Successful sons or superior daughters: sex-ratio variation in springbok.

Mothers would often benefit from producing more offspring of one sex than the other. Although some species show an astonishing ability to skew their sex ratio adaptively, the trends found in many studies on vertebrates have proved inconsistent. Furthermore, evidence for a mechanism by which such a bias is achieved is equivocal at best. Here, we examine sex-ratio variation over 30 years, both at an individual and a population level, in the highly polygynous, size-dimorphic springbok (Antidorcas marsupialis). Many previous studies of similar species have shown that mothers in superior condition preferentially produce sons, whereas those in poorer condition produce more daughters. We found the opposite to be true in springbok, perhaps because daughters provide mothers in superior condition with a more rapid and secure fitness return. This theory was supported by the findings that earlier-conceived offspring tended to be female and that an increased proportion of daughters were produced with increasing rainfall (which was likely to reduce nutritional stress). We also show that selective reabsorption of embryos is unlikely to be the main mechanism by which deviations from an equal sex ratio are achieved. Hence, either differential implantation occurs or females are able to influence the sex of the sperm fertilizing an egg.

Arthropods and helminths in springbok (Antidorcas marsupialis) at Benfontein, Kimberley.

From July 1979-December 1980, 48 springbok were culled for a parasite survey at Benfontein. Lice were the dominant parasites and reached a peak in September. Damalinia antidorcus, Linognathus antidorcitis, Linognathus armatus and Linognathus euchore were the most prevalent species, nymphs in most cases outnumbering adults. During the winter months, 2nd and 3rd instar Rhinoestrus antidorcitis and R. vanzyli (nasal botflies) were present. The number of helminths reached a peak in January and then fell, secondary peaks being recorded through autumn, winter and spring, but a rise in numbers occurred the following December. Paracooperia serrata, Trichostrongylus spp., Strongyloides spp., Cooperioides antidorca and Longistrongylus sabie adults were dominant. Fourth stage larvae (L4) of Ostertagia outnumbered adult Ostertagia hamata, but L4 of Trichostrongylus spp. were less plentiful than adults of this genus. Good spring and summer rains (414,6 mm from October-March) were probably responsible for mean total burdens exceeding 10 000 in January and April.