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Researchers take deeper look at parental care among snakes

January 24, 2011

Whether you walk, slither or fly, there is no job in the world like parenting. Parents are responsible for balancing the many needs of their children and deciding which of those needs are most important. Mammals are known for caring for their young, but reptiles aren’t often seen as nurturing parents.

ASU researchers are taking a deeper look at parental care among certain snakes, and how simple behaviors have a significant impact on developing offspring. Snakes may even offer clues about how some of our own traits developed.

Zach Stahlschmidt and Jake Brashears are graduate students in ASU’s School of Life Sciences, part of the College of Liberal Arts and Sciences. They are studying the brooding habits of both captive and wild pythons. The students work with professor Dale DeNardo at ASU’s DeNardo Lab, where undergraduate and graduate students participate in research focusing on how reptiles live and thrive in challenging environments.

Stahlschmidt recently completed a four-month field study in Australia where he examined free-ranging female water pythons, noting where they chose to lay their eggs and how long they attended them.

“Some stay for as few as three to five days, others are on the eggs for more than 50 days,” Stahlschmidt said.

Although he is still analyzing the data collected in Australia, two factors that seem to influence a mother python’s decision to stay with or leave her eggs are the temperature and humidity of the nest. Exactly how these factors are related, however, remains a mystery. In a previous study on water pythons, mothers tended to leave their eggs earlier if they laid their eggs in a nest with a warm, stable temperature. However, the pythons Stahlschmidt studied in Australia seemed to do the opposite.

“We’re still trying to figure out why that could be because it’s not as intuitive and adaptive as you would think,” Stahlschmidt said. “At ASU we have colonies of captive pythons, and the research I’ve done with them shows they use humidity and temperature cues from their environment to enhance their eggs’ water balance and temperature. To have a seemingly conflicting response out in the field is interesting.”

Despite this inconsistency, there are parental care behaviors common to both wild and captive pythons. Brooding female pythons coil tightly around their eggs to help maintain water balance within the nest. However, once eggs are enclosed in this tight space, the embryos have difficulty respiring, or exchanging oxygen and carbon dioxide with the nest environment.

“Coiling on the eggs is essential for embryo survival under some circumstances, but it comes at a cost,” Stahlschmidt said. “Since the embryos are developing in this low-oxygen environment, they become smaller, slower and weaker.”

What is a parent to do if providing one essential element of care means sacrificing another? Fortunately, female pythons have found a way to balance the needs of their developing offspring. In previous studies, Stahlschmidt found that mothers will occasionally open their coils to allow for the exchange of gases.

This parental care system can be seen in other animals as well. Like pythons, mother birds also must prioritize the needs of their eggs.

“Hatchlings need mom for protection and to keep them warm, but occasionally mom needs to leave the nest to get food, so she must balance a tradeoff between competing developmental needs,” Stahlschmidt said. This is why birds often require both parents to care for eggs – one to attend the nest while the other forages for food.

Aside from the parental care they display, birds and pythons share something else in common. “They both evolved from reptile ancestors,” Brashears said.

The earliest evidence of reptile evolution is dated at around 350 million years ago. Both mammals and dinosaurs eventually evolved from this diverse group, and birds later evolved from dinosaurs. Birds also developed a few important similarities to mammals. For example, both mammals and birds are endothermic, meaning they produce heat internally.

According to Brashears, “99.9 percent of all animals in the world, including reptiles, are ectothermic. That means they get some heat and energy from food, but mainly from outside sources like sunlight.”

Why are so few animals endothermic?

“It’s extremely expensive,” Brashears said. While humans have a high metabolic rate, requiring about 1,800 to 3,000 calories per day, a reptile of similar size might only need 300 to 400 calories per day.

Burmese pythons, however, are an exception to the rule. Although they require few calories compared to mammals, they have the ability to produce heat internally.

“It’s curious because they’re only endothermic when they are taking care of their eggs,” Brashears said. When Burmese pythons are brooding, they coil around their eggs and start twitching, generating enough heat to regulate the temperature of the eggs.

“That can go on for two months,” Brashears said. “They won’t take food or water, they’ll just brood, and they lose a lot of weight.”

It makes sense that of all the reptiles, Burmese pythons would develop endothermic properties because of their size.

They can grow up to 20 feet long, and with a smaller surface-area-to-volume ratio, they retain heat better. Brashears’ research is focused on identifying the hormones related to python endothermy to better understand why endothermy first evolved.

“Pythons are not precursors to birds and mammals, but they are ecologically and behaviorally an example of what we think birds and mammals might have been like,” Brashears said.

This makes them ideal subjects for the study of endothermy. Why did such a small percentage of animals develop a trait that requires so much energy, and why do Burmese pythons only exhibit endothermy when brooding? Brashears says that Burmese pythons are only endothermic when caring for their eggs. One wonders if the body heat that we humans enjoy today first evolved as a tool for parental care, too.

by Allie Nicodemo, Office of Knowledge Enterprise Development