Mild autumn boosts numbers of Wood Mice
THE woman ahead of me in the hardware shop bought a mouse trap and as she left, the sales assistant turned his attention to me as the next customer and volunteered: “Can’t keep traps in the place; there must be a plague of mice in the country.”
A ‘plague’ may have been a bit of an exaggeration but a spike in the numbers of mice coming indoors at this time of year is normal, though unwelcome. The autumn just past was remarkably mild and that must have contributed to the breeding success of these common rodents.
Despite its name, the Wood Mouse is by no means restricted to woodlands; it thrives in habitats ranging from moorlands to sand dunes and inhabits gardens and houses. The species will continue to breed through the winter if weather conditions remain favourable and food remains plentiful.
Wood Mice don’t live very long. Twelve months is regarded as a normal life-span so most of the mice that are around now were born earlier this year. The breeding season in Ireland begins in late March or early April and runs until October or November or whenever the weather turns very cold.
Female mice become sexually active when they are about two months old. They are on heat for only a few hours and mate with multiple males during that short time. Wood Mice groom each other and it has been noted that males tend to groom females more often than males groom other males or females groom other mice. The conclusion is that in addition to grooming, males are constantly monitoring the reproductive state of the females they encounter.
Pregnancy lasts approximately three weeks and litter sizes generally number four to seven young with a range from as low as two to as high as eleven. DNA analysis has revealed that the young in a single litter may have up to four different fathers ensuring great genetic variation.
The young are born naked and blind and weigh only two grammme. Male mice contribute nothing in the way of parental care. Mothers feed and rear their offspring and have them fully weaned after a further three weeks.
Females that have given birth breed again immediately, so the population can grow rapidly. Females may produce up to an incredible six litters per breeding season if conditions are very favourable. However, mortality for new-borns is extremely high so numbers are kept in check. Mouse traps help.