Right: high
magnification
micrograph of the
sensory epithelium
of a VNO. The
lumen of the organ
is at top right;
sensory neurons
have an ovoid-
shaped body; dark
nuclei near the
lumen are support-
ing cells; bundles of
nerve fibres appear
below the epithe-
lium. Image by Jose
Luis Clavo.
Below: snakes may
tongue-flick more
than once per
second when track-
ing prey. Image by
Eric Isselee.
conventional olfactory system, the VNO takes over
when it’s time for dinner! Because their VNO is not
accessible via the nasal cavity, the only way that it is
able to process stimuli is through the mouth. In
pursuit of prey, snakes will tongue-flick once a
second, if not more, in order to track their quarry.
When the tongue is retracted, the scent molecules can
be transferred to the VNO via the two small openings
in the palate.
About a hundred years ago, scientists developed a
very plausible theory that this accounted for the fact
that snakes (and monitors) have forked tongues –
each of the ‘tynes’ entering one of the pits to stimulate
the VNO. It was easy to extrapolate this idea to the
extent that snakes and lizards with more pronounced
forks in their tongues could be expected to have
heightened olfactory senses. The current consensus is
that this is not the case, however, it has been
speculated that the separation of the forks may allow
the reptile to determine direction to some degree –
i.e. whether the scent is stronger on the left or right.