American Toad

 

 

Common Name: American Toad – The word toad is of Old English provenance from tādie, tādige and tāde becoming tode in Middle English. According to the OED it has no known cognates in other languages. The very recognizable and common toad was likely named independently and esoterically in many languages so that there is no common root. A French toad is crapaud, a German toad is kröte and an Italian toad is rospo; the lack of semantic similarity supports the independent derivative thesis.

Scientific Name: Bufo americanus – The word for toad in Latin is bufo. The genus is being partitioned according to DNA analysis with a new genus Anaxyrus for toads.

Potpourri: There is probably no other animal that matches the toad in homeliness; the gaping maw, bulging eyes, tumid body, and warty skin – a rebarbative whole and the epitome of ugly. While this description is true from the anthropomorphic perspective, it is meaningless in the grand scheme of nature; evolutionary selection dictates the appearance, secretions and behaviors of all living things. Toads are masters of their environment to which they are ideally suited; they exude poison to ward off predators and employ their long, sticky tongues to capture hundreds of insects every day. Toads and frogs make up the order Anura (also called Salientia from the Latin word for “leaping”) of the Amphibian Class which bridges the gap between the first sea creatures of the Devonian Epoch to the reptiles of the Carboniferous. In general toads are noted for terrestrial habitation, dry skin, and short legs that are more adapted to walking than hopping; there are about 300 species worldwide of which 18 are found in North America. Frogs are more aquatic or, in the case of tree frogs, arboreal, have moist, mucus-coated skin and long legs adapted for leaping; they are much more prevalent than toads with about 2500 global species.

The “missing link” status of the anurans in the transition from water swimming to land walking animals is best exemplified by a life cycle that begins with a gilled, fish-like plant-eating tadpole and ends with a lunged, carnivore quadruped – a metamorphosis of extraordinary complexity only matched by the lepidopterans. The process begins in the spring when male toads stake out favorable watering holes as potential incubators, announcing their presence and expectations with a trilling cricket-like sound that can last for up to 20 seconds by extending and gradually contracting the dewlap pouch on their throat. For reasons that only a female toad would ever comprehend, a choice of mate is made manifest presumably based on the throatiness of the call and the quality of the proffered lair. Amplexus, the embrace of toads and frogs, ensues as the male grasps the female from behind with the specialized horny pads on the first and second toes on the forelegs. However, the foreplay is for naught, as the female responds with the release of about 6,000 eggs in two neat rows of sticky gel which are then fertilized externally by the male’s ejaculation. This is quite obviously not as unrequited as it sounds, as male toads go to great lengths to achieve this end. As one of the many methods used by toads to escape predation, the eggs are black on the top and white on the bottom and are accordingly difficult to spot from above or below. After about two weeks, the eggs hatch to tadpoles as the struggle for survival continues.

Toad tadpoles seem among the most defenseless of nature’s creations, a plump glob of nutritious protein, the beguiling lashing tail begging the attention of any watchful predator. However, all is not as it seems. In addition to staying in shallow water and swimming in schools for protection, toad tadpoles exude a toxic substance that is poisonous to at least some of its fellow pond denizens, notably fish. A relatively simple study conducted in North Dakota in 2002 demonstrated the efficacy of “chemically mediated predator avoidance” of toad tadpoles for the common yellow perch (Perca flavescens). Perch were first captured and fed non-toxic frog tadpoles to select a group that demonstrated a tadpole food preference. The tadpole eating perch were then offered toad tadpoles; every perch save two swallowed and spit out at least one tadpole. The two perch that ate the toad tadpoles died; dissection revealed that the ingested tadpoles were whole and surrounded by a thick coating of mucus as evidence of toxicity. To confirm that the toad poison was an effective deterrent, the experimenters introduced a mixture of 70 percent toad tadpoles and 30 percent frog tadpoles to the perch with the surprising result that the fish selectively ate only the non-toxic frog tadpoles – a perfect Pavlovian conditioned response. The evolution of toad toxicity was an adaptation to allow for the two-month long metamorphose period of pond vulnerability. The hind legs start to form after several weeks and the skin covered forelegs appear several weeks later at about the same time that the gills disappear and the tail is absorbed to create an air-breathing, meat-eating juvenile toad. In spite of the total transformation in morphology, the toxins are retained as adult toads are no less vulnerable.

The plodding toad has no defensive weaponry with which to deter predation and must therefore rely on a combination of apatetic methods in addition to the chemical warfare of poisons for survival. Snakes are the primary toad predators, notably hognose and garter snakes which are unaffected by toad toxins. The mottled browns, grays, and blacks of warty toad skin are ideally suited to crypsis, seeking protection in the camouflage like that of army field uniforms. There is also something of a chameleon quality, as toad skin color changes according to the colors of the habitat. The advantages of visual deception are enhanced by the toad’s behavioral adaptation of sitting absolutely still when a potential predator is detected (toads are fairly easy to photograph for this reason); snakes in particular require movement to detect prey with their notoriously poor eyesight (which is why they extend their smell sensitive tongues). As a last resort, toads will also inflate their bodies with air to deter ingestion and urinate to reduce palatability. This last observation is more likely a matter of fright, as it is hard to imagine anything more unpalatable than a toad – even to a snake. Raccoons, another key toad predator, are repelled by the skin toxins. However, with typical mammalian cleverness, they have learned to turn the toad over and avoid the skin altogether, feasting on the soft underbody.

The toad is a masterful predator in its own right, using its whip-like, sticky tongue to zap up to a thousand insects every day. The primary arthropod diet is supplemented by almost anything else that comes within range, including slugs, earthworms and snails. The omnivorous ambivalence of the toad can best be appreciated by one of its more bizarre behaviors. As the toad grows, it sheds its skin every several weeks while growing and four times annually as an adult. While most molting animals leave their discarded skin behind, toads peel it off in one piece, collect it under their tongues, and eat it. This quasi-cannibalistic practice may have evolved as a means of maximizing nutritive opportunities. However, a plausible and more logical explanation is that the retention of the molecular constituents of the poison contained in the skin is used to augment and replenish its arsenal of chemical weaponry. Toad skin is a complex organ covered with many small protuberances called warts and infused with poison producing glands. It should be duly noted that the syllogism if toads have warts, then they must cause (human) warts is incorrect. Human warts are caused by an infection with some forms of human papillomavirus, and are normally contracted by contact with another human with warts. The toad’s warts are not an abnormal growth but an essential component of their chemical defense system – poisons

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Toad poisons are generally called bufotoxins (recall bufo is Latin for toad) and consist of a variety of steroid lactones that vary according to the species. The toxic substances are produced by the skin and by two large and notable protuberances on either side of the head just behind the eyes called parotoid glands (one of the distinguishing features of Bufo americanus is that the parotoid glands do not touch the cranial ridge – if they touch it is Bufo woodhousei which is otherwise indistinguishable). The parotoid glands in toads are similar in function to the parotid glands in mammals in that they exude a secretion that is very like mammalian saliva – except it is toxic. Depending on its constituent chemistry, the toxin may be called a variety of derivative names such as bufagin and bufogenin. Suffice it to say that toads are toxic, a well-known article of folklore wisdom that extends to antiquity with a rich history that ranges from pharmaceutical application to Stygian witchcraft.

For reasons that are unclear but which may relate to medicinal uses of their toxins, toads were associated with the uterus and gynecological diseases in Ancient Greece and this association spread throughout Europe as far as Scandinavia. It may be that the midwife as the recognized expert at parturition and who perhaps used bufotoxins was the etiological origin of the witch, whose warty ugliness was mirrored in her companion toad. In most Romany dialects, the language of the gypsies who are often associated with witches, toad and devil are the same word. The witch-toad linkage may well be the origin of the wart theory, their mysterious appearance a manifestation of a spell having been cast. Witch’s marks, one of the hallmark signs sought by inquisitors as a testimony to satanic possession, were sometimes called toad’s foot. One of the more peculiar aspects of the toad and its toxins was an artifact known as the “toadstone.” It was believed that a toad would die from its own poisons if it did not have a protective antidote; this was in the form of a magical stone that the toad had in its head which could be obtained by placing an old toad on a red cloth and waiting for it to be expelled. So pervasive was this belief that it appears in Shakespeare’s As You like It Act 2, Scene 1 “Sweet are the uses of adversity, which, like the toad, ugly and venomous, wears a precious jewel in its head.” Or maybe a witch put it there.

The poisonous nature of toads, with or without the witch association, was a matter of common knowledge; Roman women are alleged to have used them to attempt to murder their adulterous husbands. At some point in the deep recesses of history, an unknown healing practitioner first used a bufotoxin as a medicinal. The first recorded European reference appears in a collection of medical practices entitled Liber de Venenis (The Treatise on Venoms) published in Padua, Italy by Pietro d’Abano in the 13th Century where toad medicine was prescribed for breathing difficulties. Toad medicine was not just a European formulation. The Chinese named their toad based pharmaceutical ch’an su and used it for the treatment of skin inflammation and toothache among other things. Toad poison was likewise used by the natives of South America as an arrow-poison. Toad based formulations were gradually adopted by most early dispensaries and national pharmacopoeias with applications that eventually coalesced into a fairly universal prescription for a very good reason: bufotoxins are good medicine. According to a reference in The Pharmacopoeia of the United States in 1919, “no essential difference in action was found between bufagin and the glucosides of digitalis.” Digitalis is a derivative of the foxglove and is widely used as the drug digoxin to increase the strength of the heart muscles. There has been a substantive amount of research primarily in Asia over the last few decades investigating the uses of bufotoxins in the treatment of breast and prostate cancer. It should come as no surprise that a chemical cocktail formulated by toads over millennia to successfully ward off predators is pretty good medicine.