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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