REPRODUCTION AND PRENATAL MORTALITY OF SNOWSHOE HARES ON MANITOULIN ISLAND, ONTARIO

Abstract

The object was to find out whether variations in reproduction occurred at different population densities in the wild, and whether these played a significant part in the 10-year cycle of numbers of Lepus americanus. Over 600 adult Females were examined from three breeding seasons, 1959–1961. Counts were made of corpora lutea, implantation sites, and living and dead embryos. The breeding season began in April and finished in July, with three highly synchronized litters. Ovulation rate showed significant seasonal and yearly variations; also it was higher both in older and in heavier hares. Loss of ova before implantation was directly proportional to the number shed. Loss of whole litters by resorption in mid-pregnancy was estimated to be as much as 20%. In litters which survived to near term, a total of about 10% of ova were lost. Results for two populations, slightly out of phase, were compared. The one that first reached a higher density started breeding a week later each year, and had consistently lower ovulation rates and greater prenatal mortality than the other. These differences in fertility were not sufficient to account for the population decline which occurred.