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Harvest Mice (Micromys minutus)


General information

Harvest mice are one of Britain's smallest mammals, and a truly fascinating creature to keep. Although many people assume they are like fancy mice, they have different behaviours and habits. I would describe them as a pet to watch rather than to handle, as although they do not bite when handled (by the tail or the scruff of the neck) they do not seem to enjoy handling in the same way as other rodents, for example rats. They are however very curious animals and will run up to you if you put your hand into the cage to see what you are doing. They are not nervous and are much more resilient than I imagined of such a small animal, handling transportation well without apparent stress. Bucks do get on and therefore there is the option to keep groups of either sex as well as colonies or breeding pairs, smell not being a problem as discussed later on.

Enclosures

When it comes to harvest mice, a taller enclosure is preferred to one with lots of floor space. A decent amount of floor space should still be provided though, with a minimum of approx 24 inches by 12 inches being suitable in my opinion. Larger tanks will also be appreciated, but a tank too large may also have a negative effect for an animal with a small natural range and who are prey by nature. Minimum height is recommended as 15 inches, more will also be appreciated if available. Glass aquariums are perfect for harvest mice as they are readily available in the correct size range and are chew proof. A wooden lid covered with 5mm wire mesh (sold in DIY shops such as B&Q and Wickes) is perfect for harvest mice as it is escape proof and offers a lot of opportunity for extra climbing as the mice frequently use the mesh. This also allows a good chance to observe your mice, especially when trying to ascertain if a doe is in the early stages of pregnancy. Taller vivariums such as those used for chameleons are also a good option, and allow for wooden structures and hanging items such as millet to be provided.

Above: Aquarium-type tank set up for harvest mice (30 x 12 x 15 inches) Below: Chameleon vivarium-type environment set up for harvest mice (approx 3 foot tall)

Harvest mice do chew wood within their enclosure so it needs to be a safe wood if you are providing any. I use apple and hazel as these are what are available to me. I provide meadow hay as bedding as this is soft and clean and therefore good for nesting and eating. As a substrate I use Aubiose, a hemp horse bedding that can be bought from equine supply shops. Luckily harvest mice have practically no odour so odour control isn't a problem regardless of which sex you are keeping. This makes buck-only groups a viable option for pet keepers, and I find the males greatly entertaining to watch as they are more active than the females. To give the mice hiding places I also provide various other items, such as natural grass tunnels, small logs with holes drilled in etc. Things like this are available in pet shops, Wilkinsons (for the drilled log) and on Ebay.

And one fantastic thing for those with many animals who will know what I mean - harvest mice do not need cleaning out very often! Every four months seems to be about right for them. Cleaning more often may discourage their breeding or make them feel insecure and is not advised by those who do conservation work. There is still no smell produced after this period, the only things I clean more often are the food bowls and water bottles, add extra hay and wipe down the inside of the glass (sometimes the mice will urinate as they climb about on the lid and this leaves streaks although again it doesn't smell!).

Feeding

A shallow bowl such as those used to feed cats is suitable for harvest mice. They do not always dig past the surface of their food very well and therefore it is advisable to make it easy for them to get to their food. They will drink from a water bottle or bowl - this is up to the owner. Mine do very well on a highly varied seed and grain mix including oats, barley, hemp, wild bird seed, sunflower, red millet, linseed, canary mix, budgie mix, plain canary, Japanese millet and niger seed to name but a few. I tend to buy every different kind of small seed I can get and make up a mix from this. They also appreciate the occasional piece of bread, chicken and (cooked) potato. I keep sprigs of millet hanging in the cages at all times. These provide opportunities for climbing as well as being a food source. If breeding the young tend to eat a lot of the hanging millet before they move onto consuming more and more of the adult mix.

Left: Young baby eating millet Centre: Pregnant doe eating millet Right: Adult buck eating bread

Breeding & behaviour

Harvest mice are notably different in both breeding and behaviour to other rodents, notably the natural comparison of fancy mice (mus musculus). The first thing to note is that only the dominant pair (alpha buck and alpha doe) will breed. The other animals in a colony do not breed if the alpha pair are still present. Therefore if you have more than a 1.1 pair and want them to breed, you must work out which pair is dominant and separate the others from them. The next most dominant pair will then pair up and breed and so on. The submissive bucks are often found sitting high up out of the way looking miserable once breeding has started! The dominant bucks will also do this when the dominant doe is the only other harvest mouse present as he is not in charge - in harvest mouse society the females are the ones who are boss. She will often chase the buck and nip at him, or adopt a threatening mouth-open soundless 'growl' when he approaches before launching herself at him. The buck is not injured in these 'attacks' but he certainly has to work hard for a mating.

Adult buck washing

Does are pregnant for approximately 18 days (it may be a day shorter or occasionally a day longer) and give birth to an average of 5 babies in my experience, although they can of course have more or fewer babies than this. The babies should not be disturbed when born in case of rejection by their mother and start to appear from the nest from the age of 12 days. At this time they are fully furred with open eyes and start to sample solid food. Leading up to their emergence and afterwards you may occasionally hear a loud squeak from the nest. You may fear that this is the doe hurting her babies, but it is actually the babies interacting with each other and sometimes squabbling - I have found the does to be very good mothers. Dad will look very interested in the nest area just after the babies are born and may tentatively nosy around it, only to be chased away by the doe. He will not hurt the babies and should remain in the cage with the doe at all times. After the litter is born the buck and doe will chase each other round the tank, the buck because he is interested in matingthe doe again and the doe will try to fight him off or keep him away from her nest. This is all normal and they will not hurt each other. They may mate again and the doe may become pregnant immediately, although not always. Sometimes there may be months between litters and sometimes they will have several in a row, with which they cope very well. Unlike with other rodents, the pair should not be separated or the doe will not breed again and she will be lost as a breeding animal.

When your juvenile mice are around 17 days old they are fully independent and should be removed from their parents. This is for several reasons - the parents can start to get annoyed with the babies as they get older and they risk harm coming to them, and if the doe is pregnant again she will be due to give birth any time and will not necessarily tolerate the presence of her older litter once the new one arrives. They should be moved to another tank as one group; they are not sexually mature until approximately 6 weeks old and there is no need to separate them before this unless some are to go to new homes, which I would say should be after 4 weeks of age.

Left: Baby perched on a twig Right: Baby emerges from the nest

Sexing

Sexing harvest mice is more difficult than with other rodents, partly because of their small size and partly because they are rather well furred under. They are also difficult to keep still, even when scruffed, so I have found the best method (and least stressful for the mice) is to let them walk around on the mesh lid of their tank and just observe their unders. This works best at 4 weeks and onwards, when it is easier to tell the genders apart. Bucks can be told apart by the position of the penis, which is well separated from the anus and is closer to the stomach. Does have nipples, although these can be difficult to see on young mice, and there is very little gap between their sexual organs and their anus in comparison to the bucks. When the does are pregnant or nursing it is very easy to see the nipples - see the photos below.

Above: Adult does - note the visible nipples due to feeding babies and pregnancy

Above: Adult doe who is not pregnant - the nipples can still be seen

Below: Adult bucks

More photos

Above: Close up of a harvest mouse's teeth and tongue

Below: When pregnant one of my does has a habit of licking the glass in her tank in a particular spot!


©2011 Cait McKeown