This is a really interesting mystery. There may be more than one answer. For example, picking up on our moods might be due to animals paying a lot of attention to our facial expressions, body movements, and voices. Social animals like dogs, horses, and dolphins depend on understanding each other without language. So they may have evolved some pretty sophisticated brain circuits to detect and interpret what other individuals are about to do. Humans may depend too much on what people say and not even notice the tiny, involuntary cues we give about our mood.

There was a famous example of this over 100 years ago. A man taught his horse (Clever Hans) to do many things. He seemed to be counting and doing math, including square roots. He used his hoof to tap answers. A team of experts observed the horse and man working together and could find no tricks. The horse’s owner was not a fraud. Then another investigator started looking at the man much more carefully. He noticed small, involuntary movements that he made, such as leaning a tiny bit forward (which signaled Hans to start tapping), and slightly relaxing when Hans had made the right number of taps (which signaled Hans to stop). Then the investigator found that if Hans couldn’t see the person who knew the answer, he’d just tap randomly. Hans was clever at reading people, but not at square roots (as far as we know).

Now experiments are designed to avoid the “Clever Hans Effect.” For example, if dolphin trainers are testing whether dolphins can learn the meaning of whistle commands, the trainer might be blindfolded so that she or he was not unconsciously looking at the hoop the dolphin was supposed to grab.

There may also be a chemical difference that animals like dogs can smell. Dogs have much better noses than humans. They also use a lot more of their brain to process smells. So if a person who is about to have a seizure has a slightly different scent, the dog may notice.

Do you think that humans consciously or unconsciously selected for the ability to “read” humans when they started breeding dogs or horses? Would wolves or wild horses be equally good at figuring out human moods?

Animals can sense human feelings the same way that humans can sense other humans' feelings, by paying attention and learning what cues humans are giving off. Dogs are especially good at this, in part because we have bred them to be, and in part because their wild ancestors (wolves) lived in social groups and were sensitive to each-other's feelings.

While many people use service dogs to detect certain diseases, the scientific jury is still uncertain about how they specifically work. We’ll use dogs that are trained to detect seizures as an example. The current hypothesis is that dogs are able to smell an unknown chemical that humans release prior to having a seizure. Another thought is that since dogs are more attuned to body language than humans are, humans may subconsciously change their actions slightly before having an attack. The service dog is trained to recognize these changes and alert their owner before the attack occurs.

While it appears that seizure service dogs often work, they are not correct 100% of the time. Because of this, seizure sufferers are encouraged to have other means of protecting themselves from attacks besides just the service animal. Thank you for the question!