Monthly Archives: November 2012

Egg-eating Snakes by Andrew Durso

This blog post comes from Andrew Durso – author of the blog ” Life is short, but snakes are long”.  You can follow Andrew and his blogs here:  http://snakesarelong.blogspot.com/

To view this blog post in Spanish, click here!   Para ver este blog en español, haga clic aquí!

I think we can all agree that amniotic eggs are delicious. They also happen to be one of the best sources of energy out there, and this is at least partially why we, and many other animals, enjoy eating them so much. In addition, they rarely fight back, and they almost never have physical defenses, such as spines, or chemical ones, such as deadly toxins. In fact, on the inside they’re pretty much all lipids (a group of molecules including fats and cholesterol), surrounded by either a leathery (in monotremes and most reptiles) or a hard, calcified (in birds) shell. I’ve already written about a species of burying beetle that specializes on snake eggs, apparently with great benefit to its fecundity relative to other burying beetles that use carrion. Turns out, snakes aren’t above specialized oophagy themselves.
There are a few snakes that eat anamniotic eggs, such as the turtle-headed sea snakes (about which I’ve written before) and the South American goo-eaters. These have many amazing adaptations to eating shell-less eggs, but I’d like to focus on the amniotic egg-eating snakes for now. To review, an amniotic egg is one with a shell and several other embryonic membranes, called the amnion, chorion, and allantois. These structures physically protect the embryo and facilitate gas and waste exchange between the embryo and its surroundings, because the shell is too thick to allow the embryo to breathe and excrete by diffusion alone. These eggs are laid by birds, many reptiles, and monotremes (egg-laying mammals such as the platypus and echidna). In placental mammals (including humans), which are also amniotes, some of these structures are part of the umbilical cord, while others are vestigial. Amniotic eggs are adapted for being laid on land, and even the most aquatic of amniotes, such as sea turtles and pelagic birds, must come to land to lay their eggs.
Because of the resilience and self-contained nature of amniotic eggs, many organisms that lay them have done away with parental care. Choosing a nest site, usually under a rock, log, or pile of poop, or in a nest dug underground, is the extent of it. Beyond that, a female snake or turtle will most likely never see her kids hatch, let alone grow up, graduate, or become successful. This also means that their eggs are basically undefended from predators, except for being concealed and not smelling very much. Birds are slightly better parents, but they risk giving away the location of their nest to predators by flying back and forth to it many times a day. Experiments conducted by herpetologist Steve Mullin and ornithologist Bob Cooper have shown that gray ratsnakes locate bird nests over twice as quickly when parents are attending than when they aren’t, a phenomenon so prevalent that it has its own name (Skutch’s hypothesis) and is thought to influence the evolution of optimal clutch size in birds (because more offspring need to be fed more often, necessitating more trips to and from the nest and increasing the likelihood of detection by a predator).

Ok, enough – let’s get to the pictures of snakes eating eggs!

African Egg-eating Snake, Dasypeltis scabra

 

Holy shit, how do they do that!? That snake is going to choke itself! Got to be a faked, Photoshopped image, right? Think again:

Damn, that’s impressive. If you watched the video above, you saw an African Egg-eating Snake, perhaps the most specialized oophagous snake there is, swallow a bird egg whole, crack it open, and regurgitate the  shell. How does it do it? The highly kinetic, flexible skull of this snake allows it to maneuver its jaws around an egg many times bigger than its head, despite the smooth, round surface and the snake’s lack of hands. It’d be like a human trying to eat a whole watermelon. Egg-eating snakes lack teeth almost entirely, not needing them for gripping their prey. In addition, the snake’s skin is stretchy enough to accommodate the egg’s passage – the scale rows are clearly visible, widely separated by the skin in between. Most of the time, this skin can’t be seen, because the skin is relaxed so that the rows of scales are in contact with one another.

 Once the egg is in the snake’s esophagus, how does it get cracked open? Snakes have strong digestive juices, but waiting for them to dissolve the shell of an egg would take too long. OK, are you ready? This is the coolest part:

Vertebral hypapophyses of
African egg-eating snakes, Dasypeltis

See those spines? Those are called hypapophyses, which is a fancy term for things that stick off the bottom (ventral side) of vertebrae. You’ve got them too – but in egg-eating snakes, they’re modified to be much larger and sharper, the better to pierce eggshells with, my dear. At least, the ones on vertebrae 17-38 are, the vertebrae that sit right above the esophagus and thus above egg once it has been swallowed. The esophagus itself is modified as well – it has loose folds, like pockets, into which each of the hypapophyses fits, so that they don’t puncture the esophagus itself. See how it works in the following video, from the BBC’s Life in Cold Blood:

Starting at 2:45, you can see the moving x-ray of the egg-eating snake swallowing the egg. Continuing through the end of the video, the snake cracks the shell, allows the yolk inside to drain into its stomach, and regurgitates the eggshell. Most amazing, young Dasypeltis don’t appear to have these hypapophyses – they grow as the snakes get older, which raises questions about what the juveniles eat. Even though eggs are nutritious, Dasypeltis must feed relatively often for a snake – one that my advisor kept in captivity ate several quail eggs a week.

Lateral view of the skull of Dasypeltis, from Gans 1952

The adaptations of the nine species of Dasypeltis allow them to eat eggs that are very large relative to their body size, and as far as we know they eat almost nothing else. Several generalist snakes also eat eggs; adult Eastern Kingsnakes (Lampropeltis getula), Western Hog-nosed Snakes (Heterodon nasicus), and Formosa Kukrisnakes (Oligodon formosanus) frequently consume reptile eggs, and many members of the rat snake genera Pantherophis and Elaphe opportunistically feed on both eggs and nestling birds. These snakes, however, have no special morphological or behavioral adaptations to assist them in the consumption of eggs. One species, the Japanese rat snake (Elaphe climacophora), can ingest relatively large eggs, and has several vertebral hypapophyses. However, E. climacophora ingests the entire egg, including the shell. Only Dasypeltis, and possibly a poorly-known species from India called Elachistodon westermanni, specialize in ingesting large eggs, then crushing the shell and retaining solely the contents.

Defensive display by Dasypeltis scabra
ACKNOWLEDGMENTS
Thanks to Armata, Tony Phelps, and the BBC for images and videos.
REFERENCES

Coleman K, Rothfuss LA, Ota H, Kardong KV (1993) Kinematics of egg-eating by the specialized Taiwan snake Oligodon formosanus (Colubridae). Journal of Herpetology 27:320-327
Gans C (1952) The functional morphology of the egg-eating adaptations in the snake genus Dasypeltis. Zoologica 37:209-244
Gans C, Oshima M (1952) Adaptations for egg eating in the snake Elaphe climacophora (Boie). American Museum Novitates 1571:1-16
Gartner G, Greene H (2008) Adaptation in the African egg-eating snake: a comparative approach to a classic study in evolutionary functional morphology. Journal of Zoology 275:368-374
Mullin SJ (1996) Adaptations facilitating facultative oophagy in the gray rat snake, Elaphe obsoleta spiloides. Amphibia-Reptilia 17:387-394
Mullin SJ, Cooper RJ (1998) The foraging ecology of the Gray Rat Snake (Elaphe obsoleta spiloides)—visual stimuli facilitate location of arboreal prey. The American Midland Naturalist 140:397-401
Savitzky AH (1983) Coadapted character complexes among snakes: fossoriality, piscivory, and durophagy. American Zoologist 23:397-409

Neonate Timber Rattlesnakes – By Shelly Cox

As many of you know I’ve been working with Dr. Mark Mills of MWSU on a rattlesnake survey on one of the farms my husbands family owns. We’ve been P.I.T. tagging rattlesnakes for 2 seasons. Nearly two weeks ago my brother-in-law found a neonate (newborn) rattlesnake hiding under a piece of tin. He did not capture it thinking we weren’t interested in tagging the babies. Little did he know it was the babies we are the most interested in tagging. I made a quick trip to the farm, and by the time I got there it was nearly dark. I prayed the snake would still be where he had found it, but had doubts as three hours had already passed. I grabbed a flashlight, my bag and my snake tongs. I flipped over the piece of tin and voila! The snake WAS still there. Such a beautiful little snake. I called Dr. Mills and told him what I had, and he was as excited as I was at the opportunity to tag a newborn. We agreed to meet at 10:00 AM the next day and process the snake.

(Mark measuring the neonate)
(Safely contained in a snake tube)
This snake measured 15.5 inches in length, weighed 1.36 ounces and is a female. She has one little button and rattle which tells us she has shed once. The average size of newborn timber rattlesnakes is 9 to 13 inches. So this snake is considerably larger than that. Our hypothesis is that this snake was born earlier than is typical for this species. Perhaps sometime the end of July or first of August. We know it is a neonate as the umbilical scar is still visible.
(Umbilical scar)
We released this little darling back to the piece of tin where she was found after P.I.T. tagging her. We took some time and searched the area for additional litter mates to this snake. We did not locate a single other snake of any kind.
Ten days later I was at the farm looking for snakes and found another neonate under the exact same piece of tin. I suspected it was probably the same snake we tagged previously, but decided to capture it anyway. I took it into work with me and called Dr. Mills. He met me at the office and we went to one of MWSU biology labs. He ran his scanner over the snake and it had no tag in it!!!! This was a new snake!!! We were so excited to have another neonate and most likely a litter mate to the one from 10 days ago. We measured her at 16.1 inches and she weighed 1.94 ounces. Again it is a female. We P.I.T. tagged her and I ran back to the farm and released her back to hiding spot. This is such an awesome opportunity for us to be able to tag not one, but two neonates. The data that we can potentially get from these snakes is invaluable. I can hardly wait to see how they do on their own.

Blog Post Written By Shelly Cox – Missouri Department of Conservation