Nowhere in the recent herpetological or herpetocultural literature regarding Butler’s Garter Snakes, Thamnophis butleri, are rodents referred to as a potential food source, either in captivity or in the wild.1 Field studies have confirmed that earthworms make up the overwhelming proportion of a Butler’s Garter Snake’s diet, followed by leeches; laboratory studies have shown that they also react to toads, small frogs, red-backed salamanders and small fish (Catling and Freedman 1980, Rossman, Ford and Seigel 1996).
Although Butler’s Garters are clearly earthworm specialists in the wild, many herpetocultural authorities, perhaps relying on dated sources that refer briefly to several prey items (e.g. Ditmars 1939, Logier 1958 and Wright and Wright 1957), seem unclear about their diet. Perlowin (1992) makes no specific comments about the species’s diet, referring only in passing to chopped earthworms and feeder guppies for neonates. Sweeney (1992) is uncertain, saying that the diet “is thought to include earthworms, leeches, small frogs and salamanders” in the wild. Rossi (1992), on the other hand, states that “earthworms are definitely the preferred food” but allows for small fish and amphibians as well. None of these authorities mention rodents. Yet we have managed to maintain six specimens of Butler’s Garters on a diet that either includes, or is mostly or even entirely based upon, domestic mice.
Jeff imported two adult T. butleri and four captive-born neonates in the summer of 2000. (Two of those neonates were later given to Jonathan.) Since earthworms are low in calcium (but see Rossi and Rossi 1995), and not always readily available in the winter, Jeff wanted to switch his Butler’s Garters to mice, as is frequently done with common garter snakes, Thamnophis sirtalis. He started the two adults with worm-scented pinky mice, and the four neonates with worm-scented pinky mouse legs and tails. All six T. butleri ate readily. After several feedings, he began trying unscented mice or mouse parts. One of the adults readily ate unscented pinky mice; the other would only take mice if they were scented with earthworm. In general, the neonates would take unscented pinky legs and tails, although some of them were better feeders than others.
When he got them, Jonathan gave his two neonates a more varied diet. Pinky parts were offered at only one quarter of the feedings, chopped nightcrawler, Lumbricus terrestris, was offered at roughly one half of the feedings, and fish fillet (ocean perch from the supermarket) was offered at the remaining one quarter of the feedings. Supplemental calcium and vitamins B1 and D3 were added occasionally. The varied diet allowed for a rough comparison of feeding responses. Not surprisingly, chopped nightcrawler elicited the strongest feeding response, followed by pinky parts, then fish fillet. It was somewhat surprising that mice seem to have been preferred over fish; more on that momentarily. In general, feeding response for these little gluttons was strong no matter what was offered, in sharp contrast to Sweeney’s (1992) comments that “in captivity the snake can prove to be a choosy feeder, often accepting only earthworms to begin with and sometimes even refusing those.”
With the onset of winter, Jeff’s four remaining T. butleri began to refuse food more frequently. Jonathan’s neonates, on the other hand, showed no sign of refusing food, even though the room in which they are kept has reached air temperatures as low as 15°C and supplemental lighting is not provided. One possible explanation is the difference in the two colonies’ diets: at that point Jeff’s T. butleri were fed pinky parts almost exclusively, whereas Jonathan’s pair were fed the varied diet enumerated above. A varied diet may ensure a good feeding response to all food items offered. Jeff’s adult T. butleri have been brumated and his two neonates have been placed with Jonathan temporarily. The neonates have been offered the varied diet and feed well.
If Butler’s Garters have such a restricted diet in the wild, why are they relatively easy to convert to a mouse-based diet? And why are their prey preferences so much broader than their actual diet in the wild? Habitat preferences are believed to limit T. butleri’s available prey in the wild (Rossman, Ford and Seigel 1996), but for our purposes that begs the question. Their evolutionary relationships with other garter snakes suggest a possible answer.
Most authorities consider Butler’s Garter Snake to be closely related to the larger Plains Garter Snake, Thamnophis radix; they refer to Butler’s Garter as a “reduced derivative” of the Plains Garter, i.e., T. butleri evolved from and is smaller than T. radix. It was, in fact, briefly considered a subspecies of T. radix at one point, along with the Short-headed Garter Snake, Thamnophis brachystoma (Ernst and Barbour 1989, Harding 1997, Rossman, Ford and Seigel 1996, Wright and Wright 1957; see Harding 1997 for a theory of how speciation occurred). A study of the diet of Plains Garters offers some possible insights into the feeding preferences of Butler’s Garters. Plains Garters reach a larger maximum length than do Butler’s Garters: 1092 mm versus 737 mm (Rossman, Ford and Seigel 1996) and as adults are quite catholic in their food preferences — mostly earthworms and amphibians (ranids, hylids, toads and various amphibian larvae), but also leeches and small mammals. As juveniles, however, they have been found to feed on earthworms almost exclusively (Rossman, Ford and Seigel 1996). Just like the closely related, and similarly sized, Butler’s Garters.
We can only speculate about what happened. The ancestors of Butler’s Garters may have been prey generalists that moved eastward into habitat in which only one kind of their preferred prey was available, and so, because of their newfound specialization in habitat, they became specialized feeders on earthworms, like the juvenile Plains Garters from which they were descended. In a way, Butler’s Garters might be seen as Plains Garters permanently stuck in childhood, never having the opportunity to graduate to an adult diet, but still capable of responding like an adult T. radix when given the chance (but see Halloy and Burghardt 1990). Notably, neither T. radix or T. butleri has been recorded as a major predator on fish in the wild, though captive maintenance is a different matter.2
While the appropriateness of a mouse-based diet for Thamnophis has been debated, Rossi and Rossi (1995) note that garter snakes have been maintained on a diet comprised entirely of mice for years with no apparent ill effects (which is more than can be said for some other captive snakes). While many natricines can be trained to eat mice (in addition to T. butleri, Jeff has trained a Western Ribbon Snake, T. proximus, and a Brown Snake, Storeria dekayi, to eat mice, but has had less success with water snakes), intuition tells us that mice should only be an occasional part of such snakes’ diets. It may be somewhat safer to use mice with Butler’s Garters if they are related to the prey generalists, but this cannot be proven without longer term study.
So far, we have observed that our captive-born neonates have proven easier to convert to mice than our wild-caught adults, but since the neonates came from a different locality, it is difficult to draw conclusions from this. We have also observed, among the neonates, that a more varied diet provokes a better feeding response on mice than a diet of mostly mice. Since a varied diet has other advantages, it makes sense to maintain Butler’s Garters in this manner.
In Ontario, Butler’s Garter Snakes are specially protected reptiles under the Fish and Wildlife Conservation Act, 1997, and they are listed as a species of special concern by COSEWIC. Both authors have permits from the Ministry of Natural Resources to keep them in captivity.
First published in The Ontario Herpetological Society News 88 (March 2001). Jeff’s snakes have since died from various causes; the last of Jonathan’s Butler’s garters died in March 2011 at more than 10 years of life in captivity, most of which was spent on an all-mouse diet.
- The single exception we have found is Wright and Wright 1957, which, citing references going back to 1908, lists field mice along with earthworms, frogs, salamanders, insects and leeches.
- Halloy and Burghardt 1990 compare T. butleri with three other Thamnophis species (T. radix, T. sirtalis and T. melanogaster) and found it to be a comparatively awkward feeder on fish. That T. radix is better at it suggests that some specialization has already taken place.
Catling, P. M. and B. Freedman. 1980. “Food and Feeding Behavior of Sympatric Snakes at Amherstburg, Ontario.” Canadian Field-Naturalist 94: 28-33.
Crowe, Jonathan. 2000. “Understanding Garter Snakes Through Their Diets.” Chorus: Newsletter of the Ottawa Amphibian and Reptile Association 17(8).
Ditmars, R. L. 1939. A Field Book of North American Snakes. Garden City NY: Doubleday.
Ernst, Carl H. and Roger W. Barbour. 1989. Snakes of Eastern North America. Fairfax VA: George Mason University Press.
Halloy, M. and G. M. Burghardt. 1990. “Ontogeny of fish capture and ingestion in four species of garter snakes (Thamnophis).” Behaviour 112: 299-318.
Harding, James H. 1997. Amphibians and Reptiles of the Great Lakes Region. Ann Arbor: University of Michigan Press.
Logier, E. B. S. 1958. The Snakes of Ontario. Toronto: University of Toronto Press.
Perlowin, David 1992. The General Care and Maintenance of Garter Snakes and Water Snakes. Lakeside CA: Advanced Vivarium Systems.
Rossi, John V. 1992. Snakes of the United States and Canada: Keeping Them Healthy in Captivity, Volume 1: Eastern Area. Malabar FL: Krieger.
——— and Roxanne Rossi. 1995. Snakes of the United States and Canada: Keeping Them Healthy in Captivity, Volume 2: Western Area. Malabar FL: Krieger.
Rossman, Douglas A., Neil B. Ford and Richard A. Seigel. 1996. The Garter Snakes: Evolution and Ecology. Norman and London: University of Oklahoma Press.
Sweeney, Roger. 1992. The Garter Snakes: Natural History and Care in Captivity. London: Blandford.
Tennant, Alan and R. D. Bartlett. 2000. Snakes of North America: Eastern and Central Regions. Houston: Gulf.
Wright, A. H. and A. A. Wright. 1957. Handbook of Snakes of the United States and Canada. 2 vols. Ithaca NY: Comstock.