ASIAN JUM­BOS LOS­ING TUSKS

The phe­nom­e­non has made re­searchers ques­tion the role of tusks in sex­ual se­lec­tion

Down to Earth - - CONTENTS - KU­MAR SAMB­HAV SHRI­VAS­TAVA | KAZI­RANGA

It is a bright af­ter­noon in the Kazi­ranga Na­tional Park in As­sam. A care­free, tusk-less male ele­phant ( makhna) is ram­bling in the shal­low wa­ters of the So­hola Beel, a sea­sonal lake in the park formed by the flood wa­ter of the Brahma­pu­tra River. A few kilo­me­tres away a herd of about 25 ele­phants—their bod­ies fully soaked in mud— slowly walks to­wards the beel. A tusker, about a foot taller than the makhna and with a pair of nicely elon­gated tusks,fol­lows the herd.It in­ter­acts with a few fe­males when other mem­bers of the herd scat­ter to for­age in the grass­land.

As the herd nears the beel, the makhna be­comes at­ten­tive. It looks at the fe­males en­ter­ing the wa­ter. A few min­utes later, it moves to­wards the herd but stops abruptly. Some fe­males from the herd look at the makhna; one of them even cov­ers a small dis­tance to­wards him. The makhna doesn’t move. The tusker was stand­ing in the mid­dle of the herd and was star­ing at the makhna. Af­ter stand­ing in the same po­si­tion for some time, the makhna turns back and walks away.

There is a good pos­si­bil­ity that the fe­male who walked to­wards the makhna was in oe­strous and the makhna was look­ing for a chance to mate with her.But at the same time, the swollen tem­po­ral gland be­tween the eye and the ear of the tusker and the small streak of fluid se­cre­tion on his cheeks from this gland in­di­cated that the tusker had just started his musth phase. Musth is a tem­po­rary but in­tense sex­ual state aris­ing due to el­e­vated testos­terone lev­els and the tusker was per­haps guard­ing the fe­male from ri­val males. The makhna could not gather the courage to chal­lenge the tusker and gave up with­out a fight.

This may sound re­as­sur­ing for the fol­low­ers of Dar­win’s the­ory of Sex­ual Se­lec­tion: the ele­phant tusk has evolved in males as a weapon to win over fe­males or as an or­na­ment to at­tract them.The tusker win­ning over the makhna in the male-male com­pe­ti­tion thus only seems a fore­gone con­clu­sion. Un­for­tu­nately, it is not.

The tusker at the So­hola Beel did not win be­cause of the tusk.The makhna was very likely in­tim­i­dated by the fact that the tusker was in the ini­tial phase of musth and that it had a larger body size. Ac­cord­ing to a study by Karpagam Chel­liah (Ph D stu­dent) and Ra­man Sukumar from Cen­tre for Eco­log­i­cal Sci­ences, In­dian In­sti­tute of Sci­ence, Ben­galuru, the musth state

and the size of bod­ies dic­tate mat­ing ad­van­tage in ele­phants. Tusk plays a rel­a­tively mi­nor role. When in state of musth, males go look­ing for oe­strous fe­males to mate with.They sig­nal their musth sta­tus by se­cret­ing a smelly fluid from a tem­po­ral gland and through a pun­gent urine drib­ble. A musth male ele­phant de­feat­ing a larger non-musth male has been ob­served in African ele­phants by sci­en­tists Joyce Poole and Cyn­thia Moss way back in the 1980s. How­ever, the im­por­tance of tusk in the male-male com­pe­ti­tion was not ex­plored.

Chel­liah and Sukumar con­ducted sys­tem­atic, be­havioural ob­ser­va­tions of male-male com­pe­ti­tion in the wild ele­phant pop­u­la­tion of Kazi­ranga, that is pos­si­bly unique be­cause of the al­most equal num­ber of tuskers and makhnas. They found that a musth male would win over a non-musth male in most cases, ir­re­spec­tive of the tusk sta­tus. And if both the males are in same musth sta­tus, the male with the larger body size would dom­i­nate, again ir­re­spec­tive of the tusk sta­tus. “Since most duels were asym­met­ri­cal, one male was in musth the other not or one male be­ing much larger in body size than the other, tusk came as an ad­van­tage only when two musth males of sim­i­lar body size fought. Such sym­met­ri­cal com­bats, how­ever, were very rare; only 12 per cent,” says Chel­liah.

The tusker that won the con­test at the So­hola Beel was larger than the makhna and in musth. So one can­not con­clude that he won only be­cause of tusks. “Had the makhna been in musth as well and of a larger body size, the tusker may not have won,” says Chel­liah.

Such re­sults were ini­tially puz­zling for her. “I saw a tusker run­ning away from a musth makhana of smaller size,” and won- dered,“how can a makhana who is just se­cret­ing a smelly fluid in­jure a tusker?” she laughs. But then we ob­served makhnas drap­ing their trunks over an op­po­nent’s tusk and press­ing down with their body weight. “Be­sides, the ten­sile strength of Asian ele­phants’ tusk is lower than that of many bam­boo species. Ele­phants can ef­fort­lessly snap bam­boo culms with their trunk and body weight, and it is pos­si­ble that they can do the same to an op­po­nent’s tusks, es­pe­cially if they are long,” says Chel­liah. “The tusk does not seem to have the additional ad­van­tage of be­ing an or­na­ment to at­tract fe­males ei­ther,” she adds.The study was pub­lished in the jour­nal An­i­mal Be­hav­iour in Septem­ber 2013.

The dis­ap­pear­ing tusk

Es­ti­mates by sci­en­tists and for­est de­part­ments sug­gest 60 per cent male ele­phants in north-east In­dia are tusk-less. This is in con­trast to the ele­phant pop­u­la­tions in south In­dia where less than 10 per cent male ele­phants are tusk-less.

Sci­en­tists at­tribute high num­bers of makhnas in the North­east to de­lib­er­ate re­moval of the tusk gene from the wild pop­u­la­tions. “Tuskers have been se­lec­tively re­moved from the wild in the North­east for about 1,000 years. There are his­tor­i­cal records to show that tuskers from As­sam and neigh­bour­ing states were sup­plied to be de­ployed in ar­mies,” says Sukumar. “Poach­ing of tuskers for ivory has also been preva­lent in these states for sev­eral hun­dred years.The re­moval of the tusk gene has been over suf­fi­cient num­ber of gen­er­a­tions to bring a ge­netic change in the ele­phant pop­u­la­tion to­wards tusk­less­ness,” he adds.

“There has been ram­pant poach­ing of tuskers for ivory in south In­dia, but only be­tween 1970 and 2000.That would have af­fected only one gen­er­a­tion and would not have caused ge­netic changes in the pop­u­la­tion,” adds Chel­liah. In the North­east the ra­tio of makhnas over tusker is only in­creas­ing. A 1995 study by sci­en­tists of Chris­tian Al­brechts Univer­sity, Kiel, Ger­many noted that in the 19th century, makhnas were rare in north-east In­dia. How­ever, by 1950 makhnas were 48.7 per cent of the male ele­phant pop­u­la­tion and about 60 per cent by 1970s. Ac­cord­ing to the pop­u­la­tion es­ti­mates of the As­sam for­est depart­ment com­piled by Bhu­pen­dra Nath Taluk­dar in his book Ele­phants in As­sam, the per­cent­age of adult and sub-adult makhnas in As­sam in--

creased from 62 per cent to 76 per cent be­tween 1993 and 2008.

Does this mean Asian ele­phants are headed to­wards a tusk-less fu­ture? Per­haps, says Sukumar. “With only slight ad­van­tage of tusk in sex­ual se­lec­tion, there is lit­tle pos­si­bil­ity of re­vival of the tusk gene.Any ex­ter­nal fac­tor, such as poach­ing, that comes as a dis­ad­van­tage for tusk will lead to ‘tusk-less­ness’,” he ex­plains. “Even­tu­ally, the trend of makhna crowd­ing out the tusker will seep into other pop­u­la­tions as well. To me, the tusk seems to be on the way out in Asian ele­phant pop­u­la­tions,” he adds.

In Sri Lanka about 98 per cent male ele­phant pop­u­la­tion is tusk-less. While the rea-

"The tusk seems to be on its way out in Asian ele­phants. The sil­ver lin­ing is that the ele­phant will be­come im­mune to poach­ing for ivory" Ð Ra­man Sukumar Cen­tre for Eco­log­i­cal Sci­ences, IISc, Ben­galuru

sons for this are not yet sci­en­tif­i­cally de­ter­mined, it could per­haps be the first pop­u­la­tion to lose the tusk en­tirely.

A mys­tery un­re­solved

Evo­lu­tion of the tusk, in fact, is one of the most in­trigu­ing ,un­re­solved mys­ter­ies of the evo­lu­tion­ary bi­ol­ogy. Fos­sils of ele­phant species that have gone ex­tinct sug­gest that all of them had very long tusks. Of the three ele­phants species that sur­vive in the world, tusk­less­ness is a char­ac­ter­is­tic of only Asian ele­phants.In mildly poached pop­u­la­tions of the African Sa­van­nah and African For­est ele­phant species,99.9 per cent of the males and 98 per cent of the fe­males have tusks.

“Six years ago when I asked Pro­fes­sor Sukumar why some Asian ele­phants do not have tusks, he asked me to find out why ele­phants have tusks in the first place,” says Chel­liah. Since evo­lu­tion of the tusk was at­trib­uted to sex­ual se­lec­tion I de­cided to ver­ify this, she adds.

When Dar­win could not ex­plain the evo­lu­tion of some of the ex­ag­ger­ated and seem­ingly use­less or­na­men­tal male traits such as the pea­cock feath­ers or the antlers of the Ir­ish elk through his the­ory of nat­u­ral se­lec­tion, he pro­posed the mech­a­nism of sex­ual se­lec­tion. He sug­gested that the males com­pete with each other to mate with fe­males and the male trait may func­tion as a weapon or the fe­males may ac­tively choose the most splen­didly or­na­mented male for mat­ing.So does Dar­win’s the­ory of sex­ual se­lec­tion ap­ply to the evo­lu­tion of the ele­phant tusk? “It does not hold true for the present pop­u­la­tion of ele­phants.And there could be two pos­si­bil­i­ties in the past: the tusk ei­ther did not evolve through sex­ual se­lec­tion or if it did, it evolved be­fore the evo­lu­tion of the musth. More ro­bust stud­ies are needed to ver­ify this,” she says.

While Chel­liah and Sukumar are still work­ing to find an­swers to why the ele­phant tusk evolved and why the fe­males in Asian ele­phants do not have tusks, their study on tusk’s role in sex­ual se­lec­tion has made it clear that the tusk is on its way out in the Asian ele­phant pop­u­la­tions. How­ever, there is a sil­ver lin­ing in this. “The ele­phant as a species will sur­vive. With this evo­lu­tion, the ele­phant will prac­ti­cally make it­self im­mune to poach­ing for ivory,” says Sukumar. As Pa­pul Rabha, a for­est guard in Kazi­ranga, puts it, “I have al­ways seen makhnas walk­ing with their head held high un­like the tuskers. It is per­haps be­cause they have noth­ing to lose.”

KARPAGAM CHEL­LIAH Even tuskers do not use tusks for fight­ing. They mostly head-butt or wres­tle with their trunks

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