Overpopulation and population control in domestic pets

Overview

The overpopulation of domestic pets such as dogs, cats, rabbits and other small animals, birds, and reptiles is a concern around the globe. In 2023, the World Animal Foundation reported a total of about 900 million dogs worldwide, and estimated that of these, 75 to 85 percent were strays. The following year, the foundation reported that out of an estimated global cat population of between 600 million and 1 billion, 480 million were strays. Based on 2019 data, the American Society for the Prevention of Cruelty to Animals (ASPCA) estimates 3.1 million dogs and 3.2 million cats enter US animal shelters annually when they are surrendered by their owners, found as strays, or rescued from hoarding cases, dog fighting rings, or puppy mills. About 810,000 strays are reunited with owners. While about 4.1 million of the remaining pets are adopted, about 920,000 are euthanized every year. Pet rabbits and other small animals also frequently are relinquished at shelters, but like all varieties of pets, sometimes they are dumped along roadways or near homes.

Globally, only about 25 percent of dogs are dedicated house pets, while 75 percent are free-roaming animals. Free-roaming populations of domestic animals have unrestricted movement but may be owned or unowned. They are generally the focus of population control methods because free-roaming animals may pose public health dangers. Various methods of dealing with overpopulation include sheltering, fertility control, and culling. Some groups engage in catch, neuter, release (CNR) programs. Methods of addressing overpopulation of cats are similar, with many communities adopting trap, neuter, return (TNR) programs. The chief method of reducing domestic pet populations in many countries is the practice of neutering and spaying pets, which most shelters require when animals are adopted. The issue of overpopulation became of even greater concern following the global COVID-19 pandemic. This event prompted many people to adopt pets in 2020 to keep them company or occupy children as they remained home and worked from home. When offices began to reopen, many shelters and rescue organizations were inundated with and overwhelmed by rabbits, dogs, cats, and other pets.

Background

Humans began domesticating animals up to seventeen thousand years ago in Asia and Europe. Hunter-gatherer groups deliberately domesticated the first species, dogs, during the last ice age. The methods and reasons are unknown, and many have varied among different regions and times, but researchers speculate that humans many have had more meat than they could eat and began sharing it with wolves. They might have had opportunities to raise wolf pups by hand. If over generations the wolves developed an affinity for the humans, they would likely have provided benefits such as keeping other predators away, warning of danger, and even in time would have aided in hunting and, even later, in herding and protecting cattle.

Researchers believe domestication of cats occurred naturally, or at least without human effort. Archaeological evidence shows that cats lived alongside humans for thousands of years, likely looking for rodents or other pests drawn to farming communities’ stores of food. Genetic research has found two lineages of cats led to modern domestic felines. About eight thousand years ago, cats were living in communities in the Fertile Crescent in the Middle East. From there they spread into southwest Asia and Europe. Another lineage of African cats common to Egypt spread through the Mediterranean region and into Europe. Scientists believe humans took cats on trade journeys and carried them aboard ships to contain rodent populations; from these forays the animals began joining new communities.

Animal pets often lived with their humans indoors, but most were not exclusively house pets. When pets gave birth, offspring were sold or given away. As human settlements grew larger, animal overpopulation was occasionally a problem. Initially governments caught and killed surplus animals, but in the late nineteenth century private shelters began to appear. While they offered more humane methods of euthanasia, they did not solve the problem of too many animals. As human populations became more urban and suburban, dogs and cats became increasingly popular as house pets. Veterinary medicine was also advancing, and in the 1930s, spaying and neutering became widely available, although many households found the procedure unaffordable. Some groups in Europe began organizing TNR programs to address overpopulation of cats in the 1950s; the practice was later adopted by some groups in the United States. Low-cost spay and neuter clinics began emerging in the late twentieth century. In 1990 the WHO published literature discouraging culling as a method of dog population control.

Until modern times, most cats were free-roaming animals. Clay cat litter emerged in the 1950s and in the 1960s households began providing pets with litter boxes. This allowed people to keep their cats indoors and increased cat ownership.

When Animals Become Strays

Animals are separated from their humans for many reasons. Sometimes they run or wander away. In the United States, the Independence Day holiday is the most common time each year for dogs to become lost. This may occur because people are distracted by family gatherings or when loud fireworks terrify the pets and cause them to run. Pet rescue groups suggest keeping pets indoors, preferably in an interior room with a radio or television playing to deaden the sound of explosives, closing windows and doors so that animals will not push out screens, and ensuring dogs are leashed securely and wearing collars with identification tags so they can be returned to their owners if they get lost. Pets also get lost when they are being cared for by others such as pet sitters or groomers or are newly adopted, when they may be confused and frightened by unfamiliar situations or people.

Many animals become lost during natural disasters such as hurricanes. People who must evacuate to shelters are often not permitted to take pets along. Many animals are later rescued from floodwaters, but local shelters may be swamped with animals or flooded themselves, so animals may be relocated far away. Often rescue groups take these pets to other states and work to get them in foster homes or adopted.

Applications

Stray or released domestic animals face many hazards. They can be injured by cars, attacked by other animals, or succumb to diseases. Many face starvation or poor nutrition; for example, about half of free-roaming dogs in India are emaciated. Stray animals can cause many problems in communities. Packs of stray or feral dogs, for example, can harass people and animals and be a safety hazard when the animals cross traffic. Dogs have reduced populations of many animals such as mountain gazelle in Israel and kiwi birds in New Zealand and have killed livestock such as sheep. Dogs, cats, and other pets can become infected with and transmit more than three hundred zoonoses, or diseases that can be transmitted from animals to humans, including rabies, and spread diseases to other animals. Researchers note 99 percent of human-rabies transmissions involve dogs. Many neighborhoods are home to feral cat colonies or community cats. Community cats are free-roaming animals that live at least part of the time outdoors and are unowned, or possibly are cared for by several households in the neighborhood. Feral cats are also free-roaming but unlike community cats are wild, either because they were born in the wild or because after some time out of doors they have reverted to being wilder; for example, they are unlikely to allow people to get close to them. Strays are owned animals that have become separated from their humans. Community cats and feral colonies are often unwelcome in neighborhoods when they dig up garden beds, mark their territory, fight loudly, or act in other ways that damage property or make people unhappy. While many neighborhoods are likewise home to community dogs, this is less common, probably because cats are crepuscular, or more active at dawn and dusk, and more frequently roam at night.

Solutions

Many countries continue to use culling as a method of animal population control, especially when dealing with an overpopulation of dogs. Higher-income countries generally use injectable barbiturates, while lower-middle and low-income countries may poison or shoot animals. Countries that use sheltering as a method may euthanize many animals that cannot be rehomed. The WHO’s recommended alternatives for controlling free-roaming dogs include public education, registration and identification, sterilization, and vaccination. Many localities focus on education. Governments and other organizations often work to inform communities about the importance of sterilization of their pets and preventing animals from roaming freely. Sometimes programs face opposition in communities where owners of free-roaming dogs do not want their animals sterilized, for example some people believe neutering causes behavioral changes they do not want or think it is against their religious beliefs. The cost of spaying and neutering can be prohibitive for some people, and owners of large dogs typically pay more for the procedures. According to GoodRx in 2024, in the United States the surgery for a dog can cost as little as $10 with a state voucher to nearly than $600 at a private animal hospital. For cats, cost ranges from $10 to more than $350. Spaying of female animals generally costs more than neutering males. The cost of sterilization of rabbits likewise varies widely.

Many organizations have called on veterinarians to educate clients and to work with rescue and CNR and TNR groups to sterilize free-roaming animals. Sterilization may be achieved using surgical or chemical methods. (Some methods of contraception using hormonal drugs are available but not usually recommended.) CNR and TNR are often performed at fixed-location facilities or mobile clinics. Many of these programs operate a discounted service for pet owners as well. Such programs involve trapping free-ranging animals; taking them to a veterinary facility or clinic where they are neutered or spayed and vaccinated against common diseases including rabies; and returning them to the location where they were trapped after a day or so of recovery time. Animals are usually marked in some way, for example it is common to clip the tip of a cat’s ear to indicate it has been sterilized. While some one- and two-year studies of TNR programs found that cat colonies increased in size, analysis of a sixteen-year-long program in California (Spehar and Wolf, 2020) shows a population decline of 99.4 percent. The study included 258 cats, 41 percent of which were adopted. Other programs have likewise seen long-term declines in free-roaming cat populations.

Issues

Many factors lead to pet overpopulation. Chief among these is failure to spay and neuter animals or delay of the procedures. Some animal owners believe their female pets should have one litter before being spayed, although this offers no benefit for the animals. Others do not organize the procedures and are faced with pregnant pets. For example, some rabbit breeds are sexually mature as young as 3.5 months; cats can become pregnant at about 4 months of age; and, depending on the breed, dogs typically reach sexual maturity between 6 and 9 months. Other species can reproduce at an even younger age; for example, hamsters can become pregnant as early as 4 weeks of age. A typical hamster litter is six to eight pups, cats have an average of four kittens in each litter, and dogs’ and rabbits’ litters vary considerably among breeds but average about five offspring. Some animals can become pregnant again almost immediately, leading to multiple litters in a year. A cat multiplication pyramid illustrates how one unspayed female, her unneutered mate, and their likewise fertile offspring can multiply. Producing two litters annually (with 2.8 surviving kittens per litter), one year results in twelve cats; by year eight, the total number of cats and kittens surpasses two million. Often times, animals are rescued from hoarders, who are overwhelmed with the number of animals they have acquired either by taking in strays or by failing, because of cost or other reasons, to have animals sterilized and, as a result, care for litter after litter of puppies or kittens. In many cases, the animals are sick, undernourished, flea ridden, or feral because they have not been socialized; some of these animals may be adopted or fostered, but if they are extremely unwell they may be euthanized.

People are often uneducated about animals or act on impulse and later regret or reconsider their decision. Rescue operations in the region of Los Angeles, California, noted in 2022 that many people purchased rabbits in the early weeks of the pandemic as pets for their bored children, often without considering or understanding the cost and commitment of owning the animals. In many cases, the rabbits were already pregnant when the person bought them. In California, sales of rabbits are illegal except in pet stores with licenses, but people often sell them on the street to impulse buyers.

Another reason for overpopulation is the status people place on owning certain breeds or a pedigreed pet. Many people believe that animals in shelters are flawed in some way. While some people have reasons for wanting a specific breed—for example, people with allergies are often advised to choose hypoallergenic dog breeds—others may select a breed because it is trendy, or because a celebrity favors it. This can lead to overbreeding of some animals. For example, unethical breeders will breed animals rapidly with little concern for the animals’ well-being or for choosing the parents carefully. The resulting puppies, for instance, may have congenital defects or be prone to ailments.

Demand for some breeds also supports puppy mills. These are breeding factories where females repeatedly give birth to litters of puppies and live in small cages. Many never leave their pens. The goal is to produce as many puppies as quickly as possible, so little care goes into selecting the parents for desirable traits and genetics.

When people buy pets from breeders or pet stores, they may not always understand the challenges they may face as a pet owner or the commitment involved. Reputable breeders work to make sure the people and pets are good fits for one another and agree to take the animal back if it does not work out. Pet stores may offer a return policy if the animal becomes ill within a specified time frame. When people decide they cannot keep the pet, they may relinquish it at a shelter, but doing so often takes a spot away from a stray or an animal a person is forced to give up, for example because they must move. Animals adopted from shelters may need time to address problems such as separation anxiety or barking, but shelters typically offer advice and even assistance to adopting families. Rescue organizations in particular try to put animals in foster homes so they are socialized and more ready for a new home. Moreover, they generally require people to return the animals to the shelter if the adoption does not work out or the individual’s life situation changes, making pet ownership difficult or impossible.

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