Why So Many Elephants Are Born Without Tusks: Natural Selection in Real Time

In Gorongosa National Park in Mozambique, about half of the older female elephants have no tusks. In most elephant populations that's rare. Here's how a civil war and the ivory trade changed an elephant population in just a few decades.

Course: Biology | Chapters: Evolution

Tusks are upper incisors that never stop growing, made mostly of dentin, the material sold as ivory. During Mozambique's civil war (1977 to 1992), elephants in Gorongosa were hunted for meat and for ivory to pay for the fighting, and the population fell from more than 2,500 to about 200. A 2021 study in Science estimated that a tuskless female was more than five times as likely to survive as a female with tusks. Before the war about 18.5% of females were tuskless; among the survivors it was about 51%, and about 33% of daughters born after the war were tuskless. That's natural selection: variation (some females were already born tuskless), a selection pressure (hunting for ivory), and inheritance (the trait is passed on). The war didn't create the trait; it changed which elephants survived to reproduce. The trait is passed on the X chromosome and appears to be lethal to males before birth, which fits the finding that tuskless mothers had about two daughters for every son (about 66% female calves). The researchers found two candidate genes, AMELX and MEP1A, both involved in tooth development. Since the war ended, the park has been protected and the elephant population is growing again.

Question to think about: suppose hunting for ivory stops completely and tusks become useful again. What would you predict about the share of tuskless females over many generations? Explain it with the words variation, selection, and inheritance.

Chapters
0:00 Elephants without tusks
0:35 What tusks are
1:08 War and ivory
1:36 Who survived
2:10 How natural selection works
2:50 Passing it on
3:22 Why only females?
4:06 Finding the genes
4:47 What happens next
5:13 Recap + quick check
6:12 Think about it: if hunting stops

Sources
Campbell-Staton, S. C. et al. (2021). Ivory poaching and the rapid evolution of tusklessness in African elephants. Science 374: 483-487. https://doi.org/10.1126/science.abe7389
Princeton University news (2021). Ivory poaching drove the rapid evolution of tusklessness in African elephants. https://www.princeton.edu/news/2021/10/21/ivory-poaching-drove-rapid-evolution-tusklessness-african-elephants
HHMI BioInteractive, The Tuskless Elephants of Gorongosa (film and data activity): https://www.biointeractive.org/
Gorongosa National Park, elephant population updates: https://gorongosa.org/
MedlinePlus Genetics, AMELX gene: https://medlineplus.gov/genetics/gene/amelx/
OpenStax Biology 2e, ch. 19 The Evolution of Populations: https://openstax.org/books/biology-2e/pages/19-introduction

Image and video credits
Gorongosa elephant and Mount Gorongosa photos: Judy Gallagher (CC BY 2.0), Wikimedia Commons
Mozambique locator map: Shosholoza (CC BY-SA 3.0), Wikimedia Commons
Elephant with tusks photo: KatamReu (CC BY-SA 4.0), Wikimedia Commons
Elephant skull cross-section photo: Aliva Sahoo (CC BY-SA 4.0), Wikimedia Commons
Elephants at a waterhole video: Axel Tschentscher (CC BY-SA 4.0), Wikimedia Commons
Elephants drinking photo: Giles Laurent (CC BY-SA 4.0), Wikimedia Commons
Elephant mother and calf photo: Charles J. Sharp (CC BY-SA 4.0), Wikimedia Commons
Elephant drinking video: Bernard DUPONT (CC BY-SA 2.0), Wikimedia Commons
Elephant herd photo: Bernard Dupont (CC BY-SA 2.0), Wikimedia Commons
Public domain photos (US government: USFWS ivory crush, USAID; Gorongosa terrain): Brian Dell, PDUSGov, USAID Africa Bureau, USFWS Mountain-Prairie (public domain), Wikimedia Commons
Elephant herd photo: cocoparisienne (CC0), Wikimedia Commons
Elephant videos from Pexels (videos 11760783, 12596712, 33660357)
Background music: "Gymnopédie No. 1" (Erik Satie), performed by Teknopazzo, CC0, Wikimedia Commons
Narration: AI voice (Microsoft Andrew). No AI-generated images or video were used.