vlei rat O. irroratus, but more precocial than young of the ice rat O. sloggetti robertsi. In terms of most reproductive parameters, O. angoniensis appears less K-selected than either O. irroratus or O.s. robertsi Resume. -On a etudie la reproduction et le developpement de 48 portees (148 petits) d'O/omys angoniensis au laboratoire. La duree de la gestation etait de 37 jours, les portees etaient de 3,1 en moyenne, le poids moyen des petits etait de 11,4 g et l'effort de reproduction etait de 1.07. Les petits naissent semi-precoces. Le developpement postnatal physique et comportemental est rapide : la locomotion est coordonnee avant le 5 e jour, les yeux s'ouvrent avant le 8 e jour et le sevrage se fait vers le 16 e jour. Le developpement postnatal precoce est caracterise par 1'accrochement des petits aux mamelles de la mere. Les femelles atteignent la maturite sexuelle ä un minimum dc 5 semaines et les males ä un minimum de 8 semaines. Les petits de O. angoniensis sont moins precoces que les petits de O. irroratus, mais plus precoces que les petits de O. sloggetti robertsi. En ce qui concerne la plupart des parametres de reproduction, O. angoniensis parait etre moins caracterise par une strategic « K » que O. irroratus ou que O.s. robertsi.

Laboratory experiments were undertaken to ascertain whether or not individuals of two allopatric Otomys irroratus populations (Kamberg and Karkloof) distinguish between mates from the same (homotype) and the other (heterotype) population. The study aimed to establish the existence of pre‐mating reproductive isolation mechanisms between the two populations. In both ‘whole‐animal choice’ trials and olfactory discrimination tests, all females significantly preferred homotype stimulus males or odour. Although males of both populations preferred homotype females, males were less discriminating at the start of experiments, possibly because of increased exploratory drive at this time. Furthermore, Kamberg males demonstrated equal preference for both stimulus females during the dark phase of the light cycle; no explanation is given for this phenomenon. Overall, the results indicate the existence of population‐specific courtship behaviour, with olfactory cues apparently playing an important role in mate recognition. It is therefore possible that, should representatives of the two populations meet in nature, they may be reproductively isolated through behavioural means.