A can be subdivided into 4 orders: Crocodilia

A pheromone is “a chemical substance produced and released into the
environment by an animal affecting the behaviour or physiology of others of its
species.” (OED, 2017). Almost all animals, perhaps even humans, use pheromones
as a method of communication. Reptiles rely more heavily on chemical
communication than most other vertebrate classes (Martin and Lopez, 2011). The
class of Reptilia can be subdivided into 4 orders: Crocodilia (crocodiles, alligators and caimans), Sphenodontia (tuataras), Squamata (snakes, lizards and
amphisbaenids) and Testudines
(turtles and tortoise) (Uetz and Hošek, 2017). All reptiles have similar
pheromone systems, but some species have differences that help them adapt to
their environments and individual needs. For many species, the chemical constituents
that mediate pheromone-controlled behaviours is unknown making them very hard to
study, meaning the majority of literature deals with snakes and lizards (Mason
and Parker, 2010).

Pheromones are
incorporated into many aspects of reptiles’ lives including individual
recognition, mate choice and territoriality (Martin
and Lopez, 2011). When detecting prey, Squamates rely on their highly
developed vomeronasal system which conveys the chemosensory information
received to the brain. The typical tongue-flicking behaviour seen in snakes and
reptiles is associated with pheromone chemoreception. It provides a method of
sampling the environment and a way to deliver the samples to the vomeronasal
organ (VNO) (Halpern, 1992). The VNO facilitates chemical cues which are
important in social scenarios and strong evidence that this functioning has
been obtained over the years. Male garter snakes (Thamnophis sirtalis)
VNO and olfactory nerves were sectioned leading to courtship behaviour deficits
(Kubie et al., 1978). Other studies obtained results proving that it was the
VNO sectioning alone, not the olfactory, causing the deficits; males with
olfactory nerve lesions were still able to show courtship behaviour to
attractive females, whereas those with just vomeronasal nerve lesions showed
the expected deficits (Mason and Parker, 2010). These findings were mirrored in
studies using the European adder (Vipera
berus) (Andren, 1982). The VNO has been shown to be important in detection
of prey. When garter snakes had there VNO altered, they could no longer
recognise earthworms as a food source (Halpern and Frumin, 1979).

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Crocodilia have a
very different system compared with Squamates. There is less literature
regarding Crocodilia perhaps due to the practical problems when trying to work
with them (Norris and Lopez, 2011). The biggest difference is that crocodilia do
not have vomeronasal systems; they therefore use olfaction and taste for
chemosensory detection (Hansen, 2007). As in squamates, pheromones are thought
to be important for reproduction and in crocodilia they are produced by the gular
and paracloacal glands. Sexual and individual variations are seen in gular
secretions and are used by females to mark nest sites with their scent (Weldon
and Tanner, 1991). Crocodiles have been shown to have biosynthetic capabilities
because occasionally new compounds such as aromatic ketones are secreted (Yang
et al., 1999). However, like many aspects of pheromones and olfaction, the function
of these compounds is currently unknown. Behaviour witnessed in different crocodilia
species has provided evidence of pheromone detection. Specific gland secretions
caused a sniffing behaviour in spectacled caimans (Caiman crocodilus) indicating pheromone detection (Huggins et al.,
1968).  imilarly, yearling alligators (Alligator mississippiensis) were observed
responding to specific airborne aromas (Johnsen and Wellington, 1982).

Pheromones have
great importance in reproduction for many reptiles. There are releaser pheromones
which cause an immediate response such as recognition of species or sex of an
individual. Pheromones can also provide information regarding desirable
individual characteristics of the signaller that the receiver will use in
reproductive decisions. At present there is no strong evidence of primer
pheromones in reptiles (Martin and Lopez, 2011). The order of Sphenodontia has only one living member,
the tuatara (Sphenodon punctatus).
Although many reptiles use pheromones in reproduction, it is unclear whether this
is the case for tuataras because the typical tongue-flicking chemosensory
exploration is not seen during mating/courtship (Gans et al., 1984). However, tuataras
do have cloacal glands which secrete pheromones such as triacylglycerols. Although
tongue flicking behaviour is not seen, airborne signals are able to be detected
via the olfactory system; tuataras bite prey chemical scented cotton balls indicating
the use of pheromones in prey detection (Cooper et al., 2001).

Like Squamates, many
species within the Testudines order have highly developed olfactory and vomeronasal
systems. Turtles also have unique specialized glands which enhances their
chemosensory detection. Some glands such as the Rathke’s glands (situated in axillary
or inguinal areas) are only active during mating season and completely dormant
in juveniles suggesting the pheromones released by these glands are important
in reproduction (Norris and Lopez, 2011). In the common musk turtle (Sternotherus
odoratus), pheromones
secreted by Rathke’s glands have dual function; during mating season,
pheromones secreted by females allow for sex recognition and all year-round predator
repellents are secreted (Bran and Fadool, 2006).

Pheromones can be
many different chemical compounds such as alkanes, acids, alcohols and
proteins. Femoral glands, found in various species of lizards, secrete a variety
of pheromone types. The femoral glands of lacertid lizards (Acanthodactylus boskianus) are
well-developed and GC-MS chemical analysis was used to study the pheromones of
this species. Avoidance behaviour towards cholesterol and alcohols was seen in
males, whereas females did not show any selection. This data supports the theory
which states that lizards utilize femoral gland secretions as territorial
markers and potentially establish dominance amongst a community. Cholesterol
and long-chain alcohol were concluded to be the most likely scent-marking pheromones
in lacertid lizards and other similar species (Khannoon et al., 2011).

Reptiles are ideal
models for the study of pheromones and chemical communication as they have a
large variety of behavioural habits and they link our understanding of
terrestrial and aquatic environments (Mason and Parker, 2010). At present, there
is controversy regarding the existence of human pheromones. Some suggest chemicals
like androstenone are human pheromones involved in social attraction and that
human menstrual synchrony pheromones explain why overtime the menstrual cycles
of roommates or close friends synchronize (Mucignat-Caretta,
2014). The study of the phylogeny and genetics
of pheromones provided a definition and the mechanisms involved consequently leading
to the discovery of human pheromones. In the future, reptiles will continue to develop
our understanding of such an important method of communication for reptiles and
other orders in the animal kingdom. 


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