Monday, February 17, 2014

What's the Fuss About Tusks?

by Rebecca Shoer

Tusks are one of the most dignified and awe-inspiring natural ornaments in the animal kingdom.  No matter how cute or cuddly an elephant can look when goofing around, there is nothing as entrancing as an individual calmly sporting a full grown pair of bone-white tusks.  (An individual with tusks is called a "tusker" by the elephant community).  Of course, in Asian elephants, only the males bear tusks; in African elephants, both males and females have tusks.  Male elephants use their tusks for a wide range of activities, from combat and defense against other males to scraping nutritious bark off of trees.  Females are able to use their small tushes, which are essentially miniature tusks, to perform some of these tasks as well.  Since the females get along fine without toting around such unwieldy , albeit impressive, burdens, why bother evolving tusks at all?  Most likely, males developed longer and longer tusks over many generations due to a process called sexual selection.  This means that, for some reason or another, female Asian elephants prefer to mate with males that have long tusks.  Possessing long tusks may signal how healthy an elephant is or how good his genes are, as they take a lot of extra energy to grow and maintain.  Tusks are oversized, living teeth, and grow at a rate of about 15 centimeters per year.  If a male can survive in the wild while also carrying an impressive set of tusks, he is most likely very healthy as well as socially dominant.

Picture by Lisa Barrett

This sort of sexual selection is happening constantly in the natural world, as mating preference subtly changed over millennia.  However, outside forces can affect the evolution of species.  Sudden environmental changes can make a once-advantageous physical trait burdensome, or a flashy physical trait that indicates health may become attractive to predators.  For elephants, such an outside force has appeared: the ivory trade.
Ivory has long been prized for its decorative and aesthetic value.  It is easily carved, and has been used for statues, jewelry, and decoration for centuries.  However, the demand for ivory has recently been increasing at a dramatic and unsustainable level, and modern technology has made it far too easy to kill and extract ivory from elephants.  Automatic weapons and abject poverty make for a dangerous combination, and anti-poaching efforts have made barely a dent in the poaching rate.  If nothing is done to reduce the demand for ivory trinkets, elephants will swiftly become extinct in the wild.

Picture by Rebecca Shoer

However, in addition to human-led efforts to combat the poaching trade, evolution has begun playing a role in this battle.  The process through which animals with advantageous adaptations survive is called natural selection.  Those individuals that survive better and for longer are most likely to pass their genetic material down to their offspring.  The poaching trade, however, is an example of artificial selection.  It is not the elephants' natural environment, but the unnatural ivory trade, that is deciding which elephants pass down their genetic material to the next generations.  Put simply, if an elephant with spectacular tusks is killed before he has a chance to mate and have offspring, the genes for his tusks are lost.  Even if elephant females would prefer to mate with such a male, he is no longer available, and females must mate with the males that are still present.

Thus, humans have created an artificial advantage for males to not possess tusks.  As the number of tuskers decreases, poachers are "forced" to kill elephants that have relatively small tusks.  Thus, those males that simply never develop tusks, and do not possess the genes necessary to grow tusks, are suddenly at a mating advantage.  The number of their tusk-ed brethren is swiftly disappearing, and tuskless males are being provided with mating opportunities they would never get in a natural, non-poaching environment.
Just how prevalent are these tuskless males?  Professor Zhang Li of Beijing Normal University reports that, of the few hundred elephants left in China, the incidence of tuskless males has increased from 2 - 5 percent of the population to 5 - 10 percent.  A study done by Oxford University also reports that the size of tusks has reduced significantly since the advent of the modern ivory trade in the 19th century.  Although these numbers may not appear particularly large, such a percent change (perhaps from 5 to 10%) is unnaturally rapid when compared to an evolutionary timescale.  Elephants are adapting to the new environmental threats that humans have created.

Photo via Centre for Ecological Sciences, Indian Indian Institute of Science

However, evolution is a notoriously slow process, and when combined with the long generations of elephants, such an adaptation is most likely too little, too late to save elephants from the threat of poaching.  Elephants can live up to their mid sixties in the wild, and at the current rate of population decrease, elephants could be extinct in a single generation.  Though the rise of tuskless males is an interesting example of a species adapting to a new threat, it will not be enough to save wild elephants alone.  It is human behavior that must change, if this species is to exist for generations to come. 

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