Denardo Lab

Click on the image to view a slide show.

Parental Care and Pythons:

Parental care is widespread among birds and mammals, and there are also considerable examples of parental care in fish and amphibians. Reptiles, however, are the only major group of vertebrates that demonstrate very little parental care. In fact, for most species, parental investment into their offspring is limited to gamete production by both sexes, yolk production by the female for embryo development, and nest site selection by the female. In some reptiles, most notably crocodilians and skinks, females may guard their eggs from predators. Beyond defense against depredation, examples of parental care in reptiles are very limited. There is evidence in some skinks and one viper of females aiding water balance of their eggs by either moving the eggs or altering their positioning relative to the eggs.

One exception to the general rule that reptiles rarely provide parental care and, when they do, it is for predator defense is egg brooding by pythons. Depending on whether your a grouper or splitter, there are 25-35 species of pythons throughout the Old World. While only a few have been scientifically studied, reports from herpetoculturists have documented brooding in all species. Unlike nest attendance in skinks, crocodilians and the few other species where it is seen, pythons maintain a very close interface with their entire clutch, tightly coiling around the eggs to the point where the clutch cannot be seen, sometimes even when the brooding female is picked up. This behavior remains relatively constant throughout an incubation characterized by self-induced anorexia, dehydration, and a 20-30% reduction in body mass. Occasionally , however, females may leave their eggs for a sure duration (e.g., an hour) to bask.

Facultative endothermy (also known as shivering thermogenesis) is a rare physiological trait where a brooding female python will produce heat (i.e., be endothermic) to regulate the developmental temperature of her eggs. While facultative endothermy is soemtimes broadly associated with pythons in general, it has been documented in only a few species and some species have been shown not to be effectively endothermic. Burmese pythons (Python molurus) is the best studied species and brooding females can raise the developmental temperature 7°C above the temperature of the nest outside the brooding female.

Our lab studies the prevelance of facultative endothermy among pythons, the functional significance of facultative endothermy, and python brooding behavior independent of endothermy.

Python Reproductive Behavior:

There are several pertinent behavioral and physiological events during the female python reproductive cycle. Each is summarized below and a slide show depicting most of them in the Children's python (Antaresia childreni) can be seen by clicking on the image above.

Body distention and increased basking during gravidity - Female pythons become greatly distended when gravid. Additonally, they alter thermoregulatory behavior to maintain a higher and more stable body temperature during this period.

Inverted basking - Numerous python species have been reported to bask "upside-down". That is, they invert their normal body position so that their back is against the substrate and their belly is facing up. The body inversion is usually limited to the caudal half of the snake, with the head and cranial body remaining in normal ventral recumbancy. While not exclusive to gravidity, this inverted basking behavior is much more prevalent at this time. The possible role of this behavior in evenly distributing heat to the female reproductive tract and incubating embryos warrants further study.

Pre-ovipoisition ecdysis - Like most snakes, pythons shed their skin prior to oviposition. However, the pre-oviposition ecdysis occurs sooner in pythons than in colubrid snakes. For example, Children's pythons typically shed 20-30 days prior to oviposition. As in other snakes, for several days prior to shedding, the snake's skin (especially the spectacles over the eyes) turn a cloudy bluish. Interestingly, the duration between ecdysis and oviposition is correlated with the average female body temeprature during that time. That is, the warmer females maintain their body temperature, the shorter the period between ecdysis and oviposition. Since developmental time is temperature dependent, the timing of the pre-oviposition ecdysis may be assoicated with a specific embryonic event, but this possibility remains untested.

Egg-laying - females lay their eggs successively with short intervals between oviposition of eggs. Immediately at oviposition the eggs is very flaccid and moist (you can see this somewhat subtle but clearly discernable difference in the oviposition photographs included in the slide show). As with most snakes, the eggs firm up and adhere to adjacent eggs within minutes of oviposition. The benefits of egg adherance is not fully known, but it keeps the clutch together, reduces water loss from the eggs, and creates a clutch geometry that is more conducive to brooding.

Pre-oviposition embryonic development - For most oviparous squamates (i.e., egg-laying lizards and snakes), a significant amount of embryonic development occurs while the eggs are still in the female. Typically, development is at approximately stage 30 at oviposition. However, python embryos develop even further (stage 35) while the eggs are still in the female's oviduct. The picture in the slide show of the embryo in the opened egg was taken within a couple hours of oviposition.

Tight coiling - After laying its last eggs, the female will coil around the clutch. For the majority of time the female is so tightly coiled around her eggs that no part of the clutch is visible. Several studies within our lab have confirmed that this tight coiled posture drastically reduces clutch water loss.

Head tucking - Typically, the female's head is positioned on top of the clutch when she is brooding. However, the female will periodically dip her nose into the clutch - sometimes momentarily, sometimes for an extended period. This implies that the female is sensing one or more conditions within her coils, but this possibility remains unexplored. The function of this behavior will be experimentally addressed in upcoming reproductive seasons.

Loose coiling - Throughout incubation, the female will periodically loosen her coil and expose as least part of the clutch to the nest environment. The amount of the clutch exposed and the duration of the exposure varies considerable, but most episodes of loosened coiling are very minor with only parts of the top egg or two becoming visible for 2-10 seconds. The purpose of these periodic exposures is unknown but is likely related to egg gas exchange. Early analysis from the most recent reproductive season indicates that as much as 50% of gas exchange between the clutch and the environment occurs duing these short opening periods. Water loss from the clutch is also elevated at these times. Despite a 5-6 fold increase in embryonic metabolic rate during incubation, the frequency and duration of loose coiling does not differ over the course of incubation. While still speculative, the loose coiling events may represent a trade-off between requirements for embryonic gas exchange and egg water conservation.

Clutch exploration - Much less frequently the female will become more active and loosen her coil to the extent that she explores the entire clutch. Although possibly related to clutch and/or nest inspection, the role of this behavior is unknown.