Understanding Garter Snakes Through Their Diets
You can learn a lot about a garter snake by what it eats.
Garter snakes are known for eating a variety of endothermic prey, such as amphibians (especially frogs and toads), fish, earthworms, and even slugs and leeches. But it’s more complicated than that. Several garter snake species specialize on only a few of these prey items and refuse the others; other species will eat all of these and more. For example, some people may not know that a few species will eat small mammals or birds, which makes it possible to feed them mice in captivity. Not only that, but the Western Terrestrial Garter Snake (Thamnophis elegans) has an exceptionally broad range of prey preferences: it also likes to eat reptiles, including snakes (so they must be kept separately in captivity). Then there are the exceptions, like the Mexican Alpine Blotched Garter Snake (Thamnophis scalaris), which is known only to eat lizards. So it’s a mistake to assume that all garters eat the same kind of food. It’s important to pay close attention to what garters eat, especially if you’re thinking about keeping one in captivity. What I will do in this article is shed a little light on the complexity and variation in garter snakes’ diets, both in the wild and in captivity.
In my presentation at the OARA meeting on September 12, I used Rossi and Rossi’s (1995) division of North American garter snakes into three categories by prey preference as a way of summarizing the genus: the big, nasty, long-headed, bug-eyed aquatic fish and amphibian specialists; the small, docile, short-headed worm and slug specialists; and, in between, several terrestrial species that, in Rossi and Rossi’s words, “will eat almost anything.” This third category of garter snake is the one with which we are most familiar, because they are common snakes with large ranges, and because they are the most likely to turn up in the pet trade. But we have to be careful about saying that they will eat anything. In fact, these generalists are very much like the two categories of specialists: they all make use of the prey available in their habitat. It’s just that the generalists are found in many different kinds of habitat, and so have developed a wide range of prey preferences. In fact, as Richard Seigel points out in his chapter on ecology in The Garter Snakes: Evolution and Ecology, the monograph he co-authored with Douglas Rossman and Neil Ford, a garter snake’s habitat has a bigger impact on its diet than its species — and other factors, such as season and age, are also significant influences. When you think about it, the reason why some species of snakes have specialized in a certain prey is because they have specialized in a given habitat, which eliminates other options for food.
Diet as a Function of Habitat
Take, for example, one of the pointy-nosed aquatic garters, the Sierra Garter Snake (Thamnophis couchii). This big, nasty snake is found in aquatic habitat in central California and Nevada, and specializes in fish and amphibians, actively hunting them even underwater. While this snake is just as big as other garter snakes that do eat mice, it refuses to eat even fish-scented mice in captivity. Because it is simply not found away from water, it does not prey on mammals in the wild. On the other hand, the Santa Cruz Garter Snake (Thamnophis atratus), another west coast species that is closely related to T. couchii, is less dependent on aquatic habitat, and reportedly can be trained to eat mice.
Sometimes a garter snake’s prey preferences are broader than its habitat allows. Butler’s Garter Snake (Thamnophis butleri), which can be found in southwestern Ontario, is one of the short-headed garter snakes; it feeds mostly on earthworms. But laboratory tests have shown that Butler’s Garters are also willing to take fish and amphibians. Butler’s Garters evolved from Plains Garter Snakes (Thamnophis radix), which are prey generalists: what probably happened is that Butler’s garters were prey generalists that moved into a habitat in which one kind of their preferred prey was available, and so, because of their habitat preferences, they became specialists in earthworms.
Even the so-called generalists among the garter snakes will specialize at the local level. The specialists usually have quite limited ranges; the generalists have such broad ranges that variation in their diet is inevitable, because the available prey will vary from place to place. So, we have some populations of Common Garter Snake (Thamnophis sirtalis) that are able to able to eat Rough-skinned Newts (Taricha granulosa), the skin toxins of which other populations of T. sirtalis could not handle. Similarly, some coastal populations of the Western Terrestrial Garter Snake in California have developed a taste for slugs, which inland populations refuse. (Interestingly, a preference for eating slugs is a heritable trait!) Inland populations, on the other hand, are more likely to eat fish. So the lesson is this: whether a wide-ranging species found in many different habitats or a habitat specialist found in a circumscribed range, garter snakes will make use of what’s available where they live.
Diet as a Function of Age
Of course a garter snake’s diet is also dependent on its age and size. It’s no surprise that smaller snakes can only handle smaller prey, but it also appears that some garter snakes’ preferences change as they grow. Let’s look at some of the generalists. Young common and plains garters live almost exclusively on earthworms in the wild. Medium-sized garters seem to have the widest variety in their diets, since they can continue to feed on earthworms, but also add tadpoles, small hylids, and recently-transformed ranids to their diets. Big garters tend to feed almost exclusively on larger ranids and mammals; one study of Wandering Garter Snakes (Thamnophis elegans vagrans) found that very large snakes ate nothing but mammals. Females tend to eat more mammals than males, probably because females are usually quite a bit larger than males. But size isn’t everything: the aquatic garters of the west coast are more than large enough to eat mice, but they don’t.
Diet in Captivity
So for the most part, the limitations on many garter snakes’ diet are based on what they’re able to eat in their habitat; their prey preferences can be much broader than what’s available. This makes some of these species very easy to feed in captivity. It’s not difficult to get young garter snakes to eat pinky mice, for example, if they belong to a species the adults of which like to eat mice in the wild. Not only that, but species that don’t normally eat mice or fish in the wild can often be trained to eat them in captivity. For those whose diets are a bit more specialized, it’s important to know which prey items are appropriate. You wouldn’t feed earthworms to Ribbon Snakes, for example, nor would you feed mice to a Sierra Garter. Obviously knowing about the habitat and diet in the wild of the species in question is essential.
Generally speaking, garter snakes are reasonably easy to feed, and yet the biggest problem in garter snake care is providing them with a healthy diet. Just because they will eat just about anything doesn’t necessarily mean that they should live on just one thing, such as fish. Significant health problems can arise if you don’t feed your garter snake a complete diet. But what does a complete diet involve?
One problem is that the ideal prey item for most garter snakes is not really a viable option. Most adult garters feed preferentially on anurans, and in an ideal situation this would make up the bulk of their diets. I’m sure they’d prefer it that way. But there are lots of reasons not to, apart from the fact that many herpers may balk at using frogs as food items! Collecting frogs as food for your garter snake is difficult to justify given declining amphibian populations worldwide; with the new bait regulations set by the Ontario Ministry of Natural Resources, it’s also more difficult from a legal perspective. Then there’s the matter of the parasites that wild-caught frogs could transmit to your snake (though freezing them first would ameliorate this). All things considered, frogs shouldn’t be used as garter food, though in some cases garters may want to eat nothing else: the British garter snake keeper Alan Francis received three plains garters that would eat nothing but frogs in their first year in captivity.
Earthworms are a somewhat better bet, though there are some concerns about parasite transmission with them as well. Some garter snakes, such as Butler’s Garter, the Short-headed Garter (Thamnophis brachystoma) and the Northwestern Garter (Thamnophis ordinoides), will want these more than anything else, except maybe slugs or leeches. Most young garters, such as our local Eastern Garter Snakes (Thamnophis sirtalis sirtalis) will happily eat them, since they probably feed mostly on worms in the wild, but Ribbon Snakes (Thamnophis proximus and Thamnophis sauritus) and aquatic garters won’t.
Worms collected from your garden will be eaten with particular enthusiasm, and nightcrawlers (Lumbricus terrestris) purchased at a bait store will also suffice. Be careful with nightcrawlers, which are big and muscular: be sure to cut them into small pieces when feeding them to small snakes. I once cut one in half and fed one to one of my baby eastern garters. It crawled back out of the snake’s mouth! (Don’t worry, the snake is fine.) Don’t use red wigglers (Eisenia), which are the worms used in vermicomposting and are sometimes sold as trout bait: they are reportedly toxic, or at the very least foul-tasting. Worms are also deficient in calcium, though there is some debate about whether this is a cause for concern; as a precaution, if your garter’s diet is mostly worm-based, supplement it periodically with calcium.
What about fish? Many keepers rely on fish because it’s very easy to find, and because it’s easier to control parasites with freezing. But in several cases you have to be careful. In terms of buying feeder fish at pet stores, using feeder guppies for baby snakes is probably all right, but from what I’ve heard about feeder goldfish, they’re best avoided as a food source. Fish fillet is very convenient, because it can be bought frozen at the supermarket, but it’s not complete nutrition: a garter that lives on fish strips misses out on the nutrients found in bone, fish guts, heads, eyeballs and so forth, so periodic calcium and vitamin supplementation will be necessary. But the big worry is an enzyme called thiaminase, which destroys vitamin B1 (thiamin) and causes thiamin deficiencies: if you feed your garter nothing but fish that contain thiaminase, this can kill your snake.
Here’s what happened to my first snake, a Red-sided Garter (Thamnophis sirtalis parietalis) I got when I was eight. During its first winter with me I fed it thawed bait minnows, which, it turned out, contained the thiaminase enzyme. The snake went into convulsions: it lost motor control and began thrashing around the cage. Luckily, it didn’t die; after my parents consulted with the local herpetologist, it recovered after being force-fed a diet rich in vitamin B1. On that herpetologist’s advice I switched the snake’s diet to ocean perch fillet from the supermarket. Ocean perch apparently doesn’t contain thiaminase.
There is some confusion in the hobby about which fish contains thiaminase. Some think all fish contain it, others think only frozen fish is at fault, still others think it’s fresh fish that is the culprit. In fact, it just depends on the fish. Many care manuals advise denaturing the enzyme by heating the fish to 80°C. This sounds messy, and can be avoided by simply using fish that doesn’t contain thiaminase. The problem is that it’s not clear which fish they are. The culprits seem to be oily fish, so whitebait and smelts are probably out. If ocean perch is fine, then other commercially-available whitefish, like catfish or snapper, are probably all right, too. Remember, fish fillet is an incomplete diet. Trout are also fine, but too big for anything but the big, nasty aquatic garters. But Alan Francis has come up with a recipe for garter food that has caught many hobbyists’ attention: it involves putting whole trout in the blender, mixing it with gelatin, and freezing the mixture. Pieces are broken off in appropriate sizes, thawed, and fed off.
In fact, the least problematic food for a garter snake is mice. Some have expressed concerns that a diet of mice alone may be too rich for garters, but Rossi and Rossi (1995) report that garters have been fed mice with no apparent ill effects for years. Besides, as we have already seen, some garters will eat rodents in the wild anyway. The main advantage of using mice is that they are more nutritious than fish or worms: they do not require supplementation and there is no risk of a thiamin deficiency. Garter snakes feeding on mice don’t need to be fed as often (the average garter would need twice-weekly feedings of fish or worms, while once weekly with mice is fine) and they grow faster, too. And another significant advantage: garter snake feces are less watery and less smelly!
Training garters to eat mice does not appear to be difficult, and involves the scent transfer techniques used to trick snakes that prefer to eat lizards into eating mice. Garters hunt by odour and motion: rubbing a pinky or fuzzy mouse with fish or earthworm can be quite effective, especially if the garter is already used to eating non-moving food like fish fillet. It’s not difficult to train young garters of species that will eat rodents in the wild, such as the Western Terrestrial, Plains, Common and Checkered garters. Eventually they will eat unscented mice, often very enthusiastically. My Wandering Garters, for example, attack their mice faster than my rat snakes and pine snakes do!
Even more specialized feeders can be trained to eat scented mice: a friend of mine has trained a Western Ribbon Snake and Butler’s Garter Snakes to eat mice scented with fish and earthworm, respectively. Scented mice should probably be used only sparingly with species that wouldn’t normally include them in their diet; think of them as nutritional insurance. With species that do include them in the diet, I believe that it’s safe to use mice most of the time, particularly if they’re fully-grown females. I suspect a large adult female garter snake should probably eat nothing but adult mice; if she’s gravid, she’ll probably need the nutrition! Otherwise, occasional feedings of fish that you know to be safe shouldn’t hurt at all. That, at least, is what I do for my own garters; other garter keepers may have their own opinions. The important thing to remember is that a diet that is complete and varied, within the parameters of the snake in question, is likely the best way to ensure a garter snake’s health in captivity.
First published in Chorus 17, no. 8 (Oct. 2001).