How wild animals die
February 25, 2025
To minimize the suffering animals experience in the wild, one thing we need to understand is how wild animals die. A fundamental reason for viewing death as a bad thing, from a welfare perspective, is that it deprives individuals of future experiences. But the process of dying is also often painful in itself. Some causes of death might entail much greater suffering than others.
It’s especially important to know which causes of death are the most common for the greatest number of animals. This can help us prioritize which hazards to protect them from so that we can reduce suffering for as many animals as possible.
Some animals die of “natural” causes like disease or starvation, while others are killed as a direct result of human activity. But it isn’t necessarily the case that “natural” deaths are preferable for the animals experiencing them. Starvation, disease, extreme weather events, and being eaten by predators can all be extremely unpleasant — possibly, even more so than hunting or vehicle collisions. If we want to minimize the suffering that wild animals experience, we need to consider the prevalence and welfare impacts of all of the ways they die.
There’s a lot we don’t know about how wild animals die, but several studies have revealed some key insights.
Most small animals and young animals are eaten by predators.
A relatively comprehensive snapshot of causes of death in the wild is emerging for terrestrial mammals, birds, and reptiles, thanks to research in ecology and conservation. Predation is the leading cause of death in small animals and juveniles.
Juvenile mammals and reptiles are twice as likely as adults to be eaten by predators. Predation accounts for more than 95% of documented deaths of juvenile reptiles, while adult reptiles are more likely to die in road traffic accidents. Birds seem to be the exception to this rule: Juveniles are no more likely than adults to be eaten, and overall, causes of death are similar across life stages.
More large, adult mammals are killed by humans than by natural causes.
The picture is different for larger animals: A 2011 study found that human-caused deaths like hunting and vehicle collisions were the most common causes of death among large mammals in North America.
However, even many large mammals die by natural causes. For example, 18% of adult New Zealand sea lions die from tuberculosis, and 24% from attacks by other sea lions. Half of American black bears die as cubs, often as a result of attacks by older male bears. Bear cubs also frequently die from disease.
Scientists know very little about how wild fish and insects die.
For fish, this is partly because it’s very difficult to track individual animals underwater. As technology improves, we should expect to see much more data on the lives and deaths of fish and other marine animals. Insects, meanwhile, are among the most numerous and diverse animals, but also the smallest. Their diversity and size makes them especially difficult to monitor on an individual level.
We need more research to understand how wild animals die and how we can help them.
Ideally, biologists could be immediately alerted to the death of an animal and perform a necropsy to determine the animal’s precise cause of death. They would know everything about the age and health of the animal, having monitored them from birth. But unfortunately, we don’t have the ability to monitor most wild populations like this. Our understanding of how and why wild animals die has so far been pieced together from statistical models and sparse data on the bodies scientists have stumbled upon by chance.
There are several things researchers can do to improve the way that cause of death is studied. Certain modeling approaches can account for some of the gaps in our knowledge, and experimental studies and improved monitoring technology can improve our ability to diagnose cause of death, estimate cause-specific mortality rates, and understand compensatory mortality.
We hope more researchers interested in wild animal welfare will identify and focus on the causes of death that involve the most suffering, as well as on the most common deaths in the wild: those of juvenile animals and members of common species. Causes of death for juvenile animals belonging to common taxonomic groups like rodents and insects are especially noteworthy because these make up the majority of wild animals. If we can find a way to reduce especially unpleasant causes of death for these animals, we’ll be able to lessen the suffering of a significant number of individuals.
Efforts to improve the health and lifespans of our fellow humans are supported by publicly available data on the most common causes of death. This is used to help governments decide which public health interventions and fields of medical research to allocate funding to. A comparable wealth of data could help us identify which wild animals need the most urgent aid, and what form that aid should take.
Actions to protect animals from specific causes of death need to be taken carefully because they could influence population sizes or be replaced by alternative causes of death. More research accounting for complexities like these is needed before we can recommend interventions.
If you’re interested in diving deeper into this topic, read our earlier three-part series exploring the science behind how wild animals die:
Part 1: Why cause of death matters for wild animal welfare