Reverse Ornamentation: Cranial Horn Regression and Emergence Across Sexes in Parson’s Chameleon

In Calumma parsonii, one of Madagascar's most iconic chameleons, a curious reversal occurs in the development of cranial horns across sexes.

Females: 

As they age, faint ridges resembling the male's horns sometimes emerge along the canthus rostralis (rostral crest) above the snout. These structures are subtle, incomplete, and appear late in life.

Males: 

In contrast, the horns that once grow into striking, shovel‑like projections gradually diminish with age, losing their prominence and definition.

Possible Explanations

• Energetic Trade‑offs: Horns are costly to maintain. In older males, resources may be redirected from ornamentation toward survival, leading to regression.
• Hormonal Shifts: Age‑related changes in endocrine balance could reduce horn growth in males while triggering vestigial expression in females.
• Sexual Signaling Dynamics: Younger males benefit from exaggerated horns as dominance and mating signals. With age, reduced reproductive competition may lessen the need for such displays.
• Developmental Plasticity: The female ridges may represent latent genetic potential—traits normally suppressed but expressed under altered hormonal or environmental conditions.
• Structural Limitations: The horns are bony cores covered only by a thin keratin layer and fine skin tissue. They are weakly vascularized, prone to abrasion, and possess limited regenerative capacity. Over time, this fragility contributes to their reduction in males and incomplete expression in females.
  

Commentary

This phenomenon illustrates how secondary sexual traits are not static but can wax and wane across the lifespan. In C. parsonii, horns serve as a vivid reminder that morphology is deeply tied to age, energy balance, and reproductive strategy. What begins as a bold ornament in males and an absence in females can, with time, invert into a subtle convergence—an evolutionary paradox written on the face of the chameleon.