Evolution of Venom: The Duvernoy's Gland

The Duvernoy’s gland is an oral gland that is located in the temporal region and is present in approximately thirty to forty percent of colubrids, snakes which are traditionally classified as non-venomous (Kardong, 2002).  The Duvernoy’s gland empties onto maxillary teeth that may or may not be grooved, but it does not open into hollow fangs as in “venomous” snakes (Hayes et al., 1993).  The function of this gland has been the source of much study and speculation, and debate is still ongoing.  It is widely recognized that the Duvernoy’s gland is the homologue of venom glands in vipers and elapids (Kardong, 2002; Fry et al., 2003).  However, the two types of glands are also “anatomically and functionally distinct,” leading some to maintain the distinction between Duvernoy’s glands and venom glands (Kardong, 2002).  For example, the Duvernoy’s gland is not typically associated with muscles that result in pressurized venom expulsion as is the case with venom glands (Kardong, 1996; Kardong, 2002).  Furthermore, Kardong (1996 and 2002) notes that just because some Duvernoy’s gland secretions are toxic in that they produce pain, swelling, and other harmful effects if injected subcutaneously, they should not be immediately considered venoms.  The word "toxic" describes the properties of the substance, but the word "venom" implies its biological use for the organism. After all, if any one of a number of substances, including human saliva, were injected subcutaneously, they would produce negative effects, but those substances are not considered venoms, nor are the organisms that produce them called venomous (Kardong, 1996; Kardong, 2002).  Others maintain that while it is obvious that the two kinds of glands often have distinct functions, Duvernoy’s glands should really be referred to a venom glands since they just represent primitive versions of the same structures found in vipers, elapids, and atractaspidids (Fry et al., 2003).

Figure 1. Anatomy of Duvernoy's gland and viperid venom gland. From Kardong, 2002 with permission from the author. Click the figure to see a larger version.

The debate over the status of the Duvernoy’s gland is complicated by the lack of concrete knowledge we have concerning its function (Kardong, 1982; Kardong, 2002).  It seems that the Duvernoy’s gland is rarely employed to rapidly kill prey, as is the case with “venomous” snakes (Hayes et al., 1993; Kardgong, 1996; Kardgong, 2002).  There are a number of other potential uses for the Duvernoy’s gland including lubrication and digestion.  Jansen (1983) showed that Duvernoy’s gland secretions inhibited bacterial growth which may contribute to the health of oral and dental structures, thus providing a survival advantage to the snake.  Shivik (2006) cites these anti-bacterial properties as evidence that the Duvernoy’s gland was used by ancestral scavenging snakes to neutralize the microbes present in their dead and decaying prey.  Hayes et al. (1993) showed that relatively little (approximately 54%) of the Duvernoy’s secretion expelled by a snake reaches the viscera of the prey, suggesting it is not efficient at inducing prey death.  Rather, they propose that these secretions play a role in chemical digestion of the prey or that they chemically open holes in the prey item, providing additional surfaces for digestive enzymes to act on (Hayes et al., 1993).