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Because o f their carnivorous habit, snakes are often considered top predators in
animal community. However, snakes are also exploited by m any other predatory
animals, and therefore, have evolved a variety o f antipredator mechanisms. Nuchal
glands are unique organs first described in a Japanese natricine snake, Rhabdophis
tigrinus in 1935. Nuchal glands, which are ontogenetically mesodermal origin, consist
o f a series o f paired organs embedded under the skin o f the neck region. These glands
contain bufadienolides, a group o f cardiotonic steroids, w hich are sequestered from
the skin toxins o f toads consumed as prey. To use these toxic chemicals effectively to
deter predation, R. tigrinus performs peculiar antipredator displays, such as neck arch
and neck butting. Several other natricine species belonging to Rhabdophis,
Macropisthodon, and Balanophis have similar organs, which are collectively called as
nucho-dorsal glands. Some o f these species have glands not only in the neck but also
in the entire length o f the body. W e provide a brief overview o f the history o f the
studies on the nuchal glands and related behavioral, ecological, and physiological
features. W e then present updated knowledge o f the nuchal glands, especially
concerning their morphological variation. Finally, w e discuss the diversity o f this
unique defensive system, which involves correlated traits among morphological,
ecological, behavioral, and physiological features.
