Chemical communication is amongst the most crucial aspects of animal behaviour. Pheromones refer to chemicals produced by an individual, which induces a change in behavior or physiology in a conspecific. As such, amongst all vertebrate taxa, chemical communication is most prevalent amongst reptiles, influencing reproduction and sexual selection significantly (Martín and López, 2011). However, knowledge regarding the significance of pheromones for non-Squamata reptiles is fairly rudimentary (Martín and López, 2011) . These include the Rhynchocephalia, Crocodilia and Testudines orders.
Not much is known about Tuataras
The Rhynchocephalia order consists of only two extant species of tuatara, which are Sphenodon punctatus and S. guentheri (Herrera-Flores et al., 2017). For this reason, there has been minimal research on the chemosensory behaviour of tuatara (Habegger et al, 2001). The influence of pheromone in tuatara reproduction remains particularly ambiguous. Tongue flicking has not been observed either during courtship or agonistic male interactions.
Furthermore, tongue flicking was also not observed in response to the detection prey chemical stimuli. However, tuatara possibly utilize pheromones while foraging. A study found that the species regularly bite cotton balls which bore prey chemicals, implying tuataras detected chemiosignals through olfactory mechanisms. (Habegger et al, 2001)
Both species possess two cloacal glands each which secrete potential pherenomes. (Weldon et al., 2008). Secretions of adult Sphenodon punctatus compromise of major glycoproteins and unusual fatty acids. There was a lack of intersexual variation in secretion composition (Weldon et al., 2008). Despite a lack of observed chemosensory behaviour in tuatara, both species do possess olfactory epitheliums structurally similar to that of squamates (Gillingham et al., 1995). However, this is possibly a vestigial adaptation.
Crocodilian Chemical Communication
Pheromones are more prevalent in the Crocodilia taxa. As such, crocodilians lack a functional vomeronasal organ. Instead, all extant species possess two distinct pairs of skin glands – gular and the paracloacal glands – which secrete potential pheromones. However, the specific functions of these chemicals remain unclear (Martín and López, 2011).
Gular glands are found on the ventral part of the lower jaw. Secretions consists of fatty acids, cholesterol, alcohols and α-tocopherol (Weldone, 2021) and exhibit extensive individual and intrasexual variation. Furthermore, gular gland secretions are possibly utilized by female crocodilians to scent mark nest sites (Martín and López, 2011) by rubbing their lower jaw on the ground (Weldon et al, 2008).
The paracloacal glands, meanwhile are located in the cloaca, and secretions consists of hydrocarbons, fatty acids and steroids and triacylglycerols (García-Rubio et al., 2002). These secretions are potentially used for mating and nesting behavior (Weldone, 2021)
Behavioural responses remain poorly understood
As such, immature crocodiles displayed elevated gular pumping in response to the detection of gular and paracloacal secretions from adult males (Johnsen and Wellington, 1982). Gular pumping is the phenomena where the pharynx floor is alternately lowered and raised to pulse air through the nasal cavity. This implies the presence of potential pheromones in these two glands. (Weldon and Ferguson, 1993).
Furthermore, gular pumping is also observed when American alligators are exposed to the scents of various meats underwater (Weldon and Ferguson, 1993). Additionally, when exposed to aqueous and chloroform extracts of meats, these crocodilians exhibited pronounced underwater head waving and mouth openings, suggesting the possibly of food chemical detection through olfaction (Weldon and Ferguson, 1993).
However, evidence of behavioural responses to pheromones remains limited in the Crocodilia order. This can be attributed to the immense challenges of studying members of this order (Martín and López, 2011). Amongst the non-Squamata reptiles, the role of pheromones is best understood in the testudines (turtle) order. Most species have elaborated olfactory and vomeronasal systems. (Martín and López, 2011)
How do turtles utilize chemical communication?
Barring members of the Testudinidae (tortoises) family, most turtle species possess Rathke’s glands (Rose et al., 1969). This gland is located in the internal lateral portion of the shell (Weldon et al., 2008). It is hypothesized that secretions from the Rathke’s glans are excreted during provocation. This is especially prevalent amongst young turtles. Therefore, it is assumed that the secretions from the Rathke’s glands are potential pheromones with the function of predator repulsion (Weldon et al., 2008). As such, secretions from the Rathke’s gland primarily constitutes of lactic acids, lipids and water-soluble protein (Weldon et al., 2008).
What are some potential phenoromes?
In the Testudinidae genera, secretions from the cloacal secrete potential pherenomes (Weldon, et al ., 2008). These are used extensively for conspecific and sex recognition (Gans and Crews, 1992). A prominent example of this are Red Footed (Chelonoidis carbonarius) and Yellow Footed Tortoises (Chelonoidis denticulatus) (Auffenberg, 1965). Males have been observed to smell the cloacal area of other tortoises. The detection of chemicals of conspecific females encourages mounting attempts by the male.
Contrastingly, heterospecific females are ignored. (Martín and López, 2011). Similar behavior has been observed in captive green sea turtles. Males were observed to smell the cloacae of breeding females more frequently than their non breeding counterparts. This suggests that female pheromones are probably crucial for both reproduction and the identification of females (Martín and López, 2011).
Mental glands are particularly prevalent across the Emydidae, Geoemydidae, Platysternidae, and Testudinidae families (Ehrenfeld,1973). Located in the throat region, the gland is thought to produce potential reproduction pherenomes. For example, mental glands in male desert tortoises (Gopherus agassizii) are enlarged during the breeding season. This corresponds with elevated quantities of secretion (Alberts et al, 1994). Furthermore, mental gland secretions are utilized to identify familiar conspecific in adult desert tortoises. (Weldon et al, 2008)] Secretions from the mental gland are also thought to induce aggression in males (Gans and Crews, 1992). In response to secretions, male Texas tortoises (Gopherus berlandieri) have been observed to exhibit combat behaviour, such as shell ramming (Weldon et al, 2008).
Conclusion
It must be noted however, that for the three orders discussed, much further research is required to truly uncover the role of pheromones on reptile behaviour. There are prominent examples of chemical communication influencing reproductive behaviour and conspecific recognition across reptilian species, regardless of order. Nonetheless, understanding chemical communication mechanism are crucial to understand interspecies interaction, which in fact has increased biodiversity.
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