The Pangolin: The Strangest Animal Known to Man

 

Pangolin Scale Evolution: Another Just-so-Story

by Jerry Bergman, PhD

 

What are Pangolins? 

Pangolins are the strangest animal you will ever meet, although you will likely see photos of them rather than the real animal unless you live in Africa or Southeast Asia. Their tough, overlapping, reptile-protective keratin scales make it look like a giant pinecone with a long, thin reptile-tail. Its profile looks very much like a miniature dinosaur and nothing like a mammal.[1] Pangolins, are scaly anteaters in the order Pholidota, Greek for ”horny scale.”[2] They are edentulous (toothless) and survive mostly on ants and termites captured using its extraordinarily long (6-to-7-inch), muscular, wormlike tongue that is almost as long as its body.[3]

They are the only known living mammals with scales, which are structurally and compositionally very different from the typical scales of reptiles.[4] The pangolin’s scales consist of hairs glommed together into large, overlapping shingles that cover all but their soft belly. The scales are soft in newborn pangolins, but harden as the animal matures. Its overlapping scales protect its body and its face when they’re both tucked under its tail.[5] The sharp scales provide extra defense from predators. For protection the pangolin can curl up into a tight ball like an armadillo (the name pangolin is from the Malay word ‘pengguling,’ meaning “one who rolls up.”) They have poor eyesight, but a very acute olfactory system which gives them an incredible sense of smell.[6]

Classification and Evolution

Classification has been a major problem as documented by many past failed attempts. They were once classified with various orders of ant-eating mammals, the Xenarthra, which includes true anteaters, sloths, and the armadillos which pangolins superficially resemble. Newer genetic evidence, however, points to their closest living relatives as the Carnivora with which forms the clade Ferae.[7] 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.[8]

A 2015 study found close affinities between pangolins and the extinct group called Creodonta.[9] In short, pangolins have features of several diverse animals. This has stymied not only their classification, but also attempts to determine their evolution, a subject largely avoided due to almost no hint of transitional forms in the fossil record, although a number of extinct pangolins have been found.

Evolution of Pangolin Scales

The selective forces underlying the origin of this unique mammalian reptile-like scale trait remain a mystery, although observations of the eight modern species suggest a defensive armor function against predators. Choo, et al. proposed that pangolin scale evolution was partly “an innovation that provided protection against injuries or stress and reduced pangolin vulnerability to infection.”[10] Choo, et al., sequenced its genome, finding the Malayan pangolin genome contains 23,446 genes and the Chinese pangolin 20,298, very close to the size of the human genome.

The Just-So-Story for Pangolin Scale Evolution

Ricki Lewis is a well-known genetics professor who authored the genetics textbook that I used to teach my college level genetics class. She reviewed the Choo, et al. study that attempted to understand the evolution of their scales, writing in her report 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 … the armor of the pangolin replaced part of its immune response.”[11] Lewis also commented,

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.[12]

She continues: the interferon epsilon gene which provides a “first line of defense” against skin infections is 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.[13]

She then gives the just-so-story of how the scales evolved:

Natural selection tells the tale. 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. Perhaps the armor also made them more attractive and they had lots of pangolin sex, assuming their privates are reachable, passing on the desirable trait.[14]

Lewis continues with her just-so-story, concluding that

clues in the pangolin genomes … suggest strongly that the 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 [in contrast to] those that have stood the test of evolutionary time to those relegated to the genomic junkyard.

The problem with this story is that a major reason why pangolins do not survive in captivity is that they often die of disease. This is possibly because their immune systems have become severely compromised due to the mutations noted. Still, they can survive in the wild with their well-designed armor. Even in the wild in a protected environment, though, disease is still a problem.[15] Nonetheless, the just-so-story ‘How the pangolin got its scales’ is, as Lewis notes, an imaginative tale not based on fact. The one factual justification for it appears to be the loss of one or more genes critical for effective immune defenses – evidence of genome deterioration and de-evolution, not the addition of new genetic information which evolutionary theory requires.

Addendum: It is a sad commentary on human nature that pangolins are being slaughtered to the endangered level by superstitious beliefs. In Vietnam, pangolin scales, which are made of the same material as fingernails, “are used in traditional medicine to treat allergies and male impotence.” Phys.org reports that Vietnamese officials seized five tons of pangolin scales shipped from Nigeria in sacks of cashews to hide them from inspectors. Last month 26 tons en route to Vietnam were seized in Singapore. “Communist Vietnam is a hotbed for the illegal wildlife trade,” the article says, “where animal products from elephant ivory to rhino horn and tiger bones are consumed domestically and also smuggled abroad.” These folk remedies are worthless, but demand is driving noble animals toward extinction.  —Ed.

References

[1] Johnson,  Jinny. 2001. 100 Things You Should Know About Mammals. New York, NY: Barnes & Noble, p. 31.

[2] Benton, M. 2005. Vertebrate Palaeontology, Third Edition. Oxford, NY: Blackwell Science, p. 348.

[3] Hutchins, Michael (Editor). 2003. Grzimek’s Animal Life Encyclopedia, Second Edition. Volume 16: Mammals V, “Pangolins.” p. 109. Farmington Hills, MI: Thomson/Gale Group.

[4] Choo, Siew, et al, 2016. Pangolin genomes and the evolution of mammalian scales and immunity. Genome Research. www.genome.org.

[5] Hutchins, Michael (Editor). 2003. Grzimek’s Animal Life Encyclopedia, Second Edition. Volume 16: Mammals V, “Pangolins.” pp. 107-113. Farmington Hills, MI: Thomson/Gale Group.

[6] Choo, Siew, et al. 2016, p. 1

[7] Penzhorn, B. 2013. In: Mammals of Africa. Volume V. Carnivores, pangolins, equids and rhinoceroses (eds Kingdon, J. & Hoffman, M.), pp. 438–443.

[8] Rook, D.L.and J.P. Hunter. 2013. “Rooting Around the Eutherian Family Tree: The Origin and Relations of the Taeniodonta.” Journal of Mammalian Evolution. 21(1): 1–17.

[9] Halliday, Thomas J. D.; Upchurch, Paul; Goswami, Anjali. 2015. “Resolving the relationships of Paleocene placental mammals”. Biological Reviews: n/a–n/a. doi:10.1111/brv.12242. ISSN 1464-7931.

[10] Choo, Siew et al, 2016.

[11] 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/

[12] Lewis, 2016. Emphasis added.

[13] Lewis, 2016.

[14] Lewis, 2016.

[15] Nick 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.

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Dr. Jerry Bergman has taught biology, genetics, chemistry, biochemistry, anthropology, geology, and microbiology at several colleges and universities including for over 40 years at Bowling Green State University, Medical College of Ohio where he was a research associate in experimental pathology, and The University of Toledo. He is a graduate of the Medical College of Ohio, Wayne State University in Detroit, the University of Toledo, and Bowling Green State University. He has over 1,300 publications in 12 languages and 40 books and monographs. His books and textbooks that include chapters that he authored, are in over 1,500 college libraries in 27 countries. So far over 80,000 copies of the 40 books and monographs that he has authored or co-authored are in print. For more articles by Dr Bergman, see his Author Profile.