The Reproductive Rate of the Hopping-Mouse Notomys Alexis and Its Ecological Significance.

Abstract

In the male hopping-mouse, spermatozoa first appeared in seminiferous tubules on day 60 and were present in the epididymis a few days later; ventral prostates markedly increased in weight between days 56 and 60. Some females had large Graafian follicles, stimulated uteri and perforate vaginae on day 40; corpora lutea were first observed on day 44, but most females did not spontaneously ovulate until after day 54. Gestation in post-partum mated, non-suckling females lasted about 32 days, with implantation on day 7 or 8. When four or more young were suckling, gestation increased to 39 days (mean); implantation took place between days 11-13 and 14-17 when there were either three or four, or five, suckling young respectively. From one to three unfertilized oocytes surrounded by cumulus cells were found in the Fallopian tubes between days 7 and 17 in some pregnant females. Post partum, the most common ovulation rate and litter size was 4; 72% of these litters fully survived to weaning; 10-15% of litters were of five or six young and nearly half of these fully survived to weaning; in the natural environment litters were of four or five young. Without post-partum mating and without suckling young, females had ovulated spontaneously by days 9-11. Significantly fewer females suckling from four to six young had oocytes at this time, but the percentage had increased by days 15-18. This indicates a delay, but not prevention, of spontaneous ovulation during lactation. Field-caught females from the southern Northern Territory were reproductively inactive on three out of four occasions. The fourth time, when there had been > 50 mm of rain in the preceding few weeks, all adult females caught had corpora lutea and some were pregnant. Greatest ovarian inhibition coincided with high population density. When compared to data for small northern temperate-zone rodents, most of the above reproductive parameters do not indicate a high reproductive rate, but data for closely related, apparently non-cyclic, Australian species appear to be similar except for, possibly, age of puberty and spontaneous ovulation during lactation. The significance of these results to r-strategy in this species is discussed.