|Dispersal of the camelids. Credit: Jerry Mann, Wikimedia|
Mammals store their fat in a variety of bodily locations, with the most common pattern being a big deposit near the belly combined with small deposits at several locations. But there is a lot of variation, both within species-- for example, people struggling with weight gain can develop “pear-shaped” or “apple-shaped” profiles, and Zebu cattle native to India, but not Zebu from Africa, can acquire a hump of fatty tissue behind their shoulders—and when comparing different species. Some mammal species store their fat in specialized deposits. Many marine mammals accumulate blubber under their skin, which improves their insulation. Dolphins further accumulate a “melon” of fatty tissue near their blowholes, which amplifies their echolocation clicks.
But probably the most familiar specialized fat deposit is the fatty hump on the back of the camel.
Present day members of the camelid family include the Bactrian and Dromedary in the Eastern Hemisphere (the Camelini) and New World camelids such as the llama and the alpaca (the Lamini). Camelids arose during the Pliestocine in North America, with later dispersals to Eurasia and to South America. Recent fossil evidence suggests that some relatives of the present-day humped camels lived in boreal forests above the Arctic Circle . Expeditions in the far north of Canada have identified 3.5 million year old fossil leg bones from a Paracamelus, a genus of camelid which is thought to include the ancestors of modern humped camels. The Paracemelus fossils were found in a context suggesting a lush boreal forest thick with larch trees.
Paracamelus in the high arctic.
Credit: Guardian/Julius Csotonyi
The two modern-day Camelini species both live in desert ecosystems, with Bactrians dominant in the high Tibetan plateau, and Dromedaries most common in desert North Africa; and it has been speculated that the camel hump would be an adaptation to these extreme environments. The recently found fossils suggest that the camel hump may instead serve as an safeguard against food shortages, which would also afflict arboreal ecosystems, as opposed to a specific feature of the desert environment.