Why Elephants Rarely Die of Cancer: Cells That Know When to Quit
An African elephant bull can weigh six tonnes and live past sixty, and its body is built from roughly a hundred times as many cells as yours. Cells in skin, gut and blood divide all the time to replace worn-out ones, and every division is a fresh chance for the instructions in a cell’s nucleus to be copied with damage. A cell with damaged instructions can start dividing when it should not, and that is how cancer begins. More cells, more divisions and more years should add up to more cancer. Across species they do not. The epidemiologist Richard Peto pointed this out in 1977, and biologists still call it Peto’s paradox.
Counting causes of death
A team at the University of Utah, led by Lisa Abegglen and Joshua Schiffman, gathered necropsy records for 644 elephants, the reports written after an animal dies and is examined. Cancer killed an estimated 4.8 percent of them. In people, cancer causes between 11 and 25 percent of deaths, depending on the population. A larger study of 110,148 zoo mammals from 191 species found the same pattern at scale: the risk of dying of cancer did not climb with body size or with lifespan.
A guard in every nucleus
Among the instructions in a cell’s nucleus is a gene for a guard protein called p53. When a cell’s instructions are damaged, p53 halts division until the damage is repaired. If the damage is too great, p53 tells the cell to self-destruct, and the cell takes itself apart without harming its neighbors. You carry one copy of the guard gene, one version from each parent. African elephants carry at least twenty copies. People born with Li-Fraumeni syndrome carry one working version and one broken one, and they develop cancer far more often than other people, many of them as children.
The Utah team put the three groups side by side. They took white blood cells from elephants, from healthy people and from people with Li-Fraumeni syndrome, damaged the cells with a dose of radiation, and counted how many self-destructed. They repeated the test with doxorubicin, a chemotherapy drug that damages a cell’s instructions. The more working copies of the guard gene a cell carried, the more readily it gave itself up.
Losing a cell to keep the tissue
A self-destructing cell sounds like a loss. For the organism it is a trade worth making. A tissue is a team of similar cells doing one job, and a skin cell or a blood cell is easy to replace. A damaged cell that keeps dividing is not. It can grow into a tumor that crowds out healthy cells, steals their blood supply and stops a whole organ from keeping the body in balance. An elephant’s cells quit early and often, and the tissue stays healthy.
| Group | Copies of the p53 guard gene | Share of deaths from cancer | Cells that self-destructed after radiation | Cells that self-destructed after doxorubicin |
|---|---|---|---|---|
| African elephants | at least 20 | about 5% (644 elephants) | 14.6% | 24.8% |
| Healthy people | 1 | 11–25% | 7.2% | 8.1% |
| People with Li-Fraumeni syndrome | 1, with one of its two versions broken | much higher than average; cancer often starts in childhood | 2.7% | — |
- Necropsy
- An examination of an animal’s body after death to find out what killed it. The animal version of an autopsy.
- Tumor
- A lump of extra cells made when a cell keeps dividing after it should have stopped.
- Gene
- One set of instructions in the DNA inside the nucleus. You will meet genes in full in the unit on nucleic acids.
- p53
- A guard protein that stops a damaged cell from dividing and, if the damage is too great, tells it to self-destruct.
- Tissue
- A group of similar cells working together on one job. Tissues build organs.
- Homeostasis
- Keeping conditions inside the body steady enough for cells to live and work.
Sources: Abegglen and others, “Potential Mechanisms for Cancer Resistance in Elephants and Comparative Cellular Response to DNA Damage in Humans,” JAMA 314 (2015): 1850–60, doi:10.1001/jama.2015.13134, for the 644 necropsies, the 4.81 percent estimate, the 11 to 25 percent human figure, the copy counts and every self-destruct percentage in Table 1; Vincze and others, “Cancer Risk across Mammals,” Nature 601 (2022): 263–67, doi:10.1038/s41586-021-04224-5, for the 110,148 zoo mammals across 191 species; Peto and others, 1975–1977, for the original observation. The elephant cell tests used blood from 8 elephants, and the authors wrote that the findings needed to be repeated. SOL codes are a judgment call — confirm against the current Curriculum Framework.
Practice: Claim, Evidence, Reasoning
Claim: one sentence that answers the question and could be shown wrong. Evidence: a number, a row of Table 1, or a line from the reading. Reasoning: the principle that ties them together.
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Claim only
A classmate says, “Elephants have so many cells that they must get cancer all the time.” Write a claim that compares how often cancer kills elephants and people.
Claim -
Full CER · from the data
Using Table 1, decide whether the number of working guard-gene copies is connected to how readily damaged cells self-destruct. Your evidence must cite all three rows of the radiation column.
ClaimEvidenceReasoning -
Full CER · from the reading
When a damaged skin cell self-destructs, is that a failure or a help for the tissue and the whole organism? Tie your answer to levels of organization and to homeostasis.
ClaimEvidenceReasoning -
Mastery · new situation
Suppose scientists study a whale that lives more than 150 years and is built from even more cells than an elephant. Its cells carry only one copy of the guard gene, yet cancer is rare in the species. Its cells repair damaged instructions about twice as fast as human cells. Decide whether extra guard-gene copies are the only way a large, long-lived animal can avoid cancer.
ClaimEvidenceReasoningWhat would change my mind -
Advanced Biology only
The self-destruct tests used white blood cells from 8 elephants, grown in a dish, and the authors wrote that the findings needed to be repeated. Decide how strongly Table 1 supports the claim that extra guard-gene copies explain why elephants rarely die of cancer, and describe one study that would make you more confident.
ClaimReasoning