The Pangolin: One of the Strangest Animals Known to Humans
Pangolins are some of the strangest animals you will likely ever see. They are covered with scales like reptiles and look like a cross between anteaters and armadillos (Kierst, 2013). Because they are like no other animal, they were put in their own taxonomic order called Pholidota, Greek for “horny scale” (Benton, 2005, p. 348). Their tough, overlapping, reptile-protective keratin scales cause it to look like a giant pinecone with a long, thin reptilian tail. Its profile looks very much like a miniature dinosaur and nothing like the mammal it is. It is a mammal because they give birth to live young that suckle mammary glands as infants (Johnson, 2001, p. 31).
Although edentulous (toothless), they have hard, rough stomach walls that contain small rocks that grind up their food. They survive mostly on ants and termites captured by their extraordinarily long (40 cm), muscular, wormlike tongue that is almost as long as its body (Hutchins, 2003). The tongue produces saliva that sticks to its main meal, ants and other small insects. Its tongue muscles are attached to, of all places, its hip bones! (Ricciuti, 1994, p. 11). Pangolins are designed with thick protective eyelids and can close their nostrils and ear openings to protect themselves from insect bites (Ricciuti, 1994, p. 16).
They are the only known living mammals with reptilian-like scales, which are structurally and compositionally very different from most reptile scales (Choo, et al., 2016). Pangolin’s scales consist of hairs glued together into large, overlapping plates that cover all but their soft belly, face, snout, and inner sides of their legs (Ricciuti, 1994, p. 8). These parts are hairy, like mammals. Its overlapping scales can protect its body and its face when they’re both tucked under its tail (Hutchins, 2003, pp. 107-113). Their sharp scales, which can be raised upward like porcupine scales to make the animal appear larger, provide critical defense from predators. For protection they can also curl up into a tight ball like an armadillo (the name pangolin is from the Malay word ‘pengguling,’ meaning “one who rolls up”). When sleeping, like a good mother, they protect their babies by curling around them. Like a skunk, they can spray enemies with their pungent spray to help defend themselves. They have poor eyesight, but a very acute olfactory system that provides them with an incredible smell ability (Choo, et al., 2016, p. 1).
Six species exist. The smallest, the long-tailed pangolin, are three feet long and weigh three to four pounds. The largest, the giant pangolin, are close to six feet long and weigh 70 pounds (Ricciuti, 1994, p. 10). Its four feet each have five claws that it uses to climb trees and rip open insect nests for meals. The claws are curled under when walking, causing it to look like a knuckle walker. When forced to run, pangolins stand up and run three miles per hour on two feet like humans! It uses its tail to help it balance (Ricciuti, 1994, p. 11). Their tails also help them climb trees by grasping the branches like monkeys!
Classification and Evolution
Classification has been a major problem as documented by the many past, failed classification attempts. They were once classified with various orders of ant-eating mammals, the Xenarthra, which include true anteaters, sloths, and armadillos, which pangolins superficially resemble. Newer genetic evidence, however, points to their closest living relatives as the Carnivora which forms, together with the order Pholidota, the clade Ferae (Penzhorn, 2013, pp. 438–443). Other evolutionists have classified the pangolins in the order Cimolesta, together with several extinct groups, though this idea has also fallen out of favor since cimolestids were not placental mammals as are pangolins (Rook and Hunter, 2013, pp. 1-17).
A 2015 study found close affinities between pangolins and the extinct group called Creodonta (Halliday, et al., 2015). In short, pangolins have features of several diverse animals that have stymied not only their classification, but also any attempts to determine their evolution, a subject largely avoided due to almost no hint of change in the fossil record. A number of extinct pangolins have been found, all modern “living fossils.”
The combination of monkey, anteater, skunk, armadillo, with a profile like a dinosaur has baffled zoologists for decades, so they just put pangolins in a class by themselves because no one can figure out what life-form they are related to! The best conclusion is the first pangolin was a pangolin and is not related to any other animal, although it has traits like several very different animal kinds [aka a ‘mosaic’ creature]. They can climb and sleep in trees, walk and sleep on land, dig burrows as deep as 30 meters deep to live in, and are even expert swimmers (Ricciuti, 1994, p. 14). The only means of travel they are unable to use is flying.
Evolution of Pangolin Scales
The selective forces underlying the origin of this unique mammalian-reptile scale trait remain a mystery, although observations of the eight modern species suggests the defensive armor functions as protection against predators. Choo, et al., proposed pangolin scale evolution was partly “an innovation that provided protection against injuries or stress and reduced pangolin vulnerability to infection.” A sequence of its genome, found the Malayan pangolin genome contains 23,446 genes and the Chinese pangolin 20,298, which is very close to the size of the human genome (Choo, et al., 2016).
The “Just-So”-Story Explanation for Scale Evolution
The well-known genetics professor Ricki Lewis reviewed the Choo, et al., study that attempted to understand how and why their scales evolved. She wrote in the new Genome Research Journal that the study “provides the basis for a ‘just-so story’ about how the pangolin — aka the scaly anteater — got its scales.” In short, the pangolin armor is said to have replaced part of its lost immune protection with scales! (Lewis, 2016) Lewis explained:
“Darwin and Lamarck pondered the advantages of the giraffe’s long legs and neck, while a few decades later Rudyard Kipling [humorously] explained how the leopard got its spots. [In short, humans painted brown spots on the leopard’s fur, so that the leopard would blend in with the environment and not be seen by its enemies] Today, genome sequencing is fleshing out what we thought we knew about some distinctive animal adaptations, from the giraffe to the leopard.” (Lewis, 2016)
She continues, noting the interferon epsilon gene which provides a “first line of defense” against skin infections was damaged in both pangolin species that Choo, et al., studied,“as well as in their African counterparts. Yet it is fully functional in 71 other species of placental mammals, where it provides a “first line of defense” against skin infections. Several other interferon genes, which deal with infection, inflammation, and skin healing, are missing too. The Malayan pangolin has three, the Chinese pangolin two, yet other mammals a full set of ten.” (Lewis, 2016)
She then gives the just-so story of how she thinks the scales evolved:
“At some point in time, a few pangolins, thanks to chance mutations, had harder hair. Other mutations somehow guided those hairs to eventually overlap, providing shielding. Individuals whose hairiness began to become overlapping scaliness were less likely to succumb to bacterial infections, and thereby more likely to survive to pass on those traits.” (Lewis, 2016)
Lewis continues with her just-so story, adding that some clues in the pangolin genomes suggest their
“armor has replaced part of the immune response. The tightly-knit, tough scales deter not only predators, but keep the animal free of infection. Although it is intriguing to imagine reasons why animals are as they are – from the giraffe’s neck to the leopard’s spots to the pangolin’s armor – clues in DNA sequences can provide a broader and less biased view [comparing] those that have stood the test of evolutionary time to those relegated to the genomic junkyard.” (Lewis, 2016)
The problem is, a major reason why pangolins do not survive in captivity is they often die of disease, possibly because their immune system is severely compromised due to the mutations noted, but still can survive because of their well-designed armor. Even in a protected environment, wild disease is a problem (Ching-Min Sun, et al., 2019). Nonetheless, this just-so story, as Lewis notes, is an imaginative narrative not based on fact except what appears to be the loss of one or more genes critical to their effective immune defenses. This is evidence of genome deterioration and de-evolution, not the addition of new information or genes as is required for ‘onward-and-upward’ evolution.
Benton, Michael. 2005. Vertebrate Palaeontology, Third Edition. Oxford, NY: Blackwell Science.
Ching-Min Sun, et al., 2019. Unprecedented account of mortality and morbidity in free ranging Formosan Pangolin (Manis pentadactyla pentadactyla). https://www.biorxiv.org/content/biorxiv/early/2018/05/17/325167.full.pdf.
Choo, Siew; et al. 2016. Pangolin genomes and the evolution of mammalian scales and immunity. Genome Research. www.genome.org.
Halliday, Thomas J. D.; Upchurch, Paul; and Goswami, Anjali. 2015. Resolving the relationships of Paleocene placental mammals.. Biological Reviews: n/a–n/a. doi:10.1111/brv.12242. ISSN 1464-7931.
Hutchins, Michael (Editor). 2003. Grzimek’s Animal Life Encyclopedia, Second Edition. Volume 16: Mammals V, “Pangolins.” Farmington Hills, MI: Thomson/Gale Group.
Johnson, Jinny. 2001. 100 Things You Should Know About Mammals. New York, NY: Barnes & Noble.
Kierst, Anastasia. 2013. P is for Pangolin. Rollinsville, CO: Eternal Summers Press.
Lewis, Ricki, 2016. “How the Pangolin Got Its Scales – A Genetic Just-So Story.” PLOS Blogs. https://blogs.plos.org/dnascience/2016/10/20/how-the-pangolin-got-its-scales-a-genetic-just-so-story/.
Penzhorn, Banie. 2013. In: Mammals of Africa. Volume V. (Eds. Kingdon, J., Hoffman, M. & Bloomsbury), London, UK, pp. 438–443.
Ricciuti, Edward R. 1994. What on Earth is a Pangolin? Woodbridge, CN: Blackbirch Press.
Rook, D.L. and J.P. Hunter. 2013. Journal of Mammalian Evolution, 21(1): 1–17.
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