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Even though ship rats have even more prominent testicles than Norway rats (which isn't easy) they can be quite hard to sex. When nervous, the bucks can withraw their testicles into the abdomen, whilst the does have a prominent bulge around the genitals. If in doubt, leave the animal to calm down for at least several hours, and then look at it from the side.
The size-difference between the sexes is not as great as in Norway rats. The smallest does are the same size as the smallest Norway does (i.e. the size of a large hamster), while the biggest bucks and does are both the size of a large Norway doe (i.e. about as big as a medium-sized grey squirrel).
Does are very playful but noisily quarrelsome, and are generally more frenetic and vocal than the males. They have no obvious sense of territory, and will invade other animals' cages without hesitation. They will also accept strangers into their own cage fairly casually.
Bucks are territorial and can be extremely aggressive towards other males (of any species) who aren't members of their own group. Within their own pack, however, once the issue of dominance has been sorted out they form harmonious friendships and are less quarrelsome than the does.
Male ship rats make excellent, loving fathers and can safely be left with their mate and babies (except that they may get the doe pregnant again - see section on reproduction).