How to sex a Slow Worm (what I'm sure you've always wanted to do!)

That rusty sheet of metal is looking at you… Could it have a lizard under it? Maybe even a snake? Unable to hold yourself back anymore you go over and lift it up, the plants growing at its edges bow backward, and underneath scores of tiny invertebrate’s squirm away into the damp mud. Then you spot

A handsome male Slow Worm emerging from hibernation

in mid-March.

it! That sleek, glistening body belonging to none other than the humble Slow Worm.

A male (right) and a female (left) basking together 

beneath a sheet of old metal.

Finding one of these beauties is like finding buried treasure, and these legless lizards are beautiful as they are scaly. Whenever you find one lounging under a stray piece of metal, you’ll get a little rush, something I’ve felt countless times, I think it’s just because they’re so different compared to things like birds. And (one of) the great things about Slow Worms is they don’t mind living with humans. This probably has something to do with the fact they spend most of their time underground, where they don’t really see or have to tolerate many of us. Either way, especially if you live near the South Coast, chances are there are some right on your doorstep.

Go on! Give it a lift, never know what could

be underneath it.

The main reason you don’t see them often is that even when not underground Slow Worms aren’t keen on basking in the open, preferring instead to do so under refuges. While corrugated iron is a favorite, any sheet of metal, rubber, and even wood could be concealing one, or several of these gorgeous creatures. But once you’ve found one, held it, put it back and done this again and again it can sometimes get a bit repetitive. So why not take a closer look at them? Because, every colour, pattern, and marking tell you a whole host of information about the animal.

In this blog post, I’ve noted down the markings and signs to look for that tell you the gender of that polished little lizard in front of you. And remember, just like it says in all the textbooks “they aren’t snakes, they’re legless lizards”.

Characteristics of a male Slow Worm:

The most immediately obvious sign to look for is the colour of the Slow Worm. While occasionally difficult to distinguish gender based on this, the males are always a beige/tan to a sickly grey colour all over the body, which is relatively pale. If you (CAREFULLY) pick up the Slow Worm, you can also look underneath at the colour of its belly. If it’s a male it’ll have a beautiful gray one with a silver sheen,

My home patch of grassland and scrub 

conceals not only Slow Worms but 

Common Lizards too.

sometimes looking icy blue, but again they’re always quite pale.

Looking at the colour of their bellies is (in my opinion) the easiest way of separating the males from the females, but a little tip: when you pick them up do not lift them above your head. This is something I’ve made the mistake of doing and there’re a couple of reasons why you should just tip them on their side instead. One, when you hold it above your head you have to hold it at a bit of an awkward angle, so if the Slow Worm decides to wriggle around you won’t have a great grip to keep a hold of it. When this happened to me, I fell over backwards and probably could’ve dropped and crushed the little thing if I

By far the largest Slow Worm I've ever found. He was a good

35cm long, although they can grow longer than 40cm!

hadn’t held on to him. (He seemed completely fine and slithered off afterward don’t worry.)  

The second reason is that when scared, one method Slow Worms use to try and shake off a predator is to excrete something called a musk. Basically, it’s a bit like on-demand explosive diarrhea. And by holding one up above your face you’re practically advertising it as a target.

With that out of the way, back to other signs to look out for. As I mentioned earlier, there are many different variants and mutations in the colour of Slow Worms, and one of these only occur in males. Normally the backs of the males will just be smooth, without any markings, but some you’ll find have conspicuous blue spots lining them. And from what I’ve read, these are exclusive to males, so it’s an immediate giveaway and is surprisingly common too. 

Slow Worms are unlike many other species in that both genders grow to be the same size, and the only

real size difference is in their heads. In order to fight off predators and rival males, the heads of the males are visibly wider than the females. This is clearest when you look at where the jaw connects to the skull, and you’ll see defined bulges in some individuals (the males) but not others. Looking from above, this gives the
males a distinct “neck”, where the body tapers inwards behind the head.

Characteristics of a female Slow Worm

Unlike the males, the females lack this "neck", having much slimmer heads, although it takes a bit of practice to get your eye in and be able to notice this.

The colour of the females clashes a bit with the males' though being anywhere from a warm gold to a dark copper on top, with the males also often being gold. Normally though the females will be a warmer tone, and the males won't shine as much. Regardless of what colour they are on top, their flanks will always be contrastingly darker, anywhere from a chestnut brown to black, compared to the males which

The black belly of a female-this one

was sadly dead. Likely killed by a cat

because it was in a very built-up area.

are a uniform colour on their top and flanks. 

If you're lucky enough to look at the underside without getting jettisoned in the face by musk, you'll see it is also very dark, usually jet black, which couldn't be more different from the pale silver gray of a male. 

And finally, the markings on females. Though lacking the males' vibrant blue spots, they do have their own unique markings, often sporting a thin black dorsal line, absent in males. Although juvenile Slow Worms have these too, they're clearer; the adults will be obviously larger, and the juveniles will have a little black face. Like with blue spots in males, only some females have a stripe, so rely more on the colour of the belly and the flanks. Are they much darker than the colour on top? Then you've got yourself a female.

I really hope this helps you next time you want to sex a Slow Worm but aren't sue what to look for (an extremely common problem in today's society).