Biology Reference
In-Depth Information
Iwasaki's slug or snail-eating snake; Iwasaki-sedaka-hebi) have a larger number of teeth
(approximately 25) in the right maxilla than in the left (approximately 17.5) (Hoso et al.,
2007). This probably allows the species to extract snails from shells that possess a right-
sided whirl (clockwise). Some snail species (e.g., the air-breathing or pulmonate snails,
Satsuma
spp.) that have evolved left-sided (or sinistral) whirls exhibit some functional
protection against predation by these snakes due to the reliance on right-dominant prey
seizure and extraction (Hoso et al., 2007). Some gastropod specialists, such as
Atractus
reticulatus
(reticulate ground snake; cobra de terra comum),
Dipsas indica
[Neotropical
snail-eater; cobra-cipo (this name is commonly used for
P. olfersii
, and other species; see
Table 4.1 for examples of commonly used regional names)], and
Sibynomorphus mikanii
(Mikan's tree snake, or dormideira preta) also exhibit wide morphological/histochemi-
cal variability in their infralabial glands that may reflect secretion constituent diversity
and dietary specialization (de Oliveira et al., 2007). In the event of biting a human vic-
tim, such dentitional modifications, adaptations, and concomitant glandular variability,
which probably facilitate specific prey capture certainly may influence the extent of any
possible local effects, with or without the introduction of Duvernoy's secretions (see
later).
A recent hypothesis suggested that an alteration of the timing of developmental
events (specifically, a heterochronic mechanism; Jackson, 2007) might provide a basis
for the appearance of specialized dentition in colubroids. This concept advanced the
development of ungrooved and grooved teeth of colubroid snakes from an ancestral
tubular fang via attachment of replacement tubular fangs to the maxilla at an ear-
lier developmental stage than previously considered (termed “precocial ankylosis”)
(Jackson, 2007). The evolution of the canaliculated fang provided a means of pene-
trating the prey's (or foe's) integument and deeply injecting venom, containing a wide
array of biologically active components, including highly potent toxins. Therefore,
the evolution of the high-pressure venom system combined with the formation of an
enclosed venom canal, or lumen (resembling the action of a hypodermic syringe nee-
dle), provided an adaptation allowing the subjugation, and in some species, prediges-
tion, of large, strong prey with a probable reduced risk of defensive or retaliatory injury
inflicted on the snake. Some studies support enclosed venom canal formation by invag-
ination of a groove along the surface of the tooth or epithelial wall of the developing
tooth with eventual fusion, thereby forming the enclosed canal, while others suggest
that direct, successive deposition of materials (e.g., dentin) form the tubular fang from
tip to base—therefore the canal develops directly, without any folding (“brick chimney
hypothesis,” Jackson, 2002; Zahradnicek et al., 2008). Using the expression of SHG,
early development of the fangs was followed in
Cryptelytrops
(Malhotra and Thorpe,
2004;
Trimeresurus
)
albolabris
(white-lipped pit viper,
Plate 2.12
) (Zahradnicek
et al., 2008). These authors found that the fang lumen was formed by an early invagi-
nation of epithelial cells into the dental mesenchyme. The epithelial cells proliferated
in order to enlarge the canal, with subsequent apoptosis (programmed cell death) form-
ing the functional lumen (Zahradnicek et al., 2008). The two sides of the invaginating
epithelium never come into contact, thus leaving the orifice open. These researchers
compared the mechanism by which the fang orifices form with that of the open groove
on the posterior maxillary teeth of
D. typus
(
Plate 2.13
). Zahradnicek et al. (2008)
