Artificial selection in action: more elephants are being born without tusks

What do you expect if hunters or poachers selectively kill elephants with big tusks—either for trophies or their ivory? This is actually a form of artificial selection, and it will have the expected results: elephants with smaller tusks will be more likely to survive and reproduce, and if there’s genetic variation for tusk size or presence, which there almost certainly is (there’s genetic variation for nearly every trait, accounting for phenomena like the ability of humans to change the gray wolf into Chihuahuas, greyhounds, sheepdogs, and so on), the “tuskiness” of elephants will change over time. Tusks will get smaller, or even disappear.

You can also predict that if tusks are more important for one sex than the other, that the natural “counterselection” against tusk reduction will be stronger in that sex, so that the reduction in size or presence over time will be slower and, ultimately, might stabilize at a larger size than in the sex having tusks less important for survival.

This is precisely what an article by Robby Berman at The Big Think reports. Berman notes that in non-poached populations of African elephants (Loxondota spp.), 2-6% of female elephants are born without tusks. I’m actually surprised that the percentage is that high given that tusks are used by both sexes to deter predators, dig water holes, clear obstacles, and strip bark from trees. But in poached populations that percentage can nearly reach 100.

But tusks are more important in males since they’re intimately connected with reproduction: males joust for mates using them, and a tuskless male is a childless male. Thus one would expect that, given equal intensity of poaching, males would still wind up with larger tusks than females—unless poachers kill every animal with tusks, which, by eliminating males, would drive the species extinct.

According to Berman, selection is indeed working this way, and because it’s strong—a large percentage of elephants are killed for their ivory—we’d expect the change to be rapid. As he writes:

In areas where there is poaching, however, the story’s very different, and the quest for elephant ivory is changing the types of offspring now being produced. In Gorongosa National Park in Mozambique, half of the older females have tusks. The situation has improved since poaching was brought under control there 20 years ago, but a third of the younger elephants are tuskless nonetheless, a meaningful increase over the historical norm.

In Zambia’s South Luangwa National Park and the Lupande Game Management Area, tuskelessness increased [JAC: read the reference] from 10·5% in 1969 to 38·2% in 1989 The numbers have improved slightly since then there as well, but only due to more tusked females migrating from nearby areas.

How strong is the selection? The Independent reports some populations have almost no females with tusks:

An increasing number of African elephants are now born tuskless because poachers have consistently targetted animals with the best ivory over decades, fundamentally altering the gene pool.

In some areas 98 per cent of female elephants now have no tusks, researchers have said, compared to between two and six per cent born tuskless on average in the past.

Almost a third of Africa’s elephants have been illegally slaughtered by poachers in the past ten years to meet demand for ivory in Asia, where there is still a booming trade in the material, particularly in China. [JAC: this trade will shut down at the end of this year by government decree.]

. . . The most striking example is in the Addo Elephant National Park in South Africa, where 98 per cent of female elephants have no ivory. Big game hunters there had killed all but 11 elephants by the time the park was created in 1931. Four of the eight surviving females were tuskless.

In 2008, scientists found that even among elephants that remained tusked, the tusks were smaller than in elephants’ a century before – roughly half their previous size.

What will happen? Given the strength of selection on tusks (ivory goes for $730 per kg on China’s black market, a 2/3 reduction since the ivory trade started to be banned), both the number of elephants and the size of their tusks will decrease. They will remain larger in males since there’s an additional penalty—a strong one—for being tuskless in that sex. One might then expect females to select for mating with those males having smaller tusks, counteracting this trend, but since females may not have a preference with whom they mate (males win in competitions), that kind of selection might not occur.

This is all speculation, but what’s not speculative is that the selective poaching of elephants with tusks is having the expected (but unwanted) evolutionary effect.

Here’s a tuskless male, thus a luckless male:

A female with small tusks:

My first thought was to anesthetize elephants and remove their tusks to foil the poachers, but that can’t be done for several reasons, most important that the tusks are alive and contain nerves and blood vessels (they are in fact incisor teeth of the upper jaw), not to mention the difficulty of doing that to a lot of elephants.

Read more about this in the article “Going tuskless” at the African Wildlife Foundation.

h/t: Steve


  1. Posted June 1, 2017 at 9:59 am | Permalink

    Reblogged this on The Logical Place.

  2. Randy schenck
    Posted June 1, 2017 at 10:09 am | Permalink

    Very interest, although sad example of selection. What I would like to see is the elimination of all poachers.

  3. Gregory Kusnick
    Posted June 1, 2017 at 10:17 am | Permalink

    I would call this natural selection, not artificial selection, since there is surely no deliberate intent on the part of the poachers to produce a breed of tuskless elephants.

    If unintended consequences count as artificial selection, then so does every instance of predation.

    • Gregory Kusnick
      Posted June 1, 2017 at 10:45 am | Permalink

      And sub.

    • dabertini
      Posted June 1, 2017 at 10:57 am | Permalink

      That is the way I look at it too.

    • Posted June 1, 2017 at 12:28 pm | Permalink

      I was thinking the same thing. This isn’t like dog breeding. In this case humans are predators and part of a changing environment that’s driving elephant evolution through natural selection.

      We aren’t doing the selecting, nature is.

    • ladyatheist
      Posted June 1, 2017 at 12:52 pm | Permalink

      That was my first thought. Predation for any reason would be natural. Aren’t there other species that kill members of another species for reasons other than food?

    • Posted June 1, 2017 at 1:41 pm | Permalink

      Yes but it works precisely as artificial selection would – a trait humans like is selected. ISTM, intent does not matter.

      For example, one of the reasons Beagles are sometimes used in cancer research is that they spontaneously develop a kind of leukemia similar to what humans get. The folks who bred Beagles did not breed for the leukemia trait, but canine leukemia in Beagles still an example of artificial selection.

      • Gregory Kusnick
        Posted June 1, 2017 at 5:06 pm | Permalink

        “a trait humans like is selected”

        But in this case, a trait humans like (ivory) is inadvertently being selected against.

        Intent matters because the key difference between natural and artificial selection is teleology: artificial selection is forward-looking, and steers toward a preconceived goal.

    • W.Benson
      Posted June 1, 2017 at 4:10 pm | Permalink

      I agree. It is anthropic selection and natural selection, selection caused by man but not intentional. When tuskless elephants are purposely bred to make them unattractive to poachers, we have artificial selection.

    • Posted June 1, 2017 at 8:41 pm | Permalink

      But intent is an illusion. Since humans do not have free will, ALL selection is natural selection. The idea that human actions of any form are somehow different than “natural” actions is just an anthropocentric conceit.

  4. Posted June 1, 2017 at 10:34 am | Permalink

    I had the pleasure of growing up in Zambia and seeing Elephants in the South Luangwa NP during the 70s and 80s. Some with glorious tusks. There is no better way to see ivory.

    This story of smaller tusks and the selection that drives it is sad and disappointing. I believe the same has also been reported in deer populations where hunters typically go for those with the more impressive antlers.

  5. Adam M.
    Posted June 1, 2017 at 10:48 am | Permalink

    Call me evil, but I wish there was an organized effort to poach the poachers.

    • rickflick
      Posted June 1, 2017 at 11:18 am | Permalink

      My sympathies lean in your direction I’m embarrassed to say.

      • Posted June 1, 2017 at 12:05 pm | Permalink

        Unfortunately, I suspect that a lot of them might be just trying to get by. I don’t actually know for sure, but in a lot of these things, the networks and such exploit the desperate and the poor.

    • Gregory Kusnick
      Posted June 1, 2017 at 11:53 am | Permalink

      The lesson of the War on Drugs is that putting selection pressure on the traffickers just makes them more organized, ruthless, and violent. It doesn’t stop the flow of contraband.

    • Kevin
      Posted June 1, 2017 at 12:32 pm | Permalink

      I have never had any ill will towards the poachers. It’s the demand. It’s the f**k shits who think the tusks give them more sexual capacity or fewer headaches or immunity to cancer.

      Here’s my evilness: for every rhino, shark, elephant, or whale slaughtered, a homeopathic imbecile deserves euthanization.

    • Posted June 1, 2017 at 12:34 pm | Permalink

      I have a plan in case someone invites me to go hunting: I’ll say “Do I get to choose which side I’m on? I want to fight for the animals.”

      Nobody ever invites me though; I don’t hang out with that type of people.

    • ladyatheist
      Posted June 1, 2017 at 12:55 pm | Permalink

      Ivory sales should be banned everywhere, and the people who buy it should be put to work in zoos cleaning elephant dung.

      Also, so-called homeopathic medicine’s ridiculous claim that you need something that resembles whatever hurts to cure it should be quashed completely. No, you can’t get a bigger ****** by ingesting ground up rhinocerous horn!

    • Posted June 7, 2017 at 1:45 pm | Permalink

      Very thoughtful article (actually an answer to a question on Quora) about combatting poaching, and specifically “shoot to kill” policies here from someone who seems to have spent plenty of time on the front line:

  6. rickflick
    Posted June 1, 2017 at 11:17 am | Permalink

    I wonder exactly how come there is a difference between male and female tusklessness. Given males are killed more frequently, if the presence of tusks was determined by one gene (probably not very likely), and the gene was not on the sex determining chromosome, the loss of tusks would be shared by both males and females, you’d think. If some genes for tusk production are on the male sex chromosome I could see how a difference would occur. I’m probably being too simplistic.

  7. Posted June 1, 2017 at 11:20 am | Permalink

    I can see why 2-6% of female elephants are tuskless in populations that were not poached. If the genes for this trait exist, they can increase in frequency by genetic drift, working against natural selection. Drift will have some significance given that the populations are not large, and selection against tuskless females may not be very strong since these social animals depend on each other.

  8. Michael Hart
    Posted June 1, 2017 at 12:07 pm | Permalink

    That pattern is very interesting but the explanation seems hard to believe. High-intensity poaching has been going on for at most about 10 elephant generations (generation time is about 25 years), so not much time for selection to have acted. There seems to be sexual selection and natural selection favouring large tusks (or high growth rates of those incisors), and that selection should have eliminated a lot of the genetic variation underlying tusk size (this is a common pattern, with traits closely related to fitness having relatively low heritability), so when poaching began in earnest the heritability of the existing variation in tusk size should have been relatively low and the potential for poaching to cause evolution should also have been low. As pointed out by Mark, elephant populations are not very large (at least by mammal standards) so the effectiveness of additional selection by poachers would be relatively weak. That could all be overcome by a very high selection coefficient against large tusks (or high growth rates of tusks), but clicking through the links in those stories doesn’t lead to any estimates of those selection coefficients. So although I loathe the poaching and love the interpretation of its effects, I’m still sort of skeptical that selection by poachers is the major cause of the pattern. I admit I don’t know what else might cause the pattern (I’m not a large-mammal ecologist). I would love for Jerry to follow up on this some time if he comes across other analyses or data. Thanks for the fun interesting post.

    • Posted June 1, 2017 at 2:35 pm | Permalink

      Well, similar observations have been made on Big Horn sheep. Intensive hunting seems have decreased both the overall size of the animals as well as the size of their horns (from the paper);

      Unrestricted harvesting of trophy rams has thus contributed to a decline in the very traits that determine trophy quality. Hunters have selectively targeted rams of high genetic quality before their reproductive peak, depleting the genes that confer rapid early body and horn growth.

      Undesirable evolutionary consequences of trophy hunting Nature; London 426.6967 (Dec 11, 2003): 655-8.

      But you do bring up a good point that this is sometimes hard to demonstrate. I should think sometimes it would be difficult to tease apart. Indeed, one paper, which looked at size-selection in fishing (among others), says in the abstract; “…determining whether the observed changes represent evolutionary response or phenotypic plasticity remains a challenge.”

      Ecological and evolutionary consequences of size-selective harvesting: how much do we know? Molecular ecology 17.1 (2008): 209-220.

      I do like these science posts at WEIT. Because when I have the time I can read up on subject I knew little about. WIN!

      • Michael Hart
        Posted June 1, 2017 at 7:12 pm | Permalink

        Yes, that’s true about similar observations in bighorn sheep. In that case generation time is much shorter, and population size is larger, so the response to selection should be faster. I don’t know if there has been a calculation of the selection coefficient against large horns (based on the hunting results) that could be compared to the response to selection (based on observing the change over time in horn size). At least there should be better records of horn size in sheep killed by hunters, compared to the data on tusk size in elephants killed by poachers (because the poaching is cryptic, whereas the sheep hunting is regulated and legit for the most part). I agree it’s a kind of corroboration to see the same kind of pattern in a different organisms under the same kind of selection, but still harder to ascribe the change over time to the specific selection pressure (hunters or poachers). It for sure seems worth following up.

      • Rob Munguia
        Posted June 1, 2017 at 10:04 pm | Permalink

        Nice post PCC and thanks Mike for the references. The sexual dimorphism in the tusks is subject to different selection pressures in females (hunting) and males (hunting vs sexual selection) so there could be different optimal thresholds of expression caused by gene modifiers.

  9. Posted June 1, 2017 at 1:45 pm | Permalink

    This arrived in time today to include it as a question on a final examination in genetics. Thanks JAC.

  10. Don Mackay
    Posted June 1, 2017 at 2:50 pm | Permalink

    Elephant tusks are used by males to uproot trees in the bushveld . The fallen trees provide cover(eg nesting) and food for other creatures, and opening the canopy lets light through to grasses.As Garret Hardin quipped, you cannot do just one thing (in ecology).

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