The harvest mouse
The word "wetlands" conjures up
various images. For you, it may be
extensive boggy grasslands, beds of
rushes, or waders and overwintering
ducks. Less romantically, cold feet
while standing with binoculars vainly
searching for the variety that a
'friend" assured you "was there
yesterday". A harvest mouse would
probably not be high on the list in the
word-association game. But signs of
harvest mice have been found at
Nansinellyn Marsh Reserve, near
Perranporth, as a result of a survey in
which 50 tennis balLs on canes were
plan ted by three intrepid volunteers
during August this year.
The harvest mouse is our smallest
rodent, weighing in at
between five and ten grams - about the
weight of a 20p piece. (The pygmy
shrew is lighter but that is an
insectivore. It lives in tall, dense
vegetation. including cereal fields,
reedbeds. sedge beds, grassy
hedgerows and bramble patches. In
shape it is, as you would expect,
mouse-like, but with small ears. I has a
long prehensile tail. Wood mice and
yellow-necked mice have an escape
mechanism whereby, if the tail is
seized, the skin strips off. Harvest
mice do not have this option. The
colour is a russet orange on the dorsal
side and pure white underneath.
Hearing is acute. eyesight is poor.
although they can detect sudden
changes in silhouette several metres
away.
Tennis ball cut open after summer to show
harvest mouse nest.
Photos: Ron Evenden
The harvest mouse is the only British
mammal to build a nest of woven grass
above ground. To find this nest is
proof of its presence in the area. A
breeding nest is about ten centimetres
across and is made by the female. This
is a clever construction. She sits on a
stem and uses her incisor teeth to split
the ends of the grass leaves and weave
them into a flamework. This is then
lined with more leaves pulled thfl)ugh
the framework. The centre has a lining
of grass or thistle down. Non-breeding
nests are also built, being only half the
diameter.
The breeding season is late May to
October with August/September as a
peak period. The average litter is five
and there can be more than one brood.
The female abandons the young at 15-
16 days. The offspring may continue
to use the nest for a couple of days or
so after this.
Harvest mouse at work.
Photos: Ron Evenden
The diet is mainly insects and seeds but
may be supplemented with fungi, moss
and root material. However, not all
that much is known about their diet.
With mammals, the ratio of surface
area to body volume is important. The
higher this ratio, the greater the heat
loss. In small mammals this is hich. so
consequently they need a
comparatively higher intake of food
than large mammals. In the case of the
harvest mouse. this means consumiuc a
third of its body weight each day. Even
so. this is only about two grams. It is
thought that this small rodent can reach
densities of 200 per hectare, but even
with those numbers they are not
considered to be a threat to food stocks
in this country. Plagues of harvest
mice in Russia and Europe have caused
extensive damace.
Their distribution is mainly flom
Yorkshire southwards, with only
isolated pockets in Wales. Perhaps the
latest survey will show this is not
correct. Who knows?! Surveys have
thrown up surprises both good and bad.
An example of good news came from
the 1970 harvest mouse survey. It was
thought up until then that it had
become rare since the nineteenth
century, but this survey showed it was
more widely spread and locally
common. Another case of under-
recording - let us hope that this is still
trile.
This handsome. clever creature
deserves our protection and is worth
more than its weight in coinage.
Ron Evenden
Retirement from the teaching profession has allowed Ron Evenden more time to persue his interest in nature.
An active member of the mammal society an the Otter Group, he has also been involved in the dormouse survey,
led the Trust's harvest mouse project, and has kindly offered to lead our new Mammal Group.
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