Boa Constrictors Get a Feel for Their Prey

What makes a snake stop squeezing? We do science to prove ourselves wrong, because the answer people predicted is not the correct answer

How do boa constrictors know when to stop constricting?
How do boa constrictors know when to stop constricting? Image courtesy of Flickr user wwarby

Ed. note: We welcome back guest blogger Greg Laden for a two-week blogging tour on Surprising Science.

This is a story of snakes, islands and students. Let’s start with the snakes.

Among the many different kinds of snakes are the constrictors: boas and pythons. They are close relatives that diverged millions of years ago. Pythons are found in the Old World (Africa and Asia) as well as Australia. Boas (family Boidae) are found in the New World (North, Central and South America including some Caribbean islands). All of them kill their prey by wrapping around it and squeezing it to death.

Among the boas there is an island-dwelling form in Belize that is the subject of interest to conservationists, ecologists and, lately, behavioral biologists. This is the miniature boa of Snake Cayes, a group of islands off the coast of southern Belize. When I say “miniature” I mean that they range in length from 30 cm to about 2 meters (1 to 6 feet). This is small compared to the mainland boas of the same species, which can reach 4 meters (13 feet) in length.

It is common for animal populations that live on islands to exhibit differences in size from those on the mainland. Medium and larger mammals like deer tend to be smaller on islands, small mammals like rodents tend to be larger. Something like this may happen with snakes as well.

Allison Hall
Allison Hall (left) says “It’s a normal thing to be a little afraid of snakes, but you really get into the project and come to love the animals.” Amanda Hayes is on the right. Image provided by Dickinson News and Events

Scott Boback is an expert on these animals, and from the time he was a graduate student at Auburn University, he’s been trying to answer the question “how and why are these snakes small?

The most likely explanations for size differences would seem to be either diet or other features of the environment, or genetics. Perhaps there is a limited food supply on the islands, so snakes grow slowly, and thus there are few or no large ones. It would take them so long reach a large size that somewhere along the line they would have met their demise. Alternatively, it could be that snakes that grow slowly or nearly stop growing as they approach a certain size survive longer or reproduce more effectively (probably owing to food supply being limited). If so, the genes involved in growth would be shaped by natural selection and over time the island snakes would be small because they are genetically different. You can easily imagine how the two processes would work together, perhaps with environmental effects working initially but genetic changes accruing over time.

Boback did eventually come to a conclusion about the small size of the island boas. He recently told me, “we determined that there is some genetic component to dwarfism on islands. However, we believe that it is actually a combination of genetic and environmental effects that ultimately determine island boa size. That is, growth rates are different between island and mainland boas and this appears to be determined partly by genetics.” (See below for the reference to his paper on this research.)

More recently, Boback and his students at Dickinson College have been addressing a different question about boas: How do they know when to stop squeezing their prey? This is an interesting question because, as you might imagine, contracting the majority of muscles in one’s body for an extended period of time is energetically costly, but letting go of prey before it is fully dead may cause the loss of a meal. As an informal experiment, I asked five different people this question over the past two days, after reading of Boback’s research, and everyone gave approximately the same answer: The snakes let go when the prey is dead and stops struggling.

Well, it turns out that we do science to prove ourselves wrong, because that is not the answer. Suspecting a particular mechanism, Boback his students, who maintain a colony of these boas in their lab at Dickinson, devised a brilliant experiment. They took a number of dead rats that would normally be fed to the snakes, and installed robotic “hearts” in them. When the snakes constricted the rats, the hearts were allowed to beat for a while, then they were turned off. Soon after, the snakes loosened their grip, then let go.

It turns out that boas have the ability to detect a heartbeat in the prey, and they use this information to determine how much pressure to apply. Snakes that had never killed or eaten live prey acted the same as snakes with experience with live prey, suggesting that this behavior is innate and not learned.

“Many of us think of snakes as audacious killers, incapable of the complex functions we typically reserve for higher vertebrates,” says Boback. “We found otherwise and suggest that this remarkable sensitivity was a key advancement that forged the success of the entire snake group.”

One of the neat things about this project is that it involved the efforts of undergraduate researchers. The undergraduates not only participated in the research, but they helped produce the peer reviewed paper and are listed as authors. Katelyn McCann, who was a student on this project and now works as a clinical-research coordinator at Children’s Hospital in Boston, notes, “I got to experience the true collaborative nature of research as well as the hours of independent work that go into the final product. Now, working in research I feel like I truly understand the scientific method and what goes into any study.” Boback adds, “student-faculty research at Dickinson is an opportunity for students to experience science in action. It is the most fundamental level of learning in science as the student actively participates in the process of discovery.”

Boback, S., Hall, A., McCann, K., Hayes, A., Forrester, J., & Zwemer, C. (2012). Snake modulates constriction in response to prey’s heartbeat Biology Letters DOI: 10.1098/rsbl.2011.1105

Boback, S. M. and D. M. Carpenter. 2007. Body size and head shape in island boas (Boa constrictor) in Belize: Environmental versus genetic contributions. Pages 102-116 in R. W. Henderson and R. Powell, editors. Biology of the boas, pythons, and related taxa. Eagle Mountain Publishing, Eagle Mountain, UT.

Additional information for this story came from Dr. Scott Boback, and a press release from Dickinson College.

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