Male or Female?

One of the earliest decisions you will need to make when you decide to get some rats is what sex you want.
Male rats are called bucks, females are called does, just as in deer and rabbits.
There are differences between the sexes, but both can make great pets; it just depends on what you personally want from your rats.

Be aware, the following descriptions are generalisations only. There are always exceptions to the rule, and you cannot ever guarantee how any rat will turn out. I've had females who acted more male, and vice versa. But the following will at least give you a starting point:

Bucks are bigger than does, sometimes even twice the size, and their fur is generally a little rougher. Bucks tend to mark their territory by leaving tiny drops of urine on whatever they wish to claim as theirs, and this will sometimes include you! Some bucks mark more than others. Some don't mark at all.
Male rats tend to be a bit soppier, more cuddly and more tolerant. A buck is the kind of rat that will want to sit on your lap and watch TV for hours. They can be thoroughly lazy, especially once they reach about 18 months old.
Bucks are the ideal gender for people who want a pet to cuddle and relax with, a lap-pet if you like. In my personal experience, boys are more outwardly affectionate than females, and seem to be less independent, more reliant on their human.

Does are smaller and sleeker than bucks and tend to be more active. They also have smoother, softer coats. They tend to be more agile and acrobatic than boys, meaning some toys are great for does but would be totally ignored by bucks (wheels are one such toy. While some boys will use them, you're more likely to get usage out of a wheel if you give it to a doe!). Does are quite independent, and like to do their own thing. They're on the go all the time, curious about everything, and generally more entertaining to observe than boys.
Does come into heat every 4 days or so for around 12 hours when they may become more jumpy and skittish and harder to handle.
Females are more prone to mammary tumours which are usually benign, but can get expensive to keep having removed. This is something to consider if you are keen on getting females, particularly if you are planning on getting pet shop or rescue girls, as incidences of tumours are higher in rats from these backgrounds. Diet also seems to play a part in this, and fat rats are more likey to develop tumours than lean rats. So if you go for girls, try not to let them get too fat.
A doe will be the rat who sits on your shoulder or uses your body like a climbing frame and explores everything on your desk. They're the ideal gender for people who like to have a pet to watch and be entertained by. They also tend to be more responsive to things like clicker training, or agility, as they like to be busy and active.

Which sex you chose is totally down to personal preference. If you want a big, laid back, squishy rat who will be content to sit on your lap then you're probably more likely to get this in a buck. If you want an interesting and active, busy pet that you can watch exploring and causing mischief, then go for a doe.

It's extremely easy to tell the difference between the sexes.
Females have nipples and males don't. This is something many people are unaware of, and they assume males also have nipples, as is the case with cats, dogs and many other species. But rats do not, and seeing nipples is a fail-safe way of ensuring you have a female.
Male rats are also very well endowed, ie, they have enormous testicals, so theres no mistaking who is who when you put them beside each other! The testicals are also quite obvious from a young age. Some people can sex rats only a few days old, so you don't have to wait for anything to 'drop' as you tend to do with puppies and some other animals. If you have a 6 week old rat and you can't see any balls.....then its a girl. It always surprises me how frequently people mis-sex rats. I've seen no end of girls sold as boys in pet shops, and staff at certain large chain pet shops are notriously bad at sexing rats properly. There have been many people over the years ring me in a panic because the rats they'd been told were boys were actually girls, but had already been introduced to the males! An alarming amount of accidental litters arise due to pet shops being thoroughly useless as sexing animals. Another reason not to buy from them! A good breeder will never sell you the wrong gender!
Rats are not hard animals to sex, compared to some species. However, I believe a lot of the confusion arises because females have a prominant urethra, which may look a little like a penis to people who are not experienced with rats, and they may just assume it is a male. But really, male rats are very obvious, frankly, you cannot miss the testicals! And if you think you can't see any, or have to 'search' for them, its not a male! While males can draw their testicals up into their body when very cold or very scared, the lack of nipples would still be a dead give away anyway!
But here is a reference picture to perhaps make it a little clearer:

If I were to give my personal opinion, I would say bucks are a better choice, generally, for a first time owner as they simply tend to be a little more amiable and easy going, and also being bigger tends to make them a little easier to handle.
Some people opt for girls because they believe males smell more.
This is not the case.
I have never noticed any difference between the two genders when it comes to smell. Perhaps this belief stems from people assuming rats are the same as mice, as male mice definitely do smell! But with rats, there really is little difference; rats kept clean should not have much of an odour, regardless of their gender.

Some people also seem to think that males are more likely to be aggressive toward people. Again, this is simply not true. In fact, if I were to be honest, I have had more nips from females than I ever have from my boys. However, while male rats aren't, in my experience, any more likely to be human-aggressive, they can tend to be rat-aggressive more commonly than females. Thats not to say boys fight all the time or cannot live together in perfect harmony, it just means that girls generally rub along together a bit easier sometimes.

I have a personal preference for boys, simply because I like pets that I can sit and cuddle with and which are happy to chill out and relax with me. Male rats have a kind of innocent, dopey charm that I love. Girls can be a bit too full on for me, sometimes!
But both genders make brilliant pets, and it is important to remember this page is purely a guide, based on generalisations about the sexes. The temperament of your rat will be detirmined by much more than just its gender. I have had some amazingly affectionate, licky, mellow girls, and I've had some crazy, live-wire boys. At the end of the day, it is completely up to you. If you're still not sure, perhaps ask the breeder or rescue if you can interact with some rats of each sex, and get a feel for which you prefer.
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