Studies on the European hare. V. Reproduction

Abstract

The breeding season in the two winter periods studies, 1958/59 and 1959/60, began in the case of the males in December. Sexually active males occurred up to September 1959. The first pregnant females were found in January 1959 and February 1960. The last pregnancies were observed in the second half of September. When comparison was made of the two successive starts of the reproduction season, distinct differences were found between them in respect of the month's delay in the appearance of pregnancies in females in 1960, and the percentage of sexually inactive males, greater than in the perceding season, in the first months of their activity. The peak valie fpr size of the gonads characterising the maximum sexual activity occurred in March in the case of males and in April in females. The maximum regression of testes was observed in the majority of the males in September and October. The dimensions of the ovaries are greatest in November and December. Two cases were observed in the material of the attainmant of sexual maturity by young females in the year of their birth. Only sporadic tendency to maturation occurred in young females, interrupted by the total regression of the gonads in the autumn. Maturation of the young hares took place normally during the oestrus at the beginning of the reproduction season later on, after the renewal of the reproductive functions in adult animals, at the minimum age of 5-7 months in males and 7-8 months in females. The ovulation level, lowest in the first month of reproduction (on an average about 1 egg per female) rises sharply up to April and is maintained on a high level (average about 4 eggs) until June. In the second half of the summer there is a decrease in the level of approximately 4 ovulated eggs per female. The number of implanted embryos is smallest in the first litter - being about 1-1.5 per female in January and February and largest in April (4.0). It then remains on a hight level (3 or more embryos) until the end of the season. Embryonic mortality (particularly resorption) was found to habe a significant effect on calculation of the size of litters. The highest mortality rate was observed at the beginning of the reproduction season and in May and June. Embryonic mortality was found to decrease in the final phase of the season. The highest percentage of pregnant females occurred in February-April and in June and July. This points to the probable occurrence of three principal litters during the year. The calculated mean number of litters is 3.4 per female during the season. The average size of the litter, after allowing for embryonic mortality, was 2.3 young per female. The annual increase in young hares per female is between 7-8 young. The second litter born during the April-May period is of fundamental importance to population increase. The ratio of males to females in the material composed of shot hares is 398:366(1: 0.92) and in the case of embryos is 75:70 (1:0.93). Out of a total number of 270 embryos studied, it was only in two cases that the possibility of superfoetation having occurred could be taken into consideration. A few isolated cases were observed of polyovularity of the Graafian follicle, migration of blastocysts and single-egg twins