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Young adult does come into season every four or five days in summer. A doe on heat will tear madly about, punctuated with 18" vertical take-offs; waggle her ears so fast they blur; and sit down on her rump and lash her tail. Does who live together come on heat together.
Ship does like to make a group of bucks chase them, so they can pick the fastest. If a buck isn't paying her enough attention, the doe will knead his shoulders with her hands (like a cat "making bread", only much faster) - or even pick up his tail in her teeth and pull it.
If there's no buck available, some does will creep up on you and start kneading you - a peculiar sensation. My steel doe Charcoal actually climbed up my back so she could knead me in the right place, between the shoulder-blades.
Many ship does living in a domestic situation are actually infertile: either they aren't interested, or they mate but don't "catch". This may well be because in the wild state they would expect to have 15 suitors chasing them over quite a long distance. Rats have induced ovulation, which means they only produce eggs if sufficiently stimulated: and it may be that without this mass marathon ship does simply don't get excited enough. If you can't provide several alternative bucks and a giant aviary, letting your mating pair of ship rats out to race round the room together should at least help.
Pregnancy lasts about 23 days. The doe does not become as obviously bulbous in front of the hips as is usual in Norway rats: instead her babies are carried quite high (or more properly, in the case of an animal which normally goes on all fours, forwards) so that the extra weight and thickness are more evenly distributed. This is presumably so that her balance isn't affected by a sagging belly while scrambling about in trees.
The father can safely be left in the cage. Ship rats form long-lasting pairs: the bucks make excellent fathers, and both parents take turns caring for the babies.
However, the doe comes on heat as soon as her babies are born (called post partum strus). If the buck is left with her she may get pregnant again immediately (implantation may be delayed up to 10 days following such a post partum mating). In practice, however, most ship does only have about 3 litters a year even if they are living with a buck full-time.
Litter-size ranges from 2-6+, normally 3 or 4. Ship rats are usually competent mothers, if not as reliably so as Norway does. Once babies are born, give feed supplements to help her produce milk - Complan, vitamin-drops etc. - and make sure she gets a lot of calcium to prevent eclampsia.
Babies are born blind and naked, but fur grows in by 10 days and their eyes open at 2 weeks. They are initially very still and placid (presumably so they don't fall out of tree-top nests while too young to climb back), but become hyperactive, playful and jumpy when they leave the nest at about 17 days. They are already almost fully weaned at that point.
Unless it is very hot, the doe stays with her babies in the nest more or less the whole time they are suckling. I suspect this is why ship rats are less greedy than Norway rats: they grow up with milk always available on tap, whereas Norway kits have to fight their 11 siblings for 5 minutes at the milk-bar. There is actually a fatal abnormality in some strains, which causes some of the kittens not to bother to feed at all.
Ship rat mothers defend their young bravely, but are more neurotic than Norway does: if you seriously disturb a mother ship rat with young babies she may eat them. While she is nursing she is likely to be startlingly aggressive - flying at the bars and swearing.
If a doe dies, or has no milk, it is possible to foster ship babies onto a nursing Norway doe. Take the new foster mother out of her cage for a bit, rub the fosterlings in dirty bedding from their new mother's nest, and then leave them in the nest for half an hour before putting the doe back in. The nest will need to be kept warm, to compensate for the fact that Norway does don't sit with their litters full-time.
If a doe dies or dries and no foster-mother is available, you can attempt to hand-feed. The babies will need to be fed every three or four hours from some sort of dropper. I personally would use a syringe with a wide-bore needle which has had the point cut off and then been filed smooth: in this case it is very important to make sure the milk is flowing freely through the needle/nozzle, as a blockage can cause the needle to be shot off the end of the syringe with some force.
Once their eyes open you can offer them some solid food, especially soft food such as human baby-food, and once they are taking it they will need less milk.
Until they are old enough to be running about and grooming themselves, you should massage the babies' stomachs gently with a bit of warm damp cottonwool or similar after every feed, to encourage them to defecate, and then wipe their behinds when they do so.
The babies will need to be kept very warm, at least until they are running about. Being in a warm room isn't enough, especially for ship rat kits whose mother would normally be in the nest with them all the time. They will need to be under an infra-red lamp or on a heat-pad or fresh hot-water bottle. Arrange their box so they have a cooler end - say two-thirds on the heat-pad and a third off it but still close to it - so they have somewhere to go if they find the pad too hot.
You should also try to keep the nest pretty humid, as the natural nest made by their mother would be. In dry conditions hand-reared ship rats often lose a small piece off the end of their tail.
It is also possible (though I personally don't fancy it) to take babies off the mother a few days before the 17-day transition, and hand-feed them. This does make for a friendly, handleable rat.
Baby Norway rats pounce and wrestle and roll round and round in a ball of assorted legs and tails, very much like cat-kittens. Baby ship rats scuttle about side by side, slapping and pulling at each other and pushing each other off things.
Until such time as a fully domesticated strain is developed, baby ship rats should never, ever be offered to a pet shop. They are only really suitable for experienced rodent fanciers (at least, people who have experience with fast-moving, semi-wild rodents such as chipmunks and spiny mice) and zoos.
There is no big hurry about separating the buck-kits from their mother and sisters. They are more or less weaned at 3 weeks - but not fertile until about 3 months.
It is easiest to sex babies when their fur is just growing in - does have nipple spots, males do not. Once they are fully furred it is hard to see the nipple-spots, and females have a prominent genital mound and a projection called the genital papilla which looks superficially like a penis. By the time you need to separate them the bucks' testicles will have dropped and it should be abundantly clear which are the males: but bear in mind that nervous bucks can withdraw their testicles into the body and keep them there for hours.