The Harvest Mouse

Description

Home Page

Introduction

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

Appendix A

Races.
Measurements.
Appearance.
Behaviour.
Food.

Races

The Harvest Mouse is the only species (in the world) in the genus Micromys though there are several subspecies throughout its range. Tropical Asiatic Harvest Mice are greyer and have longer tails whilst Japanese and east Siberian Harvest Mice have shorter tails. The British subspecies is soricinus.

Measurements

Fewer people are familiar with avoirdupois and lumping weight these days so here is a bit more detail on the Harvest Mouse’s vital statistics in more modern terms. Head and body length 50 to 70mm – that’s 2.0 to 2.7 inches, plus a tail that (as Gilbert White said) is about the same length again. The biggest of these extremes is about the length of my little finger. The same measurements for House Mice are 70 to 90mm (2.7 – 3.5 inches) or the length of my middle finger. Wood Mice (and Yellow-necked Mice) are 80 – 130mm (3.1 to 5.1 inches). The tip of my forefinger to the base of my thumb approximates to the maximum here.

Gilbert White’s measure for the Harvest Mouse (nose to rump) was 2¼ inches which is almost bang-on the mean of these scientifically achieved data. His equivalent for Wood Mouse (or could it have been Yellow-necked Mouse?) was 4¼ inches. This suggests that neither species has changed in size in the last 200 years (as would be expected) and that GW was a meticulous scientist.

Brown Rats for reference are 150 - 270mm (5.9 to 10.6 inches) nose to rump. 270mm is the length of my forearm and I’m sure that I (like you) have seen them bigger than that!

At 13-16mm and 8-9mm, the lengths of the hind-foot and ear are, respectively shorter than any of their congeners too.

Sketch of a Harvest Mouse

It is the weight of the Harvest Mouse though that seems more strikingly minimal and describes their smallness best. The adult weight of 6-8g (0.21-0.28oz) is equivalent to a twenty-pence coin and slightly less than Gilbert White’s measure of one-third of an ounce avoirdupois (which is the system we still use when defying metrication). Their summers' are spent clambering around amongst grasses where light weight is a crucial factor if all but the tips of the stalks are not to be bent down to the ground under their weight. Apart from their small size, their skeleton is notably light too, being just 5% of their total weight (ours is 15%). Pygmy Shrews average around 4.25g but may reach the mass of a Harvest Mouse after a hearty meal. Pipistrelle Bats (our smallest species of bats) weigh roughly the same as Harvest Mice. House Mice (the next smallest British mouse) weigh about 18g.

Harvest Mouse

In the far eastern extent of their range Harvest Mice are bigger. In Korea they are 84mm long plus a tail of 99mm (i.e. 117% of the body length). It is believed that shorter tails in colder climates is an adaptation for regulating body temperature.

Appearance

Adult Harvest Mice in breeding condition are a lovely russet-orange colour on their uppersides and are very distinctive, as our other mice are dull brown or grey above. The russet colour covers the distal one-third of each dorsal hair with the basal two-thirds being grey. The undersides are pure white and there is a clear line of demarcation where the two colours meet. Adults in winter lose their russet colour to some degree as they gain more black-tipped guard hairs along their back. Harvest Mice have small ears that barely extend above the profile of the head and which are quite furry. Our other mice have large, prominent ears that are less hairy and almost translucent. Harvest Mice have a flap (the anti-tragus) covering the ear opening (acoustic meatus).

Harvest Mice have a shorter muzzle than other mice, though their face is not as blunt as a vole’s. This seems to enhance their attractiveness.

The Harvest Mouse’s tail is unique in Britain in being prehensile. The distal third is used as extra support when climbing and is constantly being entwined around vegetation almost as a fifth limb. They look very endearing when they stop descending and, using their tail to grasp a grass stalk and their hind legs as pivots, they use their highly dextrous forefeet to bring food to their mouths. Usually though, the tail is used as an insurance premium and not for actual motion or support. When descending head first it serves as an emergency brake and when ascending it is held straight and stiff as a balancing aid. They certainly don’t swing through the grass stems like gibbons in a rain forest, but when needs must, they do leap about though in these circumstances they move so quickly that any sense of grace or accuracy is not apparent to the human eye.

The hind feet are bigger than the fore feet with longer toes, the two inner ones (including the “thumb”) are used to grip when climbing; both digits functioning like an opposable thumb. Both fore and hind feet have five toes but the front “thumb “ is tiny and doesn’t seem to serve any function. The fore feet do not grip, though the claws may give support.

There is an illustration in British Wildlife magazine that heads the Wildlife Reports section on mammals. It portrays a brightly pelaged Harvest Mouse descending a grass stem. Close inspection shows it to be inaccurate in many respects quite apart from the enhanced colouration. Its tail is almost in a knot – if it fell and relied on its tail it would tighten and perhaps be difficult to untie! They curl their tails around stems but do not twist it back under itself. The illustration does not show the mouse’s (semi) opposable thumbs and without them this mouse would indeed fall and they use all four legs to descend – this one seems to be signalling semaphore with its two left limbs. Otherwise though it is a charming image and captures a Harvest Mouse’s character admirably (but inaccurately).

The dentition of mammals is used extensively in systematics and identification. There is more detailed information on the subject in Appendix A where the analysis of owl pellets in assessing the distribution of small mammals is described. For now it is worth mentioning that there are characteristics of the Harvest Mouse’s dentition that separate it from other members of the muridae.

Behaviour

In captivity Harvest Mice are active in bursts of a couple of hours followed by a similar period of rest throughout the day and night. When kept together these periods synchronise. In the wild they are more nocturnal in the summer and more diurnal in the winter when the nights are cold. There may be more activity around dawn and dusk in the wild.

They are normally slow and hesitant in their movements, testing the way forward with caution. Some captive individuals though, are especially active and race around continually over the same circuit. When alarmed, they first freeze and if there is real danger they plummet down to the ground. In captivity, when confrontations occur, there is first a nose to nose assessment accompanied by gaping mouth and this is followed by a brief rapid chase with attempts to bite at the rump, tail and ears of the more timid animal. Some animals can become severely scarred during such encounters. In captivity, dead mice are consumed. This happens in the wild too when corpses of other Harvest Mice and other small mammals are encountered but in captivity the deaths can be intentionally delivered by aggressive congeners.

When crossing open ground, their locomotion is rapid as they bound along like Kangaroos using their hind legs to leap.

During courtship, copulation is preceded by brief chases and if the female is receptive, mating occurs at ground level. This is sometimes accompanied by excited chattering calls from the male. If the female is not receptive she will turn aggressively on the male and utter clicking sounds. Newborn babies when handled utter squeaks audible to humans and also ultrasonic sounds at 83-114kHz but their purpose is unknown. Adults are not known to make ultrasonic sounds.

They may use scent markings to delineate territories but little is known about this in Harvest Mice. They are not social animals and they rarely encounter other mice after they have matured except for mating, after which, the male plays no further part.

Like all mammals they can swim. This is a clear advantage to Harvest Mice as they often inhabit wetlands where their ability to clamber around in reed-beds enables them to take advantage of this niche habitat. They also use salt marshes where high spring tides can be a threat and a Harvest Mouse once seen swimming strongly three kilometres offshore was assumed to have succumbed to this hazard.

In North America there are five species of mouse known as Harvest mice but these of the genus Reithrodontomys and should not be confused with Micromys minutus. Although they are true mice and they build a nest above-ground, their behaviour and characteristics are otherwise quite different.

Food

Harvest Mice are omnivorous. In captivity they can survive very well on a good quality mixture of wild-bird seed and water. They have been observed to chase and eat insects including grasshoppers flies and moths (even large ones). They will also nibble at berries and greens such as lettuce. They especially like fresh seeding grasses.

Their small size and correspondingly low energy requirements mean that they are rarely a pest, though occasionally in Russia, they have built up to plague proportions and depleted human food supplies significantly.