A recent study found that overpopulation poses a real biological risk that could harm future generations.
For decades, scientists have observed that overpopulation weakens the ability to reproduce. Crowded chickens lay fewer eggs, crowded mice have fewer offspring, and even studies on humans have linked increased population density with decreased fertility.
Although external factors such as resource scarcity and social influences play a role, researchers have long suspected that internal biological mechanisms act as an evolutionary tool to maintain population balance.
Now, a new study from the University of Colorado Boulder, published in the journal Nature Communications , has revealed one of these key mechanisms. Researchers found that animals living in overcrowded conditions release a harmful protein that travels throughout the body, damaging eggs, harming embryos, and causing genetic mutations that are passed on to subsequent generations.
The team arrived at these results unexpectedly while studying a phenomenon known as the "radiation bystander effect," which occurs in cancer patients receiving radiation therapy, where healthy, untreated cells outside the radiation site are damaged, causing side effects such as hair loss, fatigue, and reproductive problems.
In a 2017 study on the worm C. elegans, researchers discovered that radiation-damaged cells release a protein called CPR-4, which then migrates to other healthy cells and damages their DNA. Other organisms, including humans and mice, possess a similar chemical messenger called cathepsin B.
Later, the research team observed that even in the absence of radiation, worms living in extremely crowded conditions released this same protein. At the molecular level, the crowded animals appeared as if they had been exposed to radiation.
In the new study, the team compared worms living in colonies of varying sizes. They found that the worms do not usually secrete the protein, but once the colony exceeds 3,000 individuals, they begin to do so. The more crowded the colony becomes, the more enzyme it secretes, causing DNA damage. Experiments on mice showed similar results.
On average, animals living in overcrowded conditions had 87% more gene mutations in their reproductive cells, produced far fewer offspring, and the remaining offspring often exhibited obvious deformities. Genome sequencing also showed that some of these mutations are passed down through generations, suggesting that overcrowding may drive genome evolution.
When researchers disabled this protein in animals, it prevented the negative effects of overcrowding, confirming that this enzyme plays a crucial role in reproductive problems.
This study comes at a time when the world's population is approaching 8.3 billion, three times what it was in 1950. At the same time, data indicates declining birth rates, with the global fertility rate dropping from 5 births per woman in 1950 to 2.3 births in 2021. According to the World Health Organization, one in six people suffers from infertility.
Professor Ding Xu, the study's lead author, says that while it's well-documented that population density negatively impacts fertility, the underlying mechanisms have remained unclear. He adds that their study offers new insights into how overcrowding can cause developmental disadvantages, including reduced fertility and increased mortality. He further notes that overcrowding has become a major challenge in contemporary societies, particularly in cities, where two-thirds of the world's population resides.
Researchers have already developed and patented a compound that can inhibit the Cathepsin B enzyme in animals. They believe such inhibitors could one day be used in agriculture to increase egg or fish production, for example, and that the findings could also lead to new methods for helping humans with infertility. However, they emphasize the need for further research to determine whether these results apply to humans and other animals.
