I don’t remember what the V.C.C. biology lab used as part of their semesterly experiment on adaptive behavior, but they were smalish, white, furry, and really friendly with each other. We were joking that they must have been gay until we discovered two litters of squirming, pink babies.
Turns out that they weren’t all female after all. But how, exactly, were we supposed to check?
The babies and mommies were, IIRC, eventually fed to various pet snakes. 
Genetically, rats have 21 chromosomes to the mouse 20. These are not equivalent for the most part – there have been at least 19 large scale rearrangements.
The rat genome was sequenced to approximately 6.5x coverage, with 90% of sequence assembled into contigs, by a combination of whole genome shotgun and BAC sequencing. I don’t believe it was finished or completely annotated, though. Total sequence is 2.6 gigabase pairs (Gbps) of the estimated 2.8 GBps of the genome. Mouse was sequenced by Celera and the Mouse Consortium to well over 10x coverage and 96% assembly into contigs, by whole genome shotgun coupled to a tiled BAC sequencing approach. This spans more than 2.4 of the estimated 2.6 Gbps of the genome. There has been a careful finishing and annotation process that has been nearly as intensive as the work given to the human genome project.
From this Science article (requires subscription):
That means that they are actually pretty different genetically. Compare humans and chimps, which have only 1.4% difference in their DNA, compared to the around 75% (to my rough calculations) shared between mouse and rat.
I don’t think they exist.
While mice will eat their young under stressed conditions, I would not declare them “bad mothers.” From the other perspective: when mice are not stressed, all the females in a cage (even those that have not yet bred) will take turns “babysitting” a litter of pinkies while the mother takes time out to forage for food. Mothers that give birth within a few days of each other will have their litters in the same place and the mothers appear to nurse whoever wants food until they are weaned.
I breed tons of mice in a lab environment, which is a bit different from pet mice, but I thought I’d comment.
Yes, but if you’re feeding them properly they mostly they don’t. Some strains are more prone to this than others, especially the inbred strains.
Mice will fight if over crowded. Don’t overcrowd (duh).
There is also a relatively rare dominance behavior called “barbering”, where a dominant (female, usually) will chew the fur off the shoulders of others in the cage. I get maybe 3-5 cases of this a year, while taking care of a constantly rotating population averaging ~400 mice. Maybe once a year the barbering female gets out of hand and chews skin, too - but this doesn’t seem to be a deliberate killing.
This, also, is strain-dependant.
If they’re handled regularly, they aren’t nippy at all. If they haven’t been handled, they get scared. However, they’re really not very bright, and sometimes forget that your fingers aren’t food. They’ll nibble, then let go.
Additionally, anyone who’s at all familiar with rodents can tell a mouse from a rat at a glance, even without size cues. Rats are built differently, and most have nude tails. The faces are different too.
[rant]It annoys me to no end when we get catalogs for mouse products/services in the lab with cute pictures of fuzzy little mice in cute little poses - except they’re rats. Do these morons think that their customers can’t tell the difference if there is no indication of size? Or do they not know the difference themselves? (Not very encouraging in a mouse services company.)[/rant]
yours,
mischeivous
Upon reflection, I thought I should add that male mice which have mated do fight, and kill, each other. For breeding purposes, single males need to be kept in separate cages for mating, but groups of virgin males can be kept together in a single cage (with no females, obviously).
mischievous
Thanks for enlightening me about mice, hehe. I get my information from another forum on feeder rodents for snakes, and those were the examples given to me when I asked if I should breed mice or rats. If I get into corn snakes, I’ll probably have to start breeding mice too, so it’s good to know.
And yeah, I meant mice in that one thingy. Oops!
Does anyone know if mice get stressed more easily than rats? I’ve read several posts on the other MB about how you should keep mice in a quiet, not high-flow area of the house, but nothing about that for rats. I have my rats in a high-traffic area, and my cats sleep on top of the cages, and I haven’t seen any signs of stress.
Since we’re adding anecdotal evidence, here’s what I’ve noticed. Rats are as good of mothers as mice. The difference I’ve noticed, is that usually, if there’s other adult female rats in with a rat giving birth, at least one rat will generally serve as a “midwife”, in that, she grabs the pinkies as they come, cleans them up, and stuffs them in a corner where they’re “safe”. The only trouble I’ve seen from this, is when the mother is done with labor, sometimes the midwife is a bit reluctant to turn the babies over. Have not noticed this in mice, as often. As far as baby eaters, barring overcrowding, and generally stressful conditions, I’d go with Hamsters (not the Siberian or Chinese), as the main culprits. I’ve watched a mother take her babies and munch them as they come out, on more than one occasion.
I’ve noticed male rats do tend to have a musky scent, as they mature. If you are familiar with ferrets, it’s very similar, just a lot less pungent. I have never noticed this smell with male mice. What I do notice, is that male mice are compulsive at marking territory. In an average week, I took care of anywhere from 25 to 200 mice. Once a week, I would take out everything and disinfect it. Once the water bottle and its holder was replaced (newly disinfected), the male’s one mission, was to squirt that thing until it was covered. It reeked, and consequently so did the cage.
Not true at all, in my experience. I think rats are as easily stressed, or at least, mice are not more easily stressed. I find I am more likely to stress out a mother rat by reaching in to move babies, than with a mouse, and also more likely to get bitten. I’ve seen an entire cage of rats pounce on a newcomer, as often as I’ve seen it happen with mice.
I disagree with this as well. I’d rather have a rat for a pet, but it has nothing to do with nippiness. I’ve found most mice take to flight almost every time when frightened by being handled. Rats unfamiliar with being handled, seem to resort to fighting a lot more often than mice.
I read this in the morning and later in the evening, I managed to finally watch The Princess Bride. The figurative light bulb glowed bright that night.
A classic line 
I’ve never bred rats, so I can’t compare, but I can tell you for a fact that lab mice haven’t the foggiest clue that cats are predators. I took some to a friend’s house once, and the damn things sniffed around, said hi to the cats, and then tryed to burrow under the (sitting) cat’s stomach, presumably because it looked safe and warm. The cat, freaked out by this non-prey behavior, ran off.
With regards to baby-eating, I have a little theory (take it with salt). Sometimes baby-eating seems clearly a product of stress and overcrowding, but sometimes it happens with otherwise happy mothers. The mothers do need to eat the placenta and lick the pups clean after they are born, and I think that sometimes they get a little confused as to where they are suppossed to stop eating - they’re not so smart. Second-time mothers almost never eat their litters.
I’ve also noticed that putting a piece of paper towel into the cage can be a good predictor of maternal behavior. A unhappy (or non-pregnant) mouse will rip it into large pieces and pile it flat, whereas happy, pregnant mice will completely shred it into microscopic pieces and fluff it all around them into a huge ball they can hide in. YMMV (your mice may vary), of course.
mischievous