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Saturday, June 30, 2012

Egg-eating snakes


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
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



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Life is Short, but Snakes are Long by Andrew M. Durso is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License.

Friday, June 22, 2012

Snakes that chew their food


I have to admit right up front that the title of this article is not really accurate. No snakes chew their food the way we do. Almost all snakes must swallow their food whole, which limits their (often considerable) gape to items they can jaw-walk their kinetic skulls over. Taken as a whole, there are few animals on Earth that snakes do not eat -- whales and dolphins, elephants, animals endemic to the polar regions, some very toxic millipedes. There are snakes that swallow leopards whole, snakes that eat porcupines without removing the quills, snakes that tolerate stabs from catfish spines, snakes that eat other snakes longer than they are. Here's a video of a Tantilla eating a giant centipede. As a group, they can eat nearly anything. They all swallow their prey whole. Almost.
Except for this one
Its genus name is Fordonia, which is probably meaningless, seeing as it was biologist J.E. Gray of the British Museum of Natural History,"well known for inventing many apparently meaningless scientific names", who came up with it (he also named the North American Farancia). Commonly known as the Crab-eating Water Snake or White-bellied Mangrove Snake (after the specific epithet), Fordonia leucobalia is native to the mangrove swamps and tidal mud flats of southeast Asia and northern Australia. It lives in mud lobster and fiddler crab burrows, and moves by jumping across the soft mud, into which it would sink if it tried to slither.

Part of a small but interesting group of live-bearing snakes known as homalopsids, Fordonia is southeast Asia's answer to the North American natricine Nerodia, for many the archetypical semi-aquatic snake. What sets Fordonia apart from other homalopsid snakes, which feed mostly on fishes, is that it eats crabs, an observation first made by Cantor in 1847. (This may be highly cathartic for the snakes, whose primary predators as juveniles include large crabs.)

Those are hard-shelled decapod crustaceans, for you biologists out there 
Like many other arthropods, crabs have an anti-predator adaptation called leg autotomy, similar to tail autotomy in lizards, salamanders, and some snakes. This means that their legs can break off when grabbed and will later regrow - better to lose a limb and escape than to be eaten whole. But Fordonia has evolved behaviors that exploit the crabs' ability to autotomize their legs - it pins the crab's body to the mud and pulls of its legs, eating them one at a time! Sometimes they also consume the crab's body, but often they just leave it behind. This makes Fordonia the only snake that breaks its prey apart prior to eating it, although we must admit that it is somewhat helped along by the crab's autotomy. This discovery was sufficiently exciting to be published in the prestigious journal Nature.

The five crab legs at the top, eaten by this snake, came from a crab about the size of the one on the bottom. The white circle represents the maximum-sized prey item the snake could have eaten whole. Figure from Jayne et al. 2002
The adaptations of Fordonia to cancrivory don't end there. As anyone who has eaten crab legs knows, a crab's exoskeleton is very tough - we humans must use tools to break into it. In order not to be internally lacerated by their prey, Fordonia have evolved extra tough, muscular stomach lining. Other crustacean-eating snakes, such as the North American crayfish snakes (genus Regina), as well as the arthropod-eating False Hook-nosed Snake (Pseudoficimia frontalis, a sonorine snake from western Mexican dry forests), also have thickened muscles surrounding their stomachs, to prevent internal damage from they prey's sharp exoskeletons.



Digestion in snakes is an intense process: their digestive enzymes are very strong, capable of breaking down  even bone. Still, a little mastication can help the digestive process along considerably. For most snakes this isn't an option, because their needle-like teeth and highly mobile skull bones are ill-suited to both cutting and generating bite forces. However, snake biologist Alan Savitzky reported that recently ingested crab legs extracted from Fordonia stomachs were crushed. How is this possible? In fact, Fordonia possess remarkably robust and compact teeth for a snake, almost like molars! Although this is an extreme morphological modification, Savitzky remarked that it is almost surprising that the teeth and skulls of Fordonia aren't more abnormal, considering their unusual diet. Finally, Fordonia has evolved a large salt gland to help maintain osmotic balance on a high-salt diet (crabs are isosmotic to their environment, meaning that they have the same salt content as sea water).

Left: Tooth of Cerberus rynchops; Right: Teeth of Fordonia leucobalia
While Fordonia does all this with hard-shelled crabs, another homalopsid species found in the same mangroves, the Cat-eyed Watersnake (Gerarda prevostiana), has been found to consume freshly-molted (and therefore soft-shelled) crabs in much the same way. This kind of specialization is also found among the four species of North American Crayfish Snakes (Regina) - two of which (R. rigida, R. alleni) have hinged teeth to help them consume hard-shelled crayfish, and two of which (R. grahamii, R. septemvittata) seek out freshly-molted crayfish by smelling their molting secretions. Incredibly, although Gerarda lacks the morphological adaptations for cancrivory of Fordonia, it was observed tearing apart the soft carapaces of crabs after eating their legs, which probably allows Gerarda to consume crabs that would otherwise be too large for them to swallow whole. The feeding mechanisms used by Fordonia and Gerarda differ in the modes of attack and prey restraint, the usual orientation for swallowing crabs, and how pieces were torn from prey, suggesting that they might have evolved their crab-eating habits independently and convergently, rather than inheriting them from a shared common ancestor (although they are evolutionary sisters, one another's closest relatives). Two other closely related genera of homalopsine, Myron and Cantoria, also consume some crustaceans, but are less well-known. How many snakes are out there with strange dietary adaptations that remain to be discovered? We may never know.

This snake only eats soft-shelled crabs - what a snob

ACKNOWLEDGMENTS

Thanks to A. Captain and Brendan Schembri for photographs.

REFERENCES

Alfaro ME, Karns DR, Voris HK, Brock CD, Stuart BL (2008) Phylogeny, evolutionary history, and biogeography of Oriental-Australian rear-fanged water snakes (Colubroidea: Homalopsidae) inferred from mitochondrial and nuclear DNA sequences. Molecular phylogenetics and evolution 46:576-593

Jayne BC, Voris HK, Ng PKL (2002) Snake circumvents constraints on prey size. Nature 418:143

Savitzky AH (1983) Coadapted character complexes among snakes: fossoriality, piscivory, and durophagy. American Zoologist 23:397-409

Shine R, Schwaner T (1985) Prey constriction by venomous snakes: a review, and new data on Australian species. Copeia 1985:1067-1071

Voris HK, Jeffries WB (1995) Predation on marine snakes: a case for decapods supported by new observations from Thailand. Journal of Tropical Ecology 11:569-576

Voris HK, Murphy JC (2002) The prey and predators of Homalopsine snakes. Journal of Natural History 36:1621-1632



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Life is Short, but Snakes are Long by Andrew M. Durso is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License.

Monday, June 18, 2012

Snake-eating beetles


So little is known about the parasites of snakes that we tend to discount them all together, but the ecological  and evolutionary interactions between hosts and their parasites can be very strong. This is a story about how two enterprising snake biologists solved a mystery that had been puzzling entomologists for decades.

Burying beetles (genus Nicrophorus) conceal small vertebrate carcasses underground and prepare them for consumption by their young by excavating a crypt up to 60 cm deep, removing fur or feathers from the carcass, and covering it in anal secretions to prevent fungal growth. The two parents slowly eat the carcass, defending it from other carrion eaters, and feed regurgitated bits of it to their altricial larvae, which hatch from eggs they lay in the walls of the crypt and beg to be fed like baby birds. Although feeding your babies poop-coated vomit sounds like the plot of a gruesome horror movie, it has been a successful evolutionary strategy for the burying beetles. Their complex social behavior, including biparental care and communal breeding, is unusual among insects. The whole process takes about two weeks.

Nicrophorus pustulatus
Of the nearly 75 species of burying beetle, distributed throughout the Northern Hemisphere, one in particular stands out for its unusual natural history. Entomologists studying the group use dead mice to bait traps, but one species, Nicrophorus pustulatus, never seemed attracted to the carrion. In addition, they are able to produce very large broods (up to 190 vs. 30-45 for most other species of burying beetle) of large offspring on carcasses in the laboratory. Usually, a large brood size comes hand-in-hand with a decrease in individual offspring size, but not in this species apparently. Why not?

Theories ranged from that N. pustulatus used larger carcasses, such as rabbits, without burying them, to that  it was an interspecific brood parasite, like a brown-headed cowbird, laying its eggs in the nests of other burying beetles. But in 2000, a paper in the journal Ecoscience by two snake biologists, Gabriel Blouin-Demers and Patrick Weatherhead, then of Carleton University in Ontario, revealed a surprising discovery. They were studying the nesting ecology of black ratsnakes (Pantherophis obsoletus, formerly Elaphe obsoleta) in Canada by radio-tracking adult female ratsnakes to their oviposition sites. Their purpose was to document the use of communal nests by these snakes and to collect information on clutch size and juvenile survival. When they examined the ratsnake nests they found, they discovered that many of them contained adult and larval  N. pustulatus.

Ratsnake eggs parasitized by carrion beetles
Blouin-Demers and Weatherhead found evidence of beetles in six of the seven nests they looked at. In some nests, only old eggshells with small holes evidenced the beetles' presence, but in others 100% of the eggs were destroyed by the beetles and their larvae. Because black ratsnakes nest communally in this part of the world, up to 111 eggs can constitute a nest, even though the average clutch size is only 11-15 eggs per female. The ratsnakes use the same communal nesting sites year after year, which can be highly beneficial because of increased nest temperature and shorter development time. At such northern latitudes, female ratsnakes do not lay eggs until June or July, and the babies must hatch by late August in order to avoid being killed by an early frost. A mother ratsnake's only parental care is her nest site choice, and research has shown that eggs laid in communal nests hatch earlier, grow larger in their first year, and can even swim faster than those incubated with just their clutchmates. However, the probability of a beetle infection probably increases with increasing nest size, because it only takes one infected egg to spread the beetles to the whole nest. This is why N. pustulatus is so fecund compared to other carrion beetles - because it can raise enormous numbers of larvae on large snake nests, full of nutritious eggs and already hidden away in sites with ideal thermal and humidity.

Black Ratsnake (Pantherophis obsoletus)
Based on their findings, Blouin-Demers and Weatherhead characterized N. pustulatus as a parasitoid of snakes. A parasitoid is different from a parasite because they are parasitic only as larvae (although in this case, with a little help from their parents), and they always kill their host. However, they are also different from predators, because each parasitoid larva only kills a single host individual instead of many. Blouin-Demers and Weatherhead suggested that theirs was the first example of a vertebrate being host to an arthropod parasitoid, and so far they are correct.

The full mystery is far from solved, though. Did N. pustulatus evolve this behavior by first exploiting snake eggs that failed to hatch? How do the beetles find reptile eggs? Are communal nests easier for the beetles to find, or do they simply prefer them because of their higher concentration of resources? How has parasitism by this beetle influenced ratsnake evolution? Do any other species of Nircophorus also parasitize reptile eggs? Does N. pustulatus beetles also parasitize the eggs of other species of snake? Observations of fox snake (Pantherophis vulpinus) nests in Illinois have also yielded beetle larvae. The range of N. pustulatus extends farther north than that of any oviparous snake species (snakes at high latitudes tend to be viviparous, because the females can more precisely control the temperature of their developing offspring if they carry them around). What do they use for rearing their young up there? Could it be turtle eggs, or do they use small animal carcasses like their ancestors?

Nicrophorus pustulatus with phoretic mites
From the beetle's perspective, it has arrived at a very successful reproductive strategy by shifting hosts. By moving away from nesting in carcasses, for which they must compete with flies, ants, fungi, bacteria, and scavenging vertebrates such as skunks and raccoons, it has secured an apparently unique niche. As a defense against carcass competitors, some Nicrophorus species carry phoretic mites that eat fly eggs, but lab experiments have shown that the mites sometimes eat the beetles' eggs too, so the benefit is not without risk. Additionally, not having to move or bury snake eggs saves the parent beetles a lot of energy prior to laying their eggs. Experiments have shown that N. pustulatus females oviposit rapidly in house snake (Lamprophis) eggs, and that male beetles elevate their sex pheromone emission in response to snake eggs. Other beetles in the genus Nicrophorus did not show the same response. While N. pustulatus will use mouse carcasses to rear their young in the lab, no one has ever found them doing so in the field. The entomologists who performed these lab tests also found that N. pustulatus adjusted its fecundity to the available mass of snake eggs.

As a driver of evolution in oviparous snake nesting strategies, Nicrophorus pustulatus may play an important role. Could they potentially pose a threat to egg-laying snake species that are of conservation concern, such as the Eastern Indigo Snake (Drymarchon couperi)? What might happen if they were introduced to a continent whose snakes had not evolved with parasitic beetles eating their eggs? There is still so much we don't understand about snake behavior, reproduction, ecology, and evolution, especially in the wild. Thanks to the observations of a few scientists who thought they were studying something else entirely, we are one step closer.


ACKNOWLEDGMENTS

Thanks to Joyce Gross, Loren Padelford, and Gabriel Blouin-Demers for allowing me to use their photographs.

REFERENCES

Blouin-Demers G, Weatherhead PJ (2000) A novel association between a beetle and a snake: parasitism of Elaphe obsoleta by Nicrophorus pustulatus. Ecoscience 7:395-397 <link>

Blouin-Demers G, Weatherhead PJ, Row JR (2004) Phenotypic consequences of nest-site selection in black rat snakes (Elaphe obsoleta). Canadian Journal of Zoology 82:449-456 <link>

Ikeda H, Kubota K, Kagaya T, Abe T (2006) Niche differentiation of burying beetles (Coleoptera: Silphidae: Nicrophorinae) in carcass use in relation to body size: estimation from stable isotope analysis. Applied Entomology and Zoology 41:561-564 <link>

Robertson IC (1992) Relative abundance of Nicrophorus pustulatus (Coleoptera: Silphidae) in a burying beetle community, with notes on its reproductive behavior. Psyche 99:189-198 <link>

Scott MP (1998) The ecology and behavior of burying beetles. Annual Review of Entomology 43:595-618 <link>

Smith G, Trumbo S, Sikes D, Scott M, Smith R (2007) Host shift by the burying beetle, Nicrophorus pustulatus, a parasitoid of snake eggs. Journal of Evolutionary Biology 20:2389-2399 <link

Trumbo ST (2007) Defending young biparentally: female risk-taking with and without a male in the burying beetle, Nicrophorus pustulatus. Behavioral Ecology and Sociobiology 61:1717-1723 <link>

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Life is Short, but Snakes are Long by Andrew M. Durso is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License.