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Spotting evolution can be trickier than you might think. Take African elephants. Usually they boast massively overgrown (and ever-growing) teeth—their tusks. For male elephants, these are weapons in sexual competition, but all elephants also use their tusks to scrape bark off trees, uncover roots, and dig for water during dry spells. Humans have their own use for tusks: ivory.
As a result, elephants have been subjected to centuries of hunting, and in recent years, aggressive poaching. The International Union for Conservation of Nature, which tracks the health of species across the world, estimates that elephant populations have shrunk by about 100,000 elephants across Africa since 2007, after an uptick in poaching starting around the same time. Since 2010, more than half of the dead elephants surveyed by an international monitoring program had been killed illegally.
Scientists have noticed that some elephant populations with a history of poaching show higher than expected rates of small or missing tusks, even in calves born after poaching ends. Does that mean elephants are evolving smaller or missing tusks in response to poaching? “I don’t think it’s a great stretch of the imagination to think that might be true,” says David Coltman, an evolutionary geneticist at the University of Alberta. He studies how trophy hunting affects bighorn sheep populations. “What’s difficult is identifying compelling evidence.”
The problem with sticking the label “evolution” on something like a rise in tusklessness is that terms matter, and using the wrong term can cloud our understanding of the processes at play. “I think science thrives on distinctions and being aware of differences and mechanisms,” says Nicholas Georgiadis, a researcher at the University of Washington who has studied population genetics in elephants. Evolution is grounded in genetics, not just appearance.
To sort it out, there are two types of evidence biologists can turn to: observing elephants’ appearance (their phenotype) and studying their DNA (their genotype). Picking apart genetic and environmental influence on a changing phenotype is challenging and requires carefully designed studies. “It’s probably impossible to separate it entirely,” says Coltman.
The phenotype surveys scientists have been able to put together show almost all male elephants have tusks. But rates of female tusklessness can vary dramatically between populations. Some studies conducted in the wake of poaching find higher than average rates of tusklessness lingering even in females born years after the killing stops.
Joyce Poole, co-founder of Elephant Voices, a nonprofit group, has studied elephants in Africa for nearly 40 years. She explains that tusklessness is a natural trait in elephant populations. Because poaching takes out elephants with tusks, more tuskless females survive. So a population “ends up with a higher proportion of tuskless animals who then reproduce and tend to produce tuskless offspring,” she says. “In this day and age, with all the poaching going on, tuskless elephants are at an advantage because they are not being targeted for their tusks.”
That seems like evolution, an adaptation to poaching. But Atle Mysterud, an ecologist at the University of Oslo, who studies how hunting shapes populations of antlered deer, is wary of drawing that conclusion too quickly. “Nobody doubts that if you have a very strong selective pressure, over time you will see evolution,” says Mysterud. “It’s a timescale issue.”
Calling something the result of “evolution” in biology requires checking two boxes. You need a change in the frequency of alleles, the different forms that any given section of DNA can come in; and you need selection pressure, some condition that makes it easier for some individuals, and harder for others, to successfully reproduce.
“Scientists also have trends,” Mysterud says. “It’s a bandwagon now to show harvesting effects”—like changes in fish size due to a preference for catching the largest—“that are very rapid,” says Georgiadis, “It’s possible to have huge swings in the frequency of a trait like tusklessness without any evolution.” Scientists are pretty sure that’s the case for the overwhelming amount of tuskless female elephants in Addo National Park, in South Africa. But in other small populations, it’s harder to tell.
The best way to determine whether a change in trait frequency in a group is evolution is to look at every genome in the population, says Eleftheria Palkopoulou, a geneticist at Harvard University. But that’s not easy. “We barely have elephant genomes available, so it’s a long shot,” she says. Ignoring the rest of the genome and looking at just tusk genes might make that easier—except that scientists haven’t pinned down which genes shape tusks.
Even if severe poaching continued for many generations, it’s still not necessarily the case that tusks would disappear—they may well be too important to lose. “It’s not like tusks are an ornament,” says Palkopoulou, “they actually have a function that’s very important.”
Losing a tusk underscores its importance to an elephant’s health and survival, explains Victor Muposhi, an ecologist at Chinhoyi University of Technology, in Zimbabwe. He points out that tuskless males tend to be more aggressive and may be psychologically stressed. He also expects elephants without tusks to be relatively malnourished, more susceptible to disease, and less likely to breed. Losing tusks, in other words, may not necessarily be an advantage. So unless poaching pressure eases, elephants, for now, appear stuck between a rock and a hard place.
Meghan Bartels is an editorial intern at Nautilus.
Watch: In this installment of our Spark of Science channel, Joyce Poole, the co-founder of the non-profit conservation group ElephantVoices, explains how she fell in love with elephants.
The lead image is courtesy of ENOUGH Project via Flickr.
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