Hi all, a friend was wanting to learn how to sex rodents so I pulled this from my old Rat Forum archives and figured I'd show her by sticking it up here so others can see it too. I'm just cutting and pasting it here, not entirely sure it'll work, but however well it works, here it is: Sexing Rats is something a lot of people seem to have trouble with. With a bit of experience it becomes pretty easy for most people, but not everyone has access to lots of rats of all ages to gain the familiarity needed. So, I thought I'd take pictures of male and female rats aged between about six hours and about six weeks to give everyone an idea. I took all these pictures last night. I have given three pictures for each age (apart from pinkies - we all know what a newborn jellybean looks like, right?). For each age group, all three rats are from the same litter. Day 1. These rats are a few hours old. Right away, you can quickly and easily sex rats. At this age, the easiest way to do it is to look at the distance between the anus and urethral orifice (the bit they pee out of). Unlike humans, the bit they pee out of looks pretty similar in males and females, and a common mistake is to think that they're all boys, because the pointy bit at the urethral orifice is mistaken for a penis. The males' penises come out of this spot but are usually inside the body. The females' vaginae are just posterior to the orifice (next to it on the side of the anus). MALE FEMALE A FEW DAYS OF AGE (I wasn't specific about timing, partly 'cause I'm lazy and partly 'cause different rats will develop at slightly different rates, and you shouldn't worry if yours look a little more or less developed at exactly the same age as these) When the velvety fur is starting to grow, the scrotum (ball sack/nut bag/testicle pouch) becomes fairly clearly visible as two dark lumps. Sorry about my really poor caption. After this one I decided I wasn't good at it so didn't try again. MALE The females' nipples should be quite visible by now, although they'll start getting covered by fur. The total lack of a scrotum and the shorter distance between anus and urinary orifice are the easiest ways to sex them at this stage, although if you look very closely you can see a vagina (verticle smile/playground) on the females. You probably won't be able to see the vagina without restraining the rat and looking under a microscope, which is borderline criminal perversion behaviour. If you look closely a vagina is visible in this picture. Oh, baby. FEMALE A WEEK OR A ROUGHLY BIT MORE THAN THAT OR SO At this point it can actually be a little tricky to sex rats by looking at the anus (stink hole/third option) and genitals (dangly bits/play parts), and the nipples (tuning knobs/slippery licks) themselves are obscured by fur, but the fur forms quite noticeable outlines around the nipples. MALE FEMALE GETTING A BIT CLOSER TO TWO WEEKS Pretty similar to slightly earlier. MALE FEMALE ABOUT TWO WEEKS OR A LITTLE MORE ISH This is probably the trickiest stage to sex rats, and the only one which you should have any trouble with, but with a bit of practice even at this age most people should be able to learn to quickly quickly, easily and reliably sex them. The testicles (pain centres/initiative installers) are still fairly undeveloped and the nipples have been obscured by fur. The anal/genital area of both sexes may appear sort of enlarged or swollen (not that it's actually inflamed or anything), the fur is covering all the technical gear and generally obscuring things, so even though the distance between anus and urine squirters is still greater in males and things still look different, it can be more challenging to determine. In some cases nipples may still be evident on females or male genitals more obvious, but not always. MALE FEMALE THREEISH WEEKS OR SO By now the nipples are usually getting pretty well covered by fur, but the underlying parts are getting larger relative to the layer of fur on top, and the testicles and scrota are getting larger. The easiest way to sex them at this stage is to look for testicles, but keep in mind that they can 'suck them back up' leaving an empty scrotum which some people find similar to the appearance of a female. The testicles are more likely to 'fall down into the scrotum' if the rat is held 'head up, posterior down' and also if the ambient temperature is warm. If you want to you can push the testicles down into the scrota, but I prefer to examine rat genitals without tactile work with my hands. Still, if you're not so skilled with your eyes and you want/need a more hands on approach to ensure accuracy, what you do in the privacy of your own home is disgusting, er, none of my concern. MALE FEMALE FOUR WEEKS OR A LITTLE MORE If for whatever reason you still haven't sexed your rats at this age, the good news is that it's now getting really easy if you know what you're doing (heck, you can sex the one above from the thumbnail version of the size reference picture ) I often do it around this stage because I wean around four weeks and don't care what sex most of them are before then. Be careful though, because despite being easy if you know what you're looking for, it's actually very easy to think a male is a female (the really bad type of mistake) if you don't know what you're doing. Basically, girls have a bum, a pee hole and a smaller space in between with very little in it (though you can see the vagina if you want to look closely and have good eyesight and are a freaky perv - it's fairly clear in the macro picture below). Males have a big hairy area and if you are familiar with rats you'll easily recognise the scrotum even if it's empty. If you're unsure you can hold the rat bum down, or use the more tactile approach as before. Hey, at this age the boys might be getting old enough to enjoy it. The hair isn't growing as much relative to the body as it was, and the nipples are getting relatively larger too, so once again the nipples start to become evident in the females. MALE FEMALE FIVE-SEVEN WEEKS At this stage rats sexually mature and basically become small adults. The testicles are large and once again the nipples show through the fur, becoming increasingly evident from now on. Even as old adults, males can 'suck their testicles back up' (usually when they're cold or scared) and it is a common mistake to think males are females because the scrotum is empty. For most people, this is a much larger mistake than mistaking a female for a male, because in one case it means all your females get pregnant, in the other it means just one gets pregnant, so be especially careful with your female diagnoses! These rats are a little over six weeks old. Males basically look the same as they grow larger, females look similar, but the (already obvious) nipples become more prominent with age for the next few weeks. Nipples will become obvious in all females, but some will be obvious sooner or later, and all the sexually dimorphic bits in both sexes can be more or less obvious depending on the fur colour and coat type.