The reproductive physiology of hares was studied in 760 animals shot throughout the year, and these observations were related to seasonal changes of behaviour in an attempt to explain the significance of “March madness”.

The hare was found to have a prolonged breeding season; at least some pregnant females were found in all months from January to August, and fertile males in all months except October and November. The nadir of the reproductive cycle occurred in October with no breeding amongst the females, and the males showing regressed accessory glands and involuted testes containing minimal amounts of testosterone. At this time hares were solitary and it was rare to see them active during the day.

After the autumn rest period, the first indication of a new reproductive cycle occurred in November with an increase in the size and activity of the testes. This trend continued during December, January and February by which time the reproductive tract was again fully functional. During this time there were striking changes in behaviour and matings were witnessed. Females resumed breeding in January with a marked synchrony in the timing of the first ovulation of the season. Mating preceded ovulation, and the first pregnancy normally resulted in a single fetus, although pregnancy failure was common at this time. By March and April the hares showed full reproductive activity with 100% of females pregnant and most carrying three or more fetuses. Many aspects of behaviour (March madness) reached a peak at this time, and in males this was closely related to a depletion of sperm reserves in the epididymis and the secretions of the accessory gland reservoir. During May, June and July, the females continued to be highly fertile. There was, however, already a reduction in the overt behaviour characteristic of spring, and there was a conspicuous drop in testicular testosterone production.

The reproductive cycle ended in July and August with a reversal of the changes seen earlier in the year. The testes regressed rapidly in August and sperm production ceased by September; the sperm reserves in the epididymis became progressively depleted and sperm were degenerating by October. There was a reduction in litter size towards the end of the breeding season and no pregnancies were observed after the end of August.

Of the hares under one year old shot in the spring only about 16% were apparently conceived during the period of spring madness, with the majority arising from matings in May, June and July. The age at puberty was influenced by the date of birth.

March madness appears to represent the rutting behaviour of male hares and is induced by the greatly increased testosterone production which occurs in spring. The behaviour is probably important in stimulating the females into breeding activity at the beginning of the new season.