Young kittens and puppies are cute. Most people find them utterly adorable: they make you smile and feel good about the world. But what is it about them that makes us feel so fond of them?
When you think about it, there are plenty of reasons why we shouldn't like them so much.
Kittens are juvenile predators who soon grow into independent-minded creatures with sharp teeth and claws. Before long, they'll be killing other creatures and bringing in their bloodied carcasses as gifts.
Puppies grow into less bloodthirsty animals compared to kittens, but as adult dogs they can be unruly scavengers with foul habits like rolling in poo and eating all sorts of disgusting remains.
So what is it that makes these young animals so attractive to us?
It's likely that these young pets have gradually evolved to be cute as part of their increasing domestication over the past ten thousand years. By being cute, they are assimilated far more easily into human society, which enhances the chance that they will survive, thrive, and reproduce.
There are three main features that explain the cuteness of pups and kittens: baby-like features, soft fur and an enjoyment of play.
Baby-like features are perhaps the most significant and interesting. The 20th century biologist Konrad Lorenz proposed the concept of "baby-like patterns" (Kindchenschema) which are a set of facial and body features that make some creatures appear "cute". It's these features that prompt humans (and perhaps other animals) to have the motivation to care for them. If you analyse the appearance of kittens and puppies, you'll find there are several ways that they resemble babies.
Humans have evolved an inborn desire to care for human babies, as without our close and continual care, they will not survive. So cute baby animals have evolved to exploit that instinctive human desire.
Scientifically, what has happened is that those animals that resemble babies when young are more likely to be cared for by humans, and so they have been more likely to thrive in close human company.
Just as babies have relatively large heads, flat noses and prominent eyes compared to adults, so do cute baby animals. You can pick them up, cuddle them, hold them in your arms, and make silly faces at them, just as you do with babies. And their behaviour is also baby-like: they are passive, they seem to like being cuddled, and they react positively when you pet them.
It's likely that adult animals of many species share this desire to care for baby-like creatures of their own kind, which is why adult animals tend to be more forgiving of young animals (dogs and cats tend to ignore puppies and kittens that are being annoying, whereas if an adult animal treated them with such disrespect, an aggressive response would be more likely.. And due to their baby-like appearance, animal youngsters of most species tend to evoke the "cute" response in humans.
Interestingly, it isn't just young animals that have exploited the human tendency to find baby-like features cute: the popularity of some breeds of dog and cat, such as Pugs and Persians, is partly thought to be due by their snub noses and prominent eyes, extending their baby-like appearance from youth into adulthood. And all adult dogs and cats have evolved to retain some baby-like features throughout their whole lives: this is a phenomenon known as "neoteny". Adult pets retain some degree of dependence on humans, they often enjoy playing with their owners, and they like to be petted. Wild animals don't retain these features as they grown into adulthood.
The soft fur of kittens and puppies isn't a particularly baby-like feature, but it definitely adds to the cuteness score, again, for evolutionary reasons. Primates - from monkeys to humans - have an instinctive enjoyment of the sensation of touching soft, fluffy fur. In humans, this is most obvious in children, who commonly enjoy soft blankets and fluffy toys. This may go back to the common primate behaviour of social grooming (think of monkeys picking parasites out of each other's fur). We don't do that any more, but if you feel very affectionate towards someone (a child or a loved adult) then you may find yourself running your fingers through their hair. We like doing this automatically without thinking about it. The lack of soft fur may be one of the reasons why reptiles and coarse-furred creatures (like rats) don't seem so cute. Feathers are in-between on the cuteness scale, and the fluff of day-old-chicks closely resembles soft fur, which definitely makes them "cute"
The inclination to play is the last part of the cuteness formula. Play is an interesting phenomenon: it's an enjoyable way for young animals to train for real life crises. When kittens chase a bright light shone on the ground, or a feathery toy on a wand, and when pups chase a ball, they are practicing for real life hunting. Humans like playing too, and we recognise the fun element of animals playing: it makes them all the more cute to us.
Baby-like, fluffy and playful: now you know why you love puppies and kittens.