The temporal gland or temporal pouch is a structure found in chameleons of the genus Trioceros only.
It is a holocrinous type of exocrinous glands, that discharge their products to the outer world through openings or ducts, not into the blood as is the case of the endocrinous glands.
It is a flat pocket situated bilaterally above the mouth angle. The cells of the lumen of the gland do not produce any specific secret, they simply die and fill the lumen of the gland with dead cell mass. This is then infected by unspecific aerial bacteria (e.g. Aeromonas, a genus of gram-negative, facultative anaerobic, rod-shaped bacteria) and decayed with heir help to form a cheese-like whitish or yellowish, terribly smelling substance.
This substance is used at least for two spectacular functions:
1. Antipredatory function: while being approached by a predator, the mouth is wide-opened and the lumen of the temporal gland exposed, the smell of a decaying biological substance is by the predators identified as inedible, dangerous and potentially toxic.
2. Predation function: the smelling substance is smeared in branch, the chameleon makes some steps backwards and waits till it attracts flying insects, especially Diptera, which are then shot with their tongue and eaten.
Malnourished or otherwise weakened specimens can suffer an infection of the glands, that can either damage the gland and neighboring area or even cause mouthrot…
Moreover, is has been specullated whether the chameleons do not mark their territory using temporal glands also.
For me, this function is questionable for two reasons:
1. In order to make sense, chameleons would have to have the possibility to
A/ leave odor signals – they have
B/ identify them – this is the weak point. Their olfaction is definitely absolutely weak, as they really do not use the nostrils for orientation and vomerolfaction using Jacobson’s organ at the palate is a big question, as hey do not have a forked tongue to make a use of it (as do have Varanids, Helodermatids, all ophidia etc) plus it seems to be reduced and unfunctional though he pertinent fissure on the palate are present.
2. To be able to identify fine olfactory signals, it does not make sense to block this sense by a presence of terribly and intensively smelling substance which is ever present in immediate vicinity of the possible sense organ. The presence of intense smell would very likely move the sensitivity of the organ to so low level, that it would not sense anything meaningful, moreover if it should actually identify the odor markings which are a smell of same type. Moreover, as explained, the holocrine temporal gland actually does not have any specific product itself: it is not a odor gland that produced odor. What it merely does is to produce dead cells that are base for unspecific bacteria, that decay that substance and the smell is an end product of this process…
For a theoretically possible vomerolfaction speaks however the fact, that chameleons sometimes lick the branches with their tongue. Especially when they enter a new territory… (and in captivit, it has been onserved many times also.Why they do, has not been explained yet… Tasting is one option, vomerolfaction another but they might be many other reasons…
The development of the temporal pouch varies considerably across the whole family Chamaeleonidae. Ogilvie (1966) reported it in Bradypodion, Chamaeleo, Kinyongia, Rieppeleon, and Trioceros, but seems to be absent in Calumma, Furcifer, and Rhampholeon. More recent work by Preest et al. (2016) found it to be developed to varying degrees in Chamaeleo, Furcifer and Trioceros.
Its presence does not necessarily mean its full functionality, which has really only been studied in Trioceros, having the best developed pouch.