Monday, November 4, 2013

A Brief History of the Trunk

Without a doubt, the most versatile and (in our opinion) mind-blowingly awesome appendage in the animal kingdom belongs to our humble elephants: the trunk.  It is used for everything from eating to vocalizing, tool use to heavy lifting.  We know that elephants communicate not only vocally with their trunks, but chemically and tactilely as well.  It is a highly specialized structure that marks the latest strange in the complex evolutionary history of the order Proboscidea.

Anatomically, the trunk is an amazingly complex organ.  It is comprised of over 40,000 individual muscles that run laterally, dorsally, and ventrally.  Within these muscle units are over 150,000 muscle fascicles, or individual fiber bundles.   To give you some perspective, there are approximately 643 muscles in an entire human body.  Elephants also possess a unique facial nerve, called the proboscideal nerve, which runs down the trunk and provides it with hypersensitivity for touch.  With no bones in the entire trunk, elephants possess extreme flexibility and fine dexterity, as well as tremendous strength.  An elephant can lift up to 350 kg with its trunk alone, and can also hold up to two gallons in its trunk at any time.  Thus the muscles of a trunk can not only be used to lift or stretch, but can also expand or contract the nasal passages within to suck in or spray out air and water. 
Photo by Rebecca Shoer

Only three extant (still living) species of the ancient Proboscidea order remain today, these being the two Loxodonta elephant species and our own Elephas maximus.  Although some species, like anteaters, possess similar-looking facial features, they are not members of this elite taxonomic order.  The extended nose of the anteater may look like a trunk, but it is formed by an extension of the entire skull structure: a true trunk is solely comprised of muscles.  The evolutionary history of this ancient order is a complex and puzzling one, and is constantly being questioned and revised by evolutionary biologists. 

As of 2005, scientists believe that, at one time, at least 177 species of Proboscidea existed.  All proboscids share a number of similar traits beyond the trunk, including oversized second incisors (tusks, in some species) and a uniquely rotated wrist bone.  Originating in Africa between 38 and 58 million years ago, the proboscid species adapted in three major events called "radiations."  A radiation occurs when a species moves into a novel environment and undergoes many rapid adaptations, resulting in many species quickly evolving from a single ancestor.  Rather than evolving at a steady linear pace, it seems that proboscids evolved in relatively rapid pulses.  Eventually, proboscids would be found on every continent except Australia and Antarctica.  These earliest proboscids most likely had a mobile upper lip, but did not possess the distinct trunk structure that results from the fusion of the upper lip and nose.  Even today, early fetal elephants have a separated upper lip and nose that eventually fuses in utero.

Elephant fetus from Shoshani, J. Understanding proboscidean evolution: a formidable task.

As time went on, the proboscids developed the familiar trunk structure that we know and love.  But as the trunk is entirely muscle, how can paleontologists know that these ancient animals had them?  There are a number of skeletal features that only proboscids possess, and which strongly suggest that these ancient creatures indeed had trunks.  First, proboscids possess enlarged and elevated bony openings (called "nares") at their nose, which all other mammals lack.  It is possible that a side-effect of these enlarged openings allows elephants to produce the deep infrasonic sounds to communicate across large distances.  Second, proboscids have a large intraorbital canal (an opening in the skull near the eye socket), which allows large blood vessels and nerves access to the trunk.  If skull fossils possess these two traits, scientists can conclude that the ancient animal had a trunk.

Elephant skull, from California Academy of Sciences

The evolution of the trunk may itself have driven the increased brain size of the proboscids and, eventually, of elephants.  Being able to control such a complex organ with exact precision requires massive brainpower, and elephants possess correspondingly large brain regions responsible for motor control.  Perhaps as a consequence of such increased brain mass, other areas of the brain increased as well.  The trunk is just one possible factor driving the development of large brains in today's elephants.

From over 177 species of diverse and highly specialized proboscids, only three species exist today.  Interestingly, despite an overall morphological similarity, Asian and African elephants are not as closely related as one might think.  Indeed, Asian elephants are most likely more closely related to their extinct cousins, the mammoth Mammuthus family, than to  African elephants (of course, this is still hotly contested among Proboscidea evolutionary biologists).  How could a group possessing such a versatile and useful appendage undergo such rapid rates of extinction?  We know that even the earliest humans existed alongside more probodscid species than exist today.  What happened?  There are a number of theories that biologists have posited, and most likely such rapid extinction was due to a combination of these factors.  First, proboscids have relatively long generations, and thus are unable to adapt to changing environments as rapidly as more short-lived species.  Second, the world's climate changed rapidly before and after the last Ice Age, which resulted in a massive extinction in many animal groups.  Third, the heightened specialization of this trunk may have in fact doomed some proboscid species: when an animal becomes too specialized for a single environment, they are unable to adapt to any changes that may occur.  Unfortunately, we are all too aware of the factors driving the current extinction of the remaining proboscids: poaching and habitat loss, all caused by the rapidly expanding Homo sapiens.  The fate of the last members of an entire taxonomic order rests in our hands.

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