Why animals that care for each other face less cancer

Cancer is woven into the lives of almost all mammals, yet some species seem to shrug it off. New research argues that the way animals share, compete, and care for one another helps set how much cancer their species can bear.

The scientists matched cancer records from thousands of zoo necropsies with traits like body size, litter size, and social living across many mammals.

The team built a mathematical model to test when higher cancer in older adults might actually increase total population size.

Cancer risk throughout life

Cancer starts when cells in a body keep dividing when they should stop and begin to invade nearby tissues.

Across species, cancer risk is part of an animal’s life history, the schedule of growth, reproduction, and aging.

Big animals, including whales and elephants, do not show the massive cancer burden their many cells would make you expect. Researchers call this pattern Peto’s paradox, the surprising absence of extra tumors in large animals. 

How cancer risk evolves

The work was led by Catalina Sierra, a biology researcher at the Universidad de Buenos Aires.

Her research focuses on how cancer risk evolves in mammals that live under different ecological and social conditions across the world.

Earlier comparative zoo necropsy data from 37 mammal species already showed big differences in tumor prevalence between species.

The findings hinted that how fast animals grow, how many offspring they have, and how long they live all matter for cancer.

Cancer clues in animals

In the new comparative study, Sierra and colleagues combine zoo cancer records with a three stage population model of competition and cooperation.

The dataset covers almost 200 non-domesticated mammal species with at least 20 carefully reviewed necropsies each.

The team first checked obvious suspects like body mass and metabolic rate, expecting bigger or faster burning bodies to show more cancer.

Instead, they found no strong correlation, echoing an earlier large scale survey of cancer mortality in zoo mammals. 

Social behavior and parental care

Sierra’s group then examined rates of neoplasia, abnormal new tissue growth that includes benign and malignant tumors, across species.

Litter size mattered much more than body size, with species that produce big litters showing consistently higher cancer and neoplasia levels.

Patterns like this appear beyond mammals, because a vertebrate analysis found that cancer prevalence rises with body mass but falls as gestation lengthens. 

Together, these data pointed the new team toward social behavior and parental investment as key axes for explaining cancer risk.

How behavior shifts cancer

The model treats resources as a shared pool and divides each species into juveniles, reproductive adults, and older, less reproductive seniors.

Adults who compete for that pool influence how many young survive and how many reach the stage where they can breed.

Within this framework, the hydra effect, a counterintuitive rise in population size after mortality increases, shows up under surprisingly simple conditions.

If older adults contribute little to care but still consume resources, killing more of them can free food and space for juveniles and breeders.

Cooperation lowers cancer risk

In strictly competitive settings, intraspecific competition, struggle among members of the same species for limited resources, becomes fierce.

Under those conditions, the model predicts that higher cancer mortality in seniors can raise the equilibrium population size rather than push it down.

“Using public databases, we show that species with cooperative habits have lower cancer prevalence and mortality risk,” wrote Sierra.

In the model, that pattern appears only when older individuals are not important helpers, so late life cancer simply removes weak competitors.

How care shields elders

When older animals help raise young, share food, or defend the group, their extra years boost the success of their relatives.

Evolution then favors traits that keep those elders alive, including stronger tumor suppressor systems and slower cancer progression.

Biologists describe this with inclusive fitness, genetic success achieved by helping close relatives reproduce.

In cooperative mammals, cancer that removes older helpers early cuts into that shared genetic payoff and becomes strongly disadvantageous.

The authors argue that in solitary or highly competitive species, late life cancer can work like phenoptosis, a genetically programmed form of organismal death.

Removing older, less reproductive individuals frees resources and reduces crowding, which helps younger, more fertile animals succeed.

The role of lifestyle

Naked mole rats live in large underground colonies where only a few animals breed, while many carnivores hunt and raise young alone.

Extremely cooperative and caste-based rodents almost never develop tumors in captivity, according to a zoo profile. 

The study’s results do not mean cancer is somehow good for individuals. Instead, the research highlights how evolution may have tolerated or discouraged late life tumors differently in species with contrasting social systems.

Study limitations and future research

Most of the cancer records in this work come from zoo animals, so conditions in the wild might sometimes look different.

The mathematical model also simplifies cancer into higher or lower death rates in seniors, glossing over tumor types, treatments, and complex immune responses.

Even with these limits, the work connects threads from comparative oncology, life history theory, and population ecology into a single evolutionary picture of cancer. For humans, it suggests that cancer risk reflects both biology and social history. 

Future work can test whether other late life diseases, such as neurodegeneration or heart disease, also behave differently in cooperative and competitive species.

If they do, clinicians and conservation biologists may someday treat these conditions as features shaped by evolving social lives, not only as medical problems.

The study is published in the journal Science Advances.

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