Tuskless elephants becoming more common as an evolutionary response to poaching

From 1977 to 1992, the Mozambican Civil War ravaged the park with opposing combatants poaching elephants for ivory and meat. The conflict saw a rapid decline in the elephant population, from over 2,500 to around 200 after the war.

Along with this steep decline, there was an increase in tuskless female elephants, from 18.5% to 50.9%. Male tuskless elephants have not been seen in the park, but there have been rare anecdotal sightings of them elsewhere.

A new study has found that poaching resulted in strong selection that favoured elephants that were tuskless at a time of rapid depopulation caused by poaching.

This study provides evidence for rapid, poaching-mediated selection for the loss of a prominent anatomical trait in a keystone species.

The selective killing of species that bear anatomical features such as tusks and horns is the basis of a multibillion-dollar illicit wildlife trade that poses an immediate threat to the survival of ecologically important megafauna worldwide Megaherbivores, like elephants, are especially vulnerable to overharvesting because of their large habitat requirements, small population sizes, and long generation times.

But there has been no direct genetic evidence indicating how this was happening, or why this trait was occurring exclusively in female elephants.

The team led by Princeton University researchers have now implicated two genes associated with tooth development in mammals to be at the center of the tuskless elephant phenomenon, according to a study published Oct. 21 in the journal Science. One of these genes is connected to the X chromosome and is lethal to males, while humans who have the same gene mutation exhibit similar teeth defects.

“Elephants are such an iconic species that is so important for the savanna ecosystem and now we have a better understanding of how human activity is impacting them,” said co-first author Shane Campbell-Staton, assistant professor of ecology and evolutionary biology at Princeton and associated faculty in Princeton’s High Meadows Environmental Institute (HMEI).

Campbell-Staton worked with co-first author Brian Arnold, Schmidt DataX biomedical data scientist at Princeton based in the computer science department and supported by the Center for Statistics and Machine Learning. Co-authors of the paper also include Robert Pringle, professor of ecology and evolutionary biology at Princeton and associated faculty in HMEI; Dominique Gonçalves, Elephant Ecology Project Manager at Gorongosa National Park; Petter Granli and Joyce Poole, co-founders and co-directors of ElephantVoices; and Ryan Long, associate professor of wildlife sciences at the University of Idaho.

A Response to a Population Bottleneck

Campbell-Staton, who conceived of the project while at UCLA, had a hunch that the tuskless phenotype was passed down in the X chromosome, but collecting survey data and blood samples would be key to unraveling this mystery.

To collect these samples, Campbell-Staton and other research team members focused on African bush elephants (Loxodonta africana) at Gorongosa National Park in Mozambique. African bush elephants are considered to be Earth’s largest terrestrial land animal with a height reaching 13 feet at the shoulder, distinguishing it from its smaller cousins, the African forest and Asian elephants. The animals’ tusks can measure up to six-feet-long and weigh 50 pounds each, with the tusks of some male elephants (called bulls) so large that they drag on the ground.

The rapid culling of tusked individuals changed the makeup of traits in the elephant population in only two decades, leaving behind more tuskless individuals, says Campbell-Staton of Princeton University and colleagues. The tuskless trait is heritable, and the evolutionary change in the population may stick around for several generations at least, even as poaching eases.

Poaching “changing the course of evolution” in Gorongosa’s elephants, Campbell-Staton says, can have reverberating effects through the ecosystem given elephants’ dramatic impact on their surroundings. 

“[Tusks are] not just ornamental. They serve a purpose,” he says, detailing how elephants use tusks to dig for water and strip tree bark for food. “If an elephant doesn’t have the tool to do those things, then what happens?”

Arnold, the data scientist, said they used simulated numerical models and statistical analysis to determine that a tuskless female elephant was five times more likely to survive during the war than a female with tusks, an example of a very strong natural selection caused by poachers.

“Tusks suddenly became a liability, even though in natural circumstances, tusks are very useful organs for elephants,” Arnold said. “There was intense hunting pressure on tusked females. Specifically targeting tusked females gave tuskless females a huge competitive advantage.”

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