by Greg Mayer
As the capstone to Snake Week, let’s take a closer look at how the squirrel-like mammal being eaten by Tetrapodophis in Julius Csotonyi’s striking reconstruction died. In my earlier post, I took note of the fact that the describers of the newly discovered four-legged fossil snake had inferred from its skeleton that it was a constrictor (and thus the earliest known constricting snake, implying that constriction is an ancestral characteristic of snakes), and included Csotonyi’s lovely reconstruction showing the four-legged Tetrapodophis doing in and beginning to swallow a squirrel-like mammal. Here’s a reprise of the picture.
The snake killed the ‘squirrel’, so we know who killed it, but what killed the ‘squirrel’? In police procedural talk, we’ve got the murderer, but we want to know the cause of death for the coroner’s report. Coincidentally, a new paper in the Journal of Experimental Biology, appearing at almost the same time as the description of Tetrapodophis, asks exactly that question, and shows via straight-forward and well-done experiments, what, in fact, is the ‘squirrel’s’ cause of death.
It was long thought that constricted prey died of suffocation, but it had also been suggested that the prey died of cardiac arrest due to drop in blood pressure. This had been suggested, in part, by the rapidity with which prey died, seemingly more rapidly than they would suffocate. What Scott Boback and colleagues have shown, using anesthetized rats fed to boa constrictors, with a set of catheters and probes in them to record their heart rates and blood pressures, is that there is a sharp and sudden drop in peripheral arterial pressure, an increase in central venous pressure, and a slowing of the heart rate. They conclude:
[S]nake constriction induces rapid prey death due to circulatory arrest.
I’m not sure if their experiments quite exclude asphyxia as a contributing cause, but it certainly shows the importance of the circulatory crisis caused by constriction.
Some of the media coverage has overstated the novelty of this result. For example, National Geographic headlined “Why We Were Totally Wrong About How Boa Constrictors Kill”, while Science, somewhat less over the top, headlined “Surprise: Snakes don’t kill by suffocation“. However, as Boback et al. note, circulatory collapse was first suggested over 80 years ago, and has been a viable idea for quite a while. Harry Greene, our foremost student of snake natural history, taking an ecumenical approach to the cause of death, wrote in his fine Snakes, in 1997, that constriction acted by “interfering with breathing and blood circulation so that the victim is immobilized within a minute or so”, while in a later, standard, herpetology text, Laurie Vitt and Janalee Caldwell (2009) wrote, “The tightening continues, and ultimately, circulatory failure causes death.” So Boback and colleagues have done a fine and needed study, but don’t believe the (media) hype!
In writing this post, I wondered what to call the prey in Csotonyi’s reconstruction. It could not be a rat, as in Boback’s study, as there were no rats, or rodents of any kind, in the Cretaceous. On the other hand, it does look like a squirrel (a rodent as well, again not possible for the Cretaceous), so I settled on ‘squirrel’, with scare quotes. A likely mammal for Tetrapodophis to have eaten is some sort of multituberculate, an extinct type of mammal found in the Cretaceous, and convergent on rodents in their dentition (gnawing incisors with a diastema before the molariforms). And, some of them showed arboreal adaptations, as do tree squirrels, but even more so, having prehensile tails. In the reconstruction below, accompanying a paper by Farish Jenkins and David Krause, the multiberculate Ptilodus is shown to be quite squirrel-like, except for its opossum-like prehensile tail.
Boback, S.M., K.J. McCann, K.A. Wood, P.M. McNeal, E.L. Blankenship and C. F. Zwemee. 2015. Snake constriction rapidly induces circulatory arrest in rats. Journal of Experimental Biology 218:2279-2288. abstract
Greene, H.W. 1997. Snakes: The Evolution of Mystery in Nature. University of California Press, Berkeley.
Vitt, L.J. and J.P. Caldwell. 2009. Herpetology. 3rd ed. Elsevier, Amsterdam.