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| Photo Credit: Veer |
Throughout history snake tongues have been hypothesized to be a venom delivery mechanism, a prey capture device, and a way to lubricate prey with saliva before swallowing. By the end of the 19th century the hypotheses turned more toward sensory functions, such as tactile and smell. Ever since George K. Noble's work in the 1930's it has been known that snakes used their tongues to detect molecules released by other other snakes - pheromones. Serpent tongues have been known as molecule collectors for the 70 years and have been experimentally demonstrated to be part of the vomero-nasal system. Now, new experiments demonstrate snake's don't have to pick-up pheromones with their tongue.
Rick Shine at the University of Sydney and Robert Mason at the Oregon State University have found snakes not only able to sense molecular communications through direct contact of their tongue, but are also able to 'smell' airborne molecules.
"Snakes usually depend on large fat molecules to tell them about another snake's sex and reproductive condition. Because these molecules are large, they don't float through the air - and so, a snake picks them up by tongue-flicking the body of another snake directly, or by tongue-flicking a trail that the other snake has left on the ground," said lead author Shine.
"Our work shows that snakes are more flexible that we had realised - they are quite capable of using airborne cues as well, involving different kinds of molecules, so long as these provide useful information."
Males snakes can detect that a female has mated - and will stop courtship- from the airborne scent of the fluids produced during copulation. Using red-sided gartersnakes (Thamnophis sirtalis parietalis) the authors exposed courting males to odors released by female snakes, male snakes, and two mating snakes, and recorded changes in behavior.
"We took an empty margarine container, and cut two holes (one on each side) through which we could pass hollow flexible plastic tubes. One tube ran to a portable aquarium pump, so that when we switched on the pump it pushed air through the margarine tub and out the tube on the other side," said Shine.
"We placed our 'stimulus' (say, a female snake) inside the margarine tub, and held the outlet opening close to the heads of courting male snakes at the den. That way, we could expose them to specific smells, but without giving them access to the large surface-bound molecules that snakes generally use as sex pheromones."
As predicted, when courting snakes were exposed to smells issued by the mating pair of snakes in the margarine container, the courting stopped.
The research suggests that snakes are able to adjust their behaviour to take advantage of certain cues in their environment to maximise their chances of reproductive success.
"Previous studies by these authors have shown that the presence of sperm plugs plays a significant role in the prevention of re-mating by females. The pheromonal evidence described here shows that sperm plugs are not the only deterrent employed by males to maximise their reproductive success," commented Warren Booth from North Carolina State University in the U.S., who was not involved with the study.
"Thus, we have evidence that males have evolved multiple strategies, both physical and chemical, to prevent females re-mating, and therefore to maximise their reproductive success," Booth added.
"In many species, courtship is an energetically expensive behaviour that can reduce or even eliminate a male's reproductive success if he is unable to find a responsive female. In instances where visual confirmation is not available olfactory cues in the form of airborne sex pheromones, as described here, provide the only information available to a male."
Literature
Noble, G. K. 1937. The sense organs involved in the courtship of Storeria, Thamnophis, and other snakes. Bulletin of the American Museum of Natural History, 73, 673–725.
Shine, R. and T. Mason. 2011. An airborne sex pheromone in snakes. Biology Letters October 12, 2011, doi: 10.1098/rsbl.2011.0802
Labels: pheromones, Thamnophis sirtalis parietalis
