The Rock Hyrax: Elephants’ Unexpected Relative
When looking at an elephant, it may seem logical to assume that it would be related to a large, grey animal like the rhino, but that guess would be wrong. Interestingly, the elephant’s closest relatives look nothing like them. While this topic remains somewhat controversial in the scientific community, some have pointed to the rock hyrax as the closest living evolutionary relative to the elephant¹. It is important to emphasize, however, that this argument doesn’t come without controversy, and many have pushed back on the idea of hyraxes being the elephants’ closest relatives².
The rock hyrax(Procavia capensis) is a small, furry, groundhog-like mammal that lives in rocky crevices in most parts of Africa³. Like the elephant, the rock hyrax belongs to Afrotheria, which is a superorder consisting of early mammalian ancestors of AfroArabian origin dating back from 80 to 100 million years ago⁴. They even belong to the same clade within Afrotheria, called Paenungulata. The rock hyrax belongs to the order Hyracoidea, which diverged from the elephant’s evolutionary path around 65 million years ago⁵. This may sound like a long time ago, but is actually not that distant on the evolutionary time scale. When considering the elephant’s size and appearance in comparison to the rock hyrax, this comes as quite the surprise.
The elephant and rock hyrax actually share many reproductive characteristics that indicate a common ancestor. Male hyraxes’ and elephants’ testes both remain within the retroperitoneal abdomen and do not descend into a scrotum like those of many other mammals⁶. Their female counterparts both have similar placental origins and carry pregnancies for long periods of time, 7–8 months and 21–22 months, respectively. The hyrax’s gestation period is shorter than the elephant due to its smaller size, but is very long relative to other similarly-sized animals, such as cats, who typically carry for ~2 months. Unlike most other non-primate mammals, their mammary glands are located higher up by the front legs in both of these species⁷.
While rock hyraxes may seem rodent-like in appearance, their incisors are more similar to elephant tusks than rodent teeth. Both elephant and rock hyrax “tusks” are unique compared to those of other tusked animals, as they develop from their incisors rather than their canines². Like the elephant, the rock hyrax has flattened, hoof-like nails and highly sensitive footpads, while also lacking a gallbladder and pleural space within the rib cage⁶.
In addition to these morphological similarities, the hyrax and elephant share some mitochondrial gene sequences and molecular components¹⁷. Recently, it was discovered that both species have charged myoglobin, which binds oxygen more strongly and is associated with diving behavior in aquatic and partially aquatic animals. This suggests that they both may have evolved from a common aquatic ancestor, despite their current terrestrial lifestyles⁸.
The rock hyrax, like the elephant, is an intelligent animal with a capacity for long-term memory, though, not surprisingly, the large-brained elephants exhibit higher cognitive function than their furry cousins. Despite this, they both live very social lives and use complex vocal communications within their groups⁹. This may be explained by the fact that the hyrax, like the elephant, has a relatively large hippocampus¹⁰, the region of the brain notably involved with memory formation¹¹.
The rock hyrax is not the only unusual relative to the elephant. Other notable cousins include the subungulate “sea cows”: manatees (Trichechus manatus) and dugongs (Dugong dugon). Both of these aquatic mammals have tusk-like incisors and grey, thick skin¹². Both manatees and dugongs have prehensile lips that have similar function to an elephant’s trunk¹³. Some evidence has suggested that these Sirenians are actually more closely related to the elephant than the hyrax is, but there has yet to be a final consensus. Perhaps most importantly, one thing these species have in common is that their endangerment is due to the threat of humans¹².
Seemingly far-fetched animal species relations aren’t unique to the elephant. Echidnas are the closest relative to the platypus¹⁴, seals are said to be the cousins of bears¹⁵, and horseshoe crabs are closely related to spiders¹⁶. Evolution works in mysterious ways and scientists are working harder than ever to uncover its secrets.
References:
¹Perkin, Andrew. “Why is the elephant a ‘cousin’ of the hyrax? A short introduction to the Afrotheria of the Eastern Arc and Coastal Forests”. The Arc Journal: 7.
²Safaris Thomson. “Guinea Pig Lookalikes, Elephant Cousins: Rock Hyraxes”. Thomson Safaris, February 2, 2015. http://www.thomsonsafaris.com/blog/rock-hyraxes/
³Rübsamen, K., I.d Hume, and W.v Engelhardt. “Physiology of the rock hyrax.” Comparative Biochemistry and Physiology Part A: Physiology 72, no. 2 (1982): 271–77. doi:10.1016/0300–9629(82)90219–5.
⁴Tabuce, Rodolphe, Robert J. Asher, and Thomas Lehmann. “Afrotherian mammals: a review of current data.” Mammalia 72, no. 1 (2008). doi:10.1515/mamm.2008.004.
⁵Rohland, Nadin, Anna-Sapfo Malaspinas, Joshua L. Pollack, Montgomery Slatkin, Paul Matheus, and Michael Hofreiter. “Proboscidean mitogenomics: chronology and mode of elephant evolution using mastodon as outgroup.” PLoS biology 5, no. 8 (2007): e207.
⁶Carnaby, Trevor. Beat About The Bush: Mammals. Jacana Media, 2006.
⁷"Rock hyraxes and elephants: The similarities and differences.” Second opinion doctor. Accessed September 22, 2017. http://www.second-opinion-doc.com/rock-hyraxes-and-elephants-the-similarities-and-differences.html.
⁸Mirceta, S., A. V. Signore, J. M. Burns, A. R. Cossins, K. L. Campbell, and M. Berenbrink. “Evolution of Mammalian Diving Capacity Traced by Myoglobin Net Surface Charge.” Science 340, no. 6138 (2013): 1234192. doi:10.1126/science.1234192.
⁹Kershenbaum, Arik, Amiyaal Ilany, Leon Blaustein, and Eli Geffen. “Syntactic structure and geographical dialects in the songs of male rock hyraxes.” Proceedings of the Royal Society of London B: Biological Sciences 279, no. 1740 (2012): 2974–2981.
¹⁰Hakeem, Atiya Y., Patrick R. Hof, Chet C. Sherwood, Robert C. Switzer, L.e.l. Rasmussen, and John M. Allman. “Brain of the African elephant (Loxodonta africana): Neuroanatomy from magnetic resonance images.” The Anatomical Record Part A: Discoveries in Molecular, Cellular, and Evolutionary Biology 287A, no. 1 (2005): 1117–127. doi:10.1002/ar.a.20255.
¹¹Parts of the Brain — Memory & the Brain — The Human Memory. Accessed September 29, 2017. http://www.human-memory.net/brain_parts.html.
¹²Marsh, Helene, Thomas J. O’Shea, and John E. Reynolds III. Ecology and conservation of the Sirenia: dugongs and manatees. №18. Cambridge University Press, 2011.
¹³Marshall, C. D., L. A. Clark, and R. L. Reep. “The muscular hydrostat of the Florida manatee (Trichechus manatus latirostris): a functional morphological model of perioral bristle use.” Marine Mammal Science 14, no. 2 (1998): 290–303.
¹⁴Weil, Anne. “Mammalian evolution: Relationships to chew over.” Nature 409, no. 6816 (2001): 28–31. doi:10.1038/35051199.
¹⁵Koretsky, Irina A., and Lawrence G. Barnes. “Pinniped evolutionary history and paleobiogeography.” Mesozoic and Cenozoic vertebrates and paleoenvironments: tributes to the career of Prof. Dan Grigorescu. Bucharest: Ars Docendi (2006): 143–153.
¹⁶Garwood, Russell J., and Jason Dunlop. “Three-dimensional reconstruction and the phylogeny of extinct chelicerate orders.” PeerJ 2 (2014). doi:10.7717/peerj.641.
¹⁷ Springer, Mark S., Gregory C. Cleven, Ole Madsen, Wilfried de Jong W., and et al. “Endemic African Mammals Shake the Phylogenetic Tree.” Nature 388, no. 6637 (Jul 03, 1997): 61–4. doi:http://dx.doi.org/10.1038/40386. https://search.proquest.com/docview/204499119?accountid=10267.
