A myth unleashed: Casques are used neither for water collection, nor water direction & storage in chameleons

Since the introduction of the Yemen chameleon to the collections of reptile enthusiasts, one question has persisted: Why do they have such large casques? As is often the case, people turn to science for answers; however, this quest for clarification often leads to a surge in creativity. Rather than focusing solely on factual explanations, many individuals begin to conjure myths and constructs that range from seemingly reasonable to outright fabrication and nonsense.

Myth(s)

1. People believe, the high or massive part of the head behind the orbits are used for storage of water to keep a reserve for periods with water limited or absent availability.

2. You can hear even stories like this (exact citation of an anonymous facebook member): "The way I see them put water up in their caskets through their eyes during watering when it bulges out and they suck water into it and then they pump it up into their cranial crest"...

3. Even some books and internet forums speculate about the function of high casques in the Yemen chameleons in a sense: "They collect dew which condenses on the casque and runs down the cheeks to enter the mouth at the mouth corners to be swallowed..."

All this you can find people stating totally seriously.
And, it is all a total nonsense.

Truth

1. Casques are NOT used for any kind of water storage in any form and by any means.

2. Nothing like this happens of course. There is no way ANY animal would drink with eyes and there is NO WAY how to facilitate the described process, that is totally wrong. A cranial crest is a prominent bone-based formation on the head, it can not be pumped with water!

3. The water can condense on chameleon bodies at night and in the rainy season or at very high humidity and low temperatures reaching dew-point it happens, but it is not directed anywhere as the main hydration mode in chameleons besides of the normal drinking of water drops is the breathing in of fog.

What IS stored in casques?

Under normal circumstances in the wild: 

Nothing. The content of the casque, which is often even concave, is just the flat and thin chewing muscle, jaw adductor muscle - the muscles responsible for closing the mouth and bite force.

In captivity, in extremely overfed and obese chameleons

The situation is slightly different. The progress of obesity comprises several processes: 

1. Growing of the fat-body, corpus adiposum:

   It is situated in chameleon's in the pelvic region. When growing, it proliferates into the body cavity cranially until the body cavity is filled with a huge fat body almost fully, squeezing all the organs towards the bones and against each other mutually.

2. Fat-degradation of the liver: 

   The liver becomes fat-degraded (steatosis) and tripple in size or gets even bigger. Its color changes from dark maroon-red to pink with whitish dots. It starts losing its function inevitably and irreversibly.

3. Kidney - growing and failing: 

   The kidneys usually fail to fulfill their function of detoxicating the organism and while being simultaneously squeezed by the fat-body and surrounding structures it paradogically grows, proliferates cranially  and multiples its size several times, while changing the color from dark red to pinkish.

4. Deforming of casques and cheeks:

   The casques and cheeks are often convex to bulged and heavily deformed. The content of this swelling is a tiny amount of conjuctive tissue with subcutaneous fat and a heavily hypertrophied jaw adductor muscle. The muscle grows in volume sometimes up to 5 times. As this is never present in the wild, it is a solely captivity-induced pathology that has not yet been studied in detail. 

5. Overall edematic body swelling:

   Extremities and tail and chin as well as the skin on the body demonstrates n edematic swelling. Often, a gular edema is present also.

The morbidly obese female of Chamaeleo calyptratus: LEFT with exposed fat-body (bottom left, yelow) and extremely enlarged liver (upper right, maroon), RIGHT: liver fat degeneration (steatosis)

New surprising discovery and spectacular root cause

In my quest for answers, I encountered some intriguing questions. I was fortunate to acquire several captive specimens of chameleons that exhibited significant obesity. For ethical reasons, I do not overfeed the animals in my care, so I could not obtain obese individuals from my own breeding stock. 

I conducted a detailed necropsy of these specimens and observed some interesting findings. The subcutaneous fat was minimal in volume and often barely identifiable. Fat accumulation appeared to be primarily concentrated in two areas: 

1. The fat bodies located in the pelvic region and 

2. The liver. 

During the dissection, I also examined the enlarged casques of the obese chameleons. The muscle tissue beneath the skin was significantly hypertrophied and lightly colored, exhibiting massive enlargement that exerted pressure on the skin, leading to notable deformities. 

This raised the question: What substance was causing such muscular growth? To investigate, I performed a simple experiment. I placed a piece of this muscle on a hot kitchen pan and observed the results. Contrary to my expectations, while the muscle significantly shrank in size and evaporated water, no fat was released during this process. Histological examination confirmed that the muscle was indeed hypertrophied, characterised by an abnormal swelling with a high water content.

The hypertrophied jaw adductor muscle on a pan: from left to righ: fresh, losing water, dry, powdered

This led to the consideration of whether the chameleons stored energy reserves in the form of water. However, this notion is implausible. A more straightforward explanation emerged: during periods of excessive feeding, chameleons consume a substantial amount of feeder insects that contain i.a. salt, NaCl. Feeding them highly nutritious foods, rich in fats and proteins, further contributes to an increase in salt content in their bodes. Due to the limited function of their kidneys, this elevated salt concentration likely resulted in swelling, caused by the retention of water. This physiological response serves to maintain homeostasis and prevent the salinity of bodily fluids from reaching levels that could disrupt their physiology: more salt - more water, logically. 

The deformities and swelling observed in the casques and cheeks, along with other body parts, can be attributed to intracellulary retained water resulting from increased salt levels in the chameleons' bodies.

What are casques good for, why they are often so prominent?

1. The function of casques is primarily in the process of interspecific and intraspecific identification of a potential mating partner thanks to the phenomenon called character displacement. Based on differently shaped casques and other cranial protuberances and skin derivates, the females can identify the right sexual partner of own species and opposite sex, and refuse any reproduction attempts of males belonging to another species.

2. Casques also serve to increase the force they are able to bite with. It was demonstrated by recent studies, that there is a correlation between the casque height (and logically the adductor muscle size) and the bite force.