Chris Anderson
Dr. House of Chameleons
I've been brushing up on some reproductive biology research, particularly how it relates to chameleons, and I thought it would be useful to mention a few things keepers/breeders tend to ignore.
We all know that high incubation temperatures tend to result in weak hatchlings. Our response to that knowledge, however, it simply to use cooler incubation temperatures, rather than try to think about the optimal incubation strategies.
Interestingly, many oviparous chameleon species have what is called embryonic diapause where there is an arrest of embryonic development that is controlled endogenously. Embryonic diapause isn't broken by increasing temperature or providing otherwise appropriate environmental conditions for development. This is different than other types of diapause that are induced by environmental conditions (temperature, oxygen levels, etc.). Chameleons are actually the only lizards or snakes that have embryonic diapause. Not all chameleons have embryonic diapause, however (Brookesiinae, for example, don't seem to). Clearly this embryonic diapause effects incubation though and its important to consider why such an endogenous arrest would evolve.
Its also interesting to note that chameleons are some of the few lizards or snakes that naturally tend to lay their eggs before winter, thus forcing their clutches to extend through a period where temperatures are not conducive to development. What embryonic diapause does is prevent development immediately after a clutch is laid before winter temperatures kick in and stop development until temperatures improve. It essentially protects the embryo and ensures it doesn't begin to develop before winter sets in.
Now studies have shown that when clutches are not exposed to these low temperatures shortly after a clutch is laid, embryos within a clutch break the embryonic diapause and begin development over a wide range of time. The result is that the clutches hatch out over an extended period of time rather than all at once. When these clutches are exposed to a low temperature period shortly after being laid, the entire clutch breaks the embryonic diapause some time before temperatures increase and the environmentally induced diapause holds the development of the entire clutch until temperatures increase. The result is that these clutches tend to hatch in much closer proximity to each other. Additionally, hatchlings from clutches that were not exposed to an environmental diapause but were instead incubated at a constant temperature tend to have lower masses when they hatch than clutches that are exposed to an environmental diapause.
Now, appropriate diapause periods can vary from species to species and, unfortunately, limited information is available on the appropriate diapause lengths for different species. That said, there is strong support for the idea that a constant incubation temperature for many chameleon species is far from ideal (including calyptratus and pardalis). A 3 or 4 month temperature drop shortly after a clutch is laid followed by incubating at a more typical incubation temperature may improve the strength of neonates and possibly even shorten the overall length of incubation in some species by triggering the end of embryonic diapause.
Just some food for thought, particularly for people thinking about breeding...
Chris
We all know that high incubation temperatures tend to result in weak hatchlings. Our response to that knowledge, however, it simply to use cooler incubation temperatures, rather than try to think about the optimal incubation strategies.
Interestingly, many oviparous chameleon species have what is called embryonic diapause where there is an arrest of embryonic development that is controlled endogenously. Embryonic diapause isn't broken by increasing temperature or providing otherwise appropriate environmental conditions for development. This is different than other types of diapause that are induced by environmental conditions (temperature, oxygen levels, etc.). Chameleons are actually the only lizards or snakes that have embryonic diapause. Not all chameleons have embryonic diapause, however (Brookesiinae, for example, don't seem to). Clearly this embryonic diapause effects incubation though and its important to consider why such an endogenous arrest would evolve.
Its also interesting to note that chameleons are some of the few lizards or snakes that naturally tend to lay their eggs before winter, thus forcing their clutches to extend through a period where temperatures are not conducive to development. What embryonic diapause does is prevent development immediately after a clutch is laid before winter temperatures kick in and stop development until temperatures improve. It essentially protects the embryo and ensures it doesn't begin to develop before winter sets in.
Now studies have shown that when clutches are not exposed to these low temperatures shortly after a clutch is laid, embryos within a clutch break the embryonic diapause and begin development over a wide range of time. The result is that the clutches hatch out over an extended period of time rather than all at once. When these clutches are exposed to a low temperature period shortly after being laid, the entire clutch breaks the embryonic diapause some time before temperatures increase and the environmentally induced diapause holds the development of the entire clutch until temperatures increase. The result is that these clutches tend to hatch in much closer proximity to each other. Additionally, hatchlings from clutches that were not exposed to an environmental diapause but were instead incubated at a constant temperature tend to have lower masses when they hatch than clutches that are exposed to an environmental diapause.
Now, appropriate diapause periods can vary from species to species and, unfortunately, limited information is available on the appropriate diapause lengths for different species. That said, there is strong support for the idea that a constant incubation temperature for many chameleon species is far from ideal (including calyptratus and pardalis). A 3 or 4 month temperature drop shortly after a clutch is laid followed by incubating at a more typical incubation temperature may improve the strength of neonates and possibly even shorten the overall length of incubation in some species by triggering the end of embryonic diapause.
Just some food for thought, particularly for people thinking about breeding...
Chris
