Misreading the Egg: Questioning Temperature-Driven Sex in Yemen Chameleons
The study titled "Effects of temperature on embryonic development of the veiled chameleon (Chamaeleo calyptratus)" investigates how incubation temperature influences developmental timing, hatching success, and sex ratio in this species. Researchers incubated eggs at constant temperatures ranging from 22°C to 32°C and found that higher temperatures accelerated development, reduced incubation time, and increased hatching success. They also reported a skewed sex ratio at different temperatures and suggested that temperature-dependent sex determination (TSD) might be present in C. calyptratus, a surprising claim for a species known to possess sex chromosomes.
However, the study has three significant flaws that cast doubt on its conclusions:
Unnatural Temperature Regimes: The incubation temperatures used were constant and relatively high, whereas natural nests experience lower, fluctuating temperatures with strong seasonal and circadian variation. This limits ecological relevance.
Genetic Sex Determination: Veiled chameleons have sex chromosomes, meaning sex is genetically determined. The suggestion of TSD contradicts established genetic evidence and lacks molecular validation.
Unreliable Sexing Method: The study used tarsal spurs to determine sex, a method known to be inconsistent. Both sexes can exhibit or lack spurs, and asymmetrical spur development has been documented, making this a poor diagnostic trait.
Given these issues, the study's conclusions—particularly regarding sex determination—should be interpreted with extreme caution, as they may not reflect natural developmental biology.
Original Publication:
Andrews, R. M., & Donoghue, S. (2004). Effects of temperature on embryonic development of the veiled chameleon, Chamaeleo calyptratus. Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology, 141(3), 293–301. https://doi.org/10.1016/j.cbpb.2005.01.003