The Harvest Mouse
Deaths And Entrances
1 Discovery and Context
2 Micromys - Description
3 That Wonderful Procreant Cradle
4 Where Are They Now?
5 Deaths and Entrances
6 Keeping Harvest Mice as Pets
Pregnancy and care of young.
In captivity, Harvest Mice can become pregnant at around 30 days old and they will breed throughout the year but with much reduced litter sizes in the middle of winter. In the wild, sexual maturity is normally at 45 days and the first litters are produced (in England) in June with a peak in August – September. Occasionally, breeding can be as early as April and it is probable that winter litters are produced in semi-artificial circumstances as when they used to over-winter in cereal-ricks and food supplies were generous. Drought years, which slow the maturity of grasses, delay breeding.
Harvest Mice have home ranges of about 100m2 but they may overlap with neighbouring mice.
The prelude to mating involves quick, jerky chasing by the male and this is accompanied by audible chattering sounds. If the female is not ready, she will rebuff the male's advances and there will be a “face-off” with the two mice motionless and the female with her mouth open aggressively. If she is ready for mating the chase will end at ground level where copulation, lasting just a second or two takes place.
Normally, in the wild, the male will have no further contact but in captivity, as the pregnancy progresses, the female becomes more intolerant of the male and chases him away. In the confines of a captive space, the male will attempt to avoid her notice and hide in the most distant corners whenever possible. There is more on this behaviour and its management in Chapter 6.
Pregnancy and care of young
Gestation takes 17 – 19 days and the nest is completed a day or two before birth. Litter size averages 5.4 with a range of 3 to 8 and the newborn babies are blind and naked, smaller than a little-finger nail and weighing 0.6 – 1.0 grams (or 0.02 – 0.03 oz). In captivity in mid winter, litters can be of just one or two. Given an adult’s weight of 6g, a pregnant female with an average litter is clearly around twice her normal weight just prior to the birth.
Mean litter sizes were apparently larger in 1917 when data from 16 litters gave a mean of 6.75 and a maximum of 12. No explanation for this decline has been put forward.
In some parts of the world litter size varies with season and with the age of the dam, though few actually live long enough, in the wild, to rear more than one litter. In a Russian study, 33% managed it whilst 2.8% produced 3 and 0.2% survived long enough to rear 4.
In captivity, the babies gain weight at 15% a day for the first two weeks. They are born pink and hairless but after twenty four hours their backs and tops of their heads turn blue-grey and scaley. At five days their backs are covered with short black hair though their tummies remain largely naked until the 8th or 9th day. Their eyes open at 7-9 days and their incisors erupt at about the same time.
During this time, the mother only visits them to suckle for which she has four pairs of nipples. During the visits she stimulates the babies to make bowel movements and she eats the excreta to keep the nest clean and she washes them. From around day nine she supplements her milk with finely chewed seeds.
If the mother is concerned about the security from predators she will pick up the babies with her teeth and carry them to a new location with the babies adopting a stiffened, still posture which helps make the task less difficult for her.
At 11 – 12 days they venture out of the nest, with the mother often carrying them back to protect them and the babies returning to the nest to rest. At 15-16 days they are independent with their weight around 50-60% of an adult's and their fur a dull brown colour.
The babies undergo a moult from 25 days old, which may take from 15 to 100 days because the moult is arrested in the winter and late born babies may exist in two-tone russet/brown for this time. They resemble adults once they reach full size at about 30 days and the moult is completed, but their muzzle remains short for some time.
The female is receptive to mating immediately after she has given birth (she has a post-partum oestrus) so that it is possible for her to be heavily pregnant with a subsequent litter, by the time her previous litter is becoming independent, and just 4 or 5 days after seeing the backs of them, she may have built a new nest and have given birth to a new litter.
The life expectancy of Harvest Mice is very short. Few will survive their first winter to perpetuate the population
In a study of the species in the South Downs with marked individuals (of various age) being recaptured at intervals afterwards, 30% were still alive after 6 weeks, 4.6% after 18 weeks and just 0.7% after 30 weeks. This suggests that less than 1% of the population that is alive at the end of a summer is around to breed at the beginning of the following year. It is no wonder then that populations at any one site, disappear without trace from one year to the next.
From examination of tooth wear, it has been established that maximum lifespan in the wild is 18 months, whilst in captivity they have been known to live for five years. Mine have become increasingly inactive and slow from 18 months old and lived only about 2 years. They become unable to breed at 18 months.
Only a tiny percentage (3%) survive long enough to carry 3 or more litters in a season though in captivity a single female has produced as many as 8 litters.
In population ecology, two extreme strategies are recognised; in one, the animal has long life-expectancy and few offspring which require extensive parental care until they mature. This is K strategy and is exemplified by the elephants. In the alternative, r-strategists, have brief life expectancy, large number of offspring and early maturity. Clearly Harvest Mice are r-strategists.
As babies, they are known to have been eaten by Blackbirds and Common Toads and as adults (or babies) they fall prey to all manner of predators including Pheasants, domestic cats, Weasels and Stoats, owls, hawks and crows. Their activity cycle makes them vulnerable to both diurnal and nocturnal predators but they are never a major part of any species' diet; although a study in West Sussex showed them constituting 65% of the prey of Barn Owls in November, this had reduced to less than 1% in June/July. Nationally, only 0.8% of Barn Owl pellets contain Harvest Mouse remains.
In Norfolk, Barn Owl pellet analysis has shown Harvest Mice to fluctuate in 3-year cycles and 3-4 year cycles have been identified in parts of Europe.
Their biggest killer is adverse weather with early autumn storms and frosts being particularly disastrous as October to November is the peak of their breeding period. 80% of late-born litters are killed by such conditions and even in normal condition 12% of litters found before weaning are dead, though this is probably due to the mother being predated.
Harvest Mice have a large surface to volume ratio making thermo-regulation a demanding aspect of their physiology. They need to consume as many calories as the much larger Wood Mouse each day (29-36 kJ/day) and the demands increase with every drop in ambient temperature. In order to preserve heat whilst resting they curl up into a ball exposing their backs (which are naturally cooler than their tummies) and tuck in their un-insulated feet.
The nests (both summer and winter ones) provide a measure of insulation by trapping the air in the woven grass but there is a limit to the amount of water they can shed and eventually it will cease to provide the necessary protection. Harvest Mice do not gain thick winter fur, perhaps because, in such a small animal, this would be a liability if it got wet. Instead they have proportionally more subcutaneous fat than other small mammals.
One final significant threat is the management of grassland including that on nature reserves. Without mowing (or grazing) grassland will turn to scrub and then woodland but mowing in the late summer to maintain the grassland is obviously bad news for Harvest Mice. Ideally the best approach might be to preserve some areas around the margins of a field on a rotation to permit late breeding by Harvest Mice and provide winter shelter.