Humans have spent a lot of time befriending other mammal species. Dogs started becoming our BFFs 10,000 to 40,000 years ago, while cats have been curling up in our laps for at least 50 centuries. Even goats, which were domesticated mainly for milk and meat about 10,000 years ago, make great companion animals.

But time is only part of why these relationships work. They were also pretty good matches in the first place, capitalizing on relatable traits like high intelligence or social adaptability. Wolves are clever, family-oriented hunters, so early dogs likely fit well into human homes. Cats tend to be less gregarious, yet smart, friendly and flexible enough to hobnob with benevolent apes. Goats, like horses, just seem to get us.

Thousands of mammal species are known to science, though, and a wide array live as pets, from retired livestock like pigs to captive wildlife like pangolins. The popularity of exotic pets has grown globally for years, often risking not just animal welfare, but also the conservation of wild species. The pet trade has decimated many wild fish, amphibians, reptiles, birds and mammals, even large carnivores. There are now more captive tigers in the U.S., for example, than there are wild tigers in the world.

Most people know tigers aren't good pets, but that's not always as clear for other wildlife. Mammals tend to be cute, which can overshadow needs and instincts that make them difficult or dangerous to live with. To add clarity, researchers from the Netherlands have developed a new method to assess "pet suitability" of mammal species, which they've published in the journal Frontiers in Veterinary Science.

tayra weasel, Eira barbara They may be cute, but tayras don't rank highly in a new list of pet suitability. (Photo: Matthijs Kuijpers/Shutterstock)

To be clear, their list (see below) isn't meant to advocate any particular mammals as pets. It's more about creating a standard framework to help humans understand which kinds of mammals are well-suited to life with us, which aren't and why.

"The main influence of this work is methodological," explains Paul Koene, a researcher at Wageningen University who led the new study. "In the Netherlands many mammal species are kept, and for a long time the government wanted to guarantee the welfare of animals. Therefore the Dutch Animal Act was made, stating that mammals should not be kept unless they are production animals, or are species that are suitable to be kept by anyone without special knowledge or skills."

To test that policy, Koene and his colleagues first came up with a list of candidate mammals, then devised a way to rank them from most to least suitable. They started by researching which mammals are most commonly kept as pets in the Netherlands, then added species to the list based on data from veterinarians and rescue centers.

sika deer in Nara Park, Japan Sika deer, like these at Japan's Nara Park, may be suitable as companion animals. (Photo: Teddy Leung/Shutterstock)

They came up with a list of 90 mammals, which obviously isn't comprehensive but still offers an interesting start. They left out species classified as "production animals," since their suitability is already established, as well as dogs and cats. They then collected existing data and expert opinions about all 90 mammals, creating one-line criteria statements against which each species could be graded.

(These "one-liners," as the researchers call them, were assigned a score related to behavioral needs or welfare risks. Three teams then worked together to produce the final rankings. The first team selected one-line statements for each species, and the second team assessed the strength of those statements in regard to behavior, health, welfare and relationships with humans, both in captivity and the wild. The third team then used those strength summaries to gauge each mammal's pet suitability.)

So, minus dogs and cats, which mammals rank highest? Using this method, the researchers highlight five of their 90 mammals that seem suitable:

  1. Sika deer (Cervus nippon)
  2. Agile wallaby (Macropus agilis)
  3. Tamar wallaby (Macropus eugenii)
  4. Llama (Lama glama)
  5. Asian palm civet (Paradoxurus hermaphroditus)

Here's a chart showing the top 25:

chart of mammals' suitability as pets Species with an AS3 score above zero are considered more suitable as pets, researchers say. (Image: Koene et. al.)

Again, this is not a suggestion that anyone adopt a sika deer or agile wallaby. It just offers perspective, hinting at the relative rarity of animals that make good human companions — and how lucky we are to have pets like dogs and cats.

The full list includes lots of mammals whose unsuitability is obvious, like bears or screaming hairy armadillos, but also some popular exotic pets such as ferrets and sugar gliders. And while a low rank doesn't necessarily mean a species should never be a pet, it does point to higher odds of problems for both parties.

Species were deemed unsuitable for many different reasons — some need too much space, too much exercise or too specific of a diet, and some just have social requirements that clash with ours. And others, of course, pose a danger to themselves or to humans when living in captivity.

sugar glider in a tree Sugar gliders take care of themselves in the wild, but some humans find the job challenging. (Photo: Praisaeng/Shutterstock)

After this initial list, the researchers plan to expand their rankings. A team is already analyzing 270 other mammals, Koene says, and looking ahead to even broader lists that are relevant to more people. "They are also looking at how to determine the suitability of birds and reptiles in the future," he says. "So the impact of the study is that there is a framework and shared database that could be further developed in a more widely used context, for instance across the E.U., the U.S. or even worldwide."

It may seem strange for a list of most suitable pets to omit dogs and cats, but did it really need to include them? It's obvious that dogs and cats mix well with humans, and while Koene says they should be included in future versions of this list, he also notes that their roles as top pets remain virtually unrivaled.

"Dogs and cats are a special kind of pets, because of their way of housing (free roaming), of variation in breeds, the vast amount of literature and of the delicacy of the subject," Koene says, "and so were not analyzed."

Anyway, he adds, "wallabies will certainly not replace them."

Russell McLendon ( @russmclendon ) writes about humans and other wildlife.