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Harvest Mouse
Micromys minutus
The harvest mouse is the smallest British rodent. It’s also the only British
animal with a truly prehensile tail, being almost the same length as the head
and the body. Harvest mice are extremely active climbers.
Head
Its face is blunt and rounded;
and its eyes are black, small and
round (about 3mm in diameter).
Climbing up
Feet
The feet are fairly broad and
specially adapted for climbing,
with the outer of the five toes on
each foot being large and
more-or-less opposable. This
mouse can grip a stem with each
hindfoot and its tail, leaving the
forepaws free for collecting food.
Hind foot 1.7 cm
Head & body 5.7 cm
Skull 2 cm
Climbing down
Ears
Tail
The auditory bullae are large, and
it is thought that the size of these
resonating chambers enables the
mouse to detect low frequency
sounds carried over great
distances, and therefore better
able to escape predation.
The prehensile tail
is bicolored and
lacks hair on tip.
Climbing up
Tail 5.5 cm
The tail is used as
a balancing organ.
Fur
Climbing down
They use their tails
as a brake.
The harvest mouse has distinctive
fur, its back is russet in colour and
the belly is white and clearly
demarcated from the flanks.
Weight 6-8g
DEVELOPMENT OF CLIMBING SKILLS
Because of the short lactation
period, it’s likely that the pups
rapidly develop the ability to
climb, being able to climb a
vertical stem at 10 days old,
despite their immaturity at
birth.
WEANING
Righting
10–12 days
Tail prehension
10–11 days
Quadruped stance
6–11 days
Foot grasping
6–9 days
BIRTH
The strong foot grasping and
tail prehension characteristic
of harvest mice is not
common in murids.
Hand grasping
3–7 days
DAYS 1
2
3
4
5
6
7
8
9
10
11
12
13
14
Nest building & reproduction
BUILDING THE BREEDING NEST
Step 4
A pregnant female harvest mouse starts building
her nest 10 days before giving birth. Breeding
nests (6 to 10 cm in diameter) are made of woven
grass. They tend to be found at least 30 cm above
ground in dense vegetation such as grasses,
rushes, brambles, cereals, etc.
She then throws her head and
forequarters backwards,
thereby causing the leaf to
split longitudinally.
Step 1
The pregnant mother first bites
the grass stem along part of its
length, thereby weakening it so
that the stem can be bent or
pulled into position more easily.
These nests are mainly made from grass leaves,
but they also may include leaves from other plants.
Sometimes other materials are incorporated into
the nest wall, such as bird feathers, heads of reeds
and pieces of string. Also, there’s no definite
entrance to the nest.
Step 2
Sitting on the grass stem
with her hind feet and
tail, the mouse then
takes hold of a nearby
leaf with her forefeet.
Step 3
She bites into the leaf so
that her incisors are around
the outermost vein.
Grass leaf with a split narrow strip (containing a single vein)
Step 5
Sitting in roughly the same position,
the mother continues until several
leaves have been shredded.
Step 6
Step 7
Working from the inside of the next, she
then pulls the shredded leaves together
and loosely weaves the shredded ends
into the framework of the nest. The nest
is loose and untidy in appearance.
From the inside, she pulls the ends of more
leaves, shredding them and incorporates these
strips into the wall of the nest. When the
framework of the nest is complete, it’s then
stuffed with chewed grass or thistledown.
6-10 cm
The structure of the harvest
mouse nest is important for
thermoregulation.
30 cm
If the nest gets soaked through, it’ll increase heat
loss. This is why in Britain heavy rain and sudden
frosts during the autumn and early winter can kill
80% of the late-born litters of mice before they
leave the nest.
NON-BREEDING NESTS/SLEEPING NESTS
These nests are built very quickly by individual mice of
both sexes throughout the year. They are constructed
of loosely woven and shredded grass and serve as a
temporary shelter.
4 cm
During the summer they can be found in thick
vegetation at the bottom of plant stems. During the
winter they may be found under rocks, amongst stacks
of bales, in holes or in abandoned birds nests.
6-10 cm
Non breeding nest
Breeding nest
TIMELINE: BUILDING THE BREEDING NEST, GESTATION AND BREEDING
BUILDING THE BREEDING NEST
The time building depends
on weather conditions.
3-4 days
The mother stops suckling
and her mammary glands
cease to be active. Then
she leaves the nest.
6-7 days
9
16
19
When young mice
leave the nest
other mammals
may occupy it.
34
2 WEEKS
40
gestation period
lactation period
Female mouse starts building her
nest 10 days before giving birth.
GESTATION
The gestation period is 17
to 19 days. The weight of a
pregnant female is up to
15g before giving birth.
GIVING BIRTH
INDEPENDENCY
Birth is a rapid process, with
only a minute or two between
the birth of each mouse. It
usually occurs at night, often in
the early hours of the morning.
At 15 or 16 days old
the young mice
become independent.
The daily weight gain is about 15%
Weigth: 0.7-1 g
Length: 15-22 mm
Tail: 40%
1 Number of days
Weigth: 3-3.5 g
Length: 35-45 mm
Tail: 50%
New-born mouse
In calm, rain-free weather.
They are born blind and
completely naked. At birth the
tail isn’t prehensile.
In windy, rainy conditions.
2-week old mouse
Their fur is dull brown or sandy colour.
The tail is fully prehensile.
Harvest mouse lifespan up to 18 months
BIRTH AND GROWTH
Harvest mice breed before they are one year old and can
produce 3-7 litters a year (4-8 mice per litter). New-born
harvest mice are comparatively large animals as they are
approximately 10% of the weight of the female.
Their mating season is between May and October, but can continue into
December depending upon the weather.
Breeding season
JAN
FEB
MAR
MAY
APR
JUN
JUL
AUG
SEP
OCT
NOV
DEC
Food & metabolism
HARVEST MOUSE DIET
eds
The diet of the harvest mouse is very
varied, and the animal is truly
omnivorous. They eat a wide variety
of seeds (grass, millet, oats and
wheat); fruits, berries and a
variety of insects, including
moths, caterpillars, flies and
grasshoppers.
Moss, roots, fungi and grain from
cereal heads (leaving characteristic
sickle-shaped remains*) may also be
taken. Noticeable damage to cereal
crops is extremely rare.
Se
45%
25%
L
eav
es
1
5%
s
ect
Ins
6% Berries & fruits
9% Other (fungi, moss, grain, etc)
ENERGY REQUIREMENTS
For most of the year, seeds are the most important item in the
diet of the harvest mouse, and these have a high energy content.
When living on seeds, harvest mice consume about 30% of
their own weight each day. Insects have a much higher water
content than seeds, and so a greater weight must be eaten to
supply the same amount of energy. Hence common shrews eat
the equivalent of their own body weight in insects each day, and
pygmy shrews twice their own weight.
Weight
Harvest mouse
8g
Common shrew
5-15g
Pygmy shrew
2.3-5g
30%
100%
200%
Main diet: Seeds
Equivalent of their own weight
Main diet: Insects
Daily energy intake
By selectively feeding on high-energy foods and
digesting them efficiently, the harvest mouse can
spend less time feeding and more time conserving
energy and warmth by sheltering in its nest.
With an enviromental temperature of 20˚C, the daily energy
intake of an 8g harvest mouse is 7.8 to 8.4 kilocalories. This
energy requirement is the same as that of a 20g vole or wood
mouse, only half that of the common shrew, and a third of the
calorie intake of the pygmy shrew.
Calorie intake
7.8-8.4 KCal
Weight
2.3-5g
8g
Harvest mouse
5-8cm head-body
5-15g
20g
20g
Bank vole
8-12cm head-body
Wood mouse
8-11cm head-body
Daily energy intake
Common shrew
6-8cm head-body
Pygmy shrew
4-6.5cm head-body
x2
Daily energy intake
x3
Daily energy intake
Habitat & population
STATUS & DISTRIBUTION
POPULATION OF INSECTIVORES AND RODENTS IN ENGLAND
Since 2007 harvest mice are a UK BAP Priority
Species, which means that these species have
been identified as being the most threatened and
requiring of conservation action under the UK
Biodiversity Action Plan (UK BAP).
The numbers represent the population estimate of each species.
(*)This graphic doesn’t include the numbers of water shrews,
water voles, dormice and squirrels.
INSECTIVORA
51,650,000*
The distribution of the harvest mouse is generally
limited to southern England and coastal Wales,
with scattered colonies further north. The species
is likely to be sensitive to climate change and, with
a preference for dry conditions, may be limited by
summer rainfall. Under threat from agricultural
practices such as planting crops right to the edge of
the field, which means leaving no grass margin for
small mammals to use, in addition, numerous large
fields mean less unmanaged habitat for them. Also,
pesticide use, hedge management and possibly the
flooding of reedbeds are possible threats.
Common shrew
26,000,000
Mole
19,750,000
Harvest mouse distribution
Wood mouse
19,500,000
RODENTIA
66,602,500*
Map: The IUCN Red List
of Threatened Species
Bank vole
17,750,000
Field vole
17,500,000
Common rat
5,240,000
Map: The Tracking Mammals Partnership
Pygmy shrew
4,800,000
HABITAT
House mouse
4,535,000
As the seasons change, harvest mice modify their
lifestyle. In summer they primarily live a
stalk-zone existence, inhabiting in grassy
habitats, where the vegetation is tall; including
reed beds, hedgerows, roadside verges, gardens,
cereal sown fields and any area of rough grassland
with tussock-forming species, such as Cock's Foot.
Harvest mouse
71% decline over
18 years. Results
from partial survey
published in a
Mammal Society
newsletter.
During winter they adopt a ground-dwelling
lifestyle, although they don't dig their own burrows
and will use the holes of other small mammals.
Thereby they avoid cold winds and this lets them
utilise the warm micro-climate at ground level.
Data source: UK mammals: Species status and population trends
by the Tracking Mammals Partnership
Harvest mouse
1,415,000
Hedgehog
1,100,000
Yellow-necked
mouse
662,500
Things to do
HABITAT MANAGEMENT
Conditions for harvest mice may therefore be improved by:
For landowners (such as farmers): Sensitive management
that promotes harvest mice and other mammals'
existence on their farms. Plant wild bird or cover crops
with millet as part of the mix, it’s ideal for harvest mice
to nest and feed in.
Also hedgerows are important, these allow small
mammals to move between sites and also enable
them to get away from floods - especially if the
farm has ditches that flood on a regular basis.
For wet areas such as ponds, ditches, rivers
or dykes, leave the surrounding vegetation
to grow taller to provide nesting places.
Allow the grass around
the trees to become long.
Agri-environment schemes provide funding for grass
margin creation and management, soft-grazing (cows,
not sheep) thinking about grass species planted in the
margin, etc. The choice of grass species is important,
as harvest mice prefer tussock-forming species such
as Deschampsia, Reed Canary Grass and Cock's Foot.
2m
Leave areas of rough grass and cut them
on a 3-5 year rotation (at least 2m wide).
SURVEYING
Taking part in nest searches will help ecologists
to build a picture of the current distribution of
harvest mice.
For more information see:
www.mammal.org.uk/harvestmouse_survey
Surveying for harvest mouse nests is not a difficult
task. If you find a small woven nest, from about 6 cm
to about 10 cm in diameter, built from strips of grass
or similar vegetation, with or without an entrance
hole and in suitable harvest mouse habitat, this is
likely to be a harvest mouse nest.
Indirect methods, such as raptor and owl pellet
analysis, can be useful indicators of the presence of
harvest mice in an area.
The MISE project is testing innovative
techniques using millet-baited pots. The
initial survey method, trialled in 2012 at
Chester Zoo’s release scheme, involved
attaching pairs of plastic pots horizontally
to bamboo canes. Droppings found within
the pots were collected and DNA tested by
the MISE team at Waterford Institute of
Technology (WIT). All the samples that
tested positive for harvest mouse DNA
came from the upper-level bait pots.
For more info see: www.miseproject.ie
ACKNOWLEDGEMENTS
SOURCES
(*) ‘Secret Life of the Harvest Mouse’
by Stephen Harris
www.wildlifetrusts.org
‘Early development of climbing
skills in harvest mice’
by Reiko Ishiwaka & Takayuki Mori
www.oxonmammals.org
‘UK Mammals: Species Status and
Population Trends’
First Report by the Tracking
Mammals Partnership
www.iucnredlist.org
‘Harvest mouse’
by Suffolk Wildlife Trust
www.mammal.org.uk
www.berksmammals.org.uk
ptes.org
jncc.defra.gov.uk
www.miseproject.ie
Oxon Mammal Group & Berkshire Mammal Group
Amanda Lloyd. Ecologist specialising
in British mammals.
The Mammal Society
John Dobson. Essex Mammal Surveys.
Derek Crawley. Staffordshire Mammal Group.
Ruth Brandt. MSc (Res).
Text correction: Maria Turnock
Illustrations and design: Clara Prieto