One of the questions I get most frequently, particularly from prospective garter snake buyers, is whether there is any difference between males and females.

For most snakes, the sex doesn’t matter much, unless you’re trying to breed them (or, conversely, trying to avoid breeding them, if you want to house two in the same cage and therefore want them both to be the same sex). With garter snakes, though, there are a few differences; some of them may matter to you.

First of all, garter snake females get much larger than males. A good rule of thumb for the most commonly kept species is that males reach a length of about two feet (60 cm), while females can reach around three feet (90 cm).

Why is this so? Garter snakes bear live young rather than lay eggs. A larger female is capable of carrying more babies; natural selection encourages larger females, because larger litters mean a greater chance that some of the babies will survive. In most live-bearing species — for example, other natricines, such as water, brown, red-bellied and queen snakes, as well as most boas — the females get considerably larger, and in roughly the same proportion: a female boa constrictor, for example, may reach a length of 11 feet (3.3 m), but a comparable male only 8 feet (2.5 m).

Female garter snakes are also much bulkier than males, for the same reason. Females tend to be much less active than males because they’re conserving their energy for reproduction. The more they burn, the less they can put into babies. Males, on the other hand, are much skinnier because they’re much more active: in the wild, they’re spending a good deal of their time chasing girls. Evolution favours skinny, fast males that can catch females, and fat females that can pump out a lot of babies.

(The exception to this rule is rattlesnakes, which are also live bearers. Male rattlesnakes engage in ritual combat with one another during mating season; as a result, natural selection encourages large males as well as large females.)

What are the implications of these differences in size and behaviour on keeping garter snakes in captivity?

For one thing, females may need a larger cage. Zoo guidelines suggest that a snake should be kept in a cage with a combined length and width that is equal to or greater than the length of the snake — for example, a three-foot snake would be properly housed in a cage two feet long and one foot deep. On the other hand, males are so much more active that they might need more room than their length might indicate; mine, for example, frequently race around their cages, and I suspect they’d use as much room as I give them.

Several years ago, my advice was that females were calmer and males were more active, so that someone wanting a calm, friendly snake might want a female. After a few more years of observing my garter snakes, I don’t think this is true any more.

Male garter snakes may be more active — they move around a lot when you try to handle them, and it can be a bit of a challenge to hang on to them sometimes — but they’re not necessarily jumpy, nervous or aggressive; if anything, they may actually be friendlier than female garter snakes. Just quick.

Female garter snakes, on the other hand, may be more sluggish, but that doesn’t necessarily mean calm. I’ve found some of my female garter snakes to be more aggressive when disturbed, and more prone to bite and musk. My yearling female Blue-striped Garter Snake (Thamnophis sirtalis similis) is incredibly jumpy and given to strike at you through the glass, and I once had a female Eastern Black-necked Garter Snake (Thamnophis cyrtopsis ocellatus) that was, quite frankly, insane.

But these are generalizations based on patterns I’ve observed in my garter snake collection. There are always exceptions. For example, my late breeder female Red-sided Garter Snake (Thamnophis sirtalis parietalis) was quite possibly the sweetest-tempered garter snake on the planet, and my female Wandering Garter Snake (Thamnophis elegans vagrans) is pretty good, too. Still, the tame garter snakes have invariably been male.

So, if all of the above has helped you decide whether you want a male or female garter snake, how do you tell them apart? It’s not easy, especially if you’re dealing with very young snakes.

The standard methods of sexing snakes — probing, popping and by visual appearance — apply equally to garter snakes. Alan Francis has a good summary on his web site. I don’t probe or pop snakes myself, so I have to rely on visual sexing, which is the least reliable method. In a nutshell, males have longer and thicker tails than females, whose tails are narrower, shorter and taper more sharply. It’s much easier to tell the difference when you have one of each sex, and it’s much more obvious with adult snakes; with baby snakes it’s very hard to tell.

Sexing a garter snake by size alone is not a sure thing: while a robust three-foot garter snake is almost certainly a female, a young, anorexic, hyperactive female could conceivably be very difficult to tell from a male. If, on the other hand, you’re raising several baby garter snakes from infancy (for example, a litter), you will begin to see differences in size when they’re about one year old, at which point the females will start to grow larger and huskier than the males.

When you have several garter snakes, there’s one other, somewhat more unofficial method of sexing them. A year ago, I was at a reptile show with the last four snakes from my 2002 litter of Red-sided Garter Snakes. I had not taken the trouble to sex them, but knew that they were old enough that I could visually sex them for prospective buyers as needed. In the event, though, the snakes sexed themselves by behaviour — I found out that I had three males and one female, because the males kept jumping on the single female (as it turned out) in the display cage and began courtship behaviour. In other words, they were sexed by the “hey, get off your sister” method.