Progression and Regression of Onthogeny of Mid-Dorsal Crests in Calumma brevicorne and its Alies
Semiadult (left) and adult (middle) and hatchling (right) of Calumma crypticum, courtesy Patrick Andriamihaja
The Ontogeny of External Morphological Features
in chameleons typically follows a pattern of progressive development that begins at hatching and continues until adulthood. This process is reflected in the consecutive growth of skin derivatives, including enlarged scales and cranial protuberances such as casques, horns, crests, and tubercles. Juvenile chameleons often exhibit a primitive morphology, characterized by a lack of visible enlarged features. The aforementioned skin derivatives and cranial protuberances grow step-by-step, following a pattern influenced by sexual dimorphism. These features are generally finalized upon reaching sexual maturity but may continue developing throughout adulthood. The horns become more elongate and more prominent, while the enlarged scales become larger, more pointed, taller, and generally more pronounced. The head crests also typically grow higher and more distinct during this period.
Medial Crests
Most chameleon species, possess prominent mid-dorsal crests that are formed by one or more rows of modified, enlarged, often conical scales. Along the midventral line, these scales can create a gular crest that continues as a ventral crest towards the vent, though it does not carry on to the ventral side of the tail. This ventral crest typically splits at the level of the umbilical scar into two rows, merging again caudally. The dorsal crest begins as a standard dorsal crests situated just behind the casque along the mid-dorsal line of the back, usually continuing to the tail as a tail crest, which typically reaches the proximal part of the tail but does not extend to its tip.
Functional Significance
The primary function of these prominent scales is to aid adult females in recognizing whether the individual chameleon belongs to their species during mating encounters and to help identify suitable sexual partners. Furthermore, these features may play a secondary role in crypsis and mimicry, as the structures can imitate spiky formations found within their environment, enhancing their camouflage.
The Enigma
In Calumma brevicorne and Calumma crypticum, mid-dorsal crests undergo particularly dramatic development that deviates from the typical ontogenetic pattern described above. Unlike other chameleons, juveniles of these species are initially covered with almost equally sized scales upon hatching with one exception: their dorsal crest is composed of solitary enlarged scales. During the first weeks of life, the dorsal and caudal crests undergo a rapid growth characterized by an increase in the prominence of conical enlarged scales. This development creates a saw-like appearance on the back, as larger, conical pointed scales are interspersed with three to four scales of more standard size.
At approximately half the total length of the maximum size of adults, these prominent scales stop growing and are caught up more or less in the development by the scales between them, which then adjust their size to reach the size of the prominent scales, so that the serrated pattern becomes inconspicuous or even disappears. Moreover, step by sep, these scales lose their pointed appearance and end up being of approximately equal size and reach the final stage when they are merely conical and are about same high as they are wide at heir base. This way, the semi adults of approximately half size, or the maximum size of the adults seem to be much more adorned by prominent scales, which are enlarged in comparison to the standard size of the body scales than the adults, in which the scallation appears significantly less heterogeneous than in the case of the sub adult specimens.
Unclear Function
We can only speculate about the function of this enigma. It might e.g. have the function either in the crypsis to resemble some thorny twigs or serrated margins of certain plants like for instance, Pandanus, or may play a role in the antipredatory mechanisms to appear spiky and undigestible. The main function of these in large scales, which is generally considered the identification of the belonging to the same species and opposite sex by the females, does not apply in this case, because this choice is made by adult females, and not by juvenile ones.