Duvernoy’s Gland Secretions in Colubrid Snakes

—This article by Jerry Cates, first published on 20 March 2010, was last revised on 24 April 2016. © Bugsinthenews Vol. 11:03(18).

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The Colubridae family of snakes includes 320 genera and about 700 species. Until the 1920’s (some authorities say the 1950’s) these snakes were considered utterly non-venomous. Not harmless, just non-venomous, as the word “harmless” cannot accurately describe a family, wherein certain members bite with teeth that pierce the skin and splatter blood. The wound often heals quickly, precisely because profuse bleeding cleans the punctures, and for true herpetologists that makes an occasional bite from a “non-venomous” snake an acceptable part of herping, not to mention a badge of distinction. Herping, one might say, isn’t for sissies.

Many colubrids, however, are anything but non-venomous. O’Donnel et al mention, with particular reference to ring-necked snakes in the genus Diadophis, that despite an absence of specialized venom delivery systems like those of vipers and elapids, many colubrids have advanced salivary glands (Duvernoy’s) that produce proteins with phospholipase A₂ activity. These scientists then note that phospholipase A₂ is a common component of elapid and viperid venom.

When serpents with phospholipase A₂ activity bite their prey the mechanical effects of the bite are synergized by the chemical effects of their saliva. For many if not most of these snakes, the phospholipase A₂ activity in their salivary secretions isn’t merely sufficient to anesthetize, but to actually kill that prey.  Michael Smith, in his humorous paper on Duvernoy’s glands and what he calls “warm” herping (as distinguished from “hot” herping, i.e., the keeping of highly venomous serpents), discusses this topic as it pertains to hog-nosed snakes in the genus Heterodon, and garters in the genus Thamnophis. For Smith, who writes tongue-in-cheek, discovering that his beloved garter snakes were mildly venomous was a true esteem-raising experience.

The question is, how dangerous is this venom to man?

The answer to that question varies. It depends, for one thing, on who you ask. Most importantly, it also depends on the snake, both in terms of its personality, the chemistry of its Duvernoy’s gland secretions (the salivary secretions of some colubrids are absolutely non-toxic), the architecture of its mouth and jaw, and the size and location of its teeth. In Texas, and presumably throughout North America, none of the native colubrids are thought to be dangerously venomous (though they may pose hazards to children, elderly persons, and individuals with unusually sensitive or compromised immune systems). This is not true in other parts of the world, where certain native colubrids are considered nearly as dangerous as the vipers and elapids they coexist with.

But, before we go into those issues, we first need to ask which colubrids have Duvernoy’s glands that produce venomous secretions. This discussion on that is confined to the snakes found in North America. First, we need to look at the kinds of Duvernoy’s glands that North American snakes possess.

Duvernoy’s glands are actually an advanced kind of salivary gland (see Kardong, 1982). These were modified, many geneticists believe, in accord with what has come to be called the Red Queen Principle of selective adaptation, to produce toxic secretions. This is a controversial subject, however, as some consider the distinction between Duvernoy’s glands and venom glands a non sequitur. I am presently undertaking an in depth review of the literature on this subject and will flesh my findings out in this thread as my grasp on this topic improves.