Animals have both positive and negative effects on human mood. Let's start with the negative effects. Many people have deep-seated, almost irrational fears of certain types of animals, especially snakes and spiders. This may have an evolutionary connection to days when we lived much closer to nature and where medical care was not available to treat poisonous bites. Whatever the case, there are many people who suffer from animal phobias which cause the person's heart rate to rise, mouth to go dry, adrenalin to be released and a fright response to kick in, such as running away, or jumping up in the air. Even people who do not suffer such phobias can react this way when the encounter is sudden and unexpected.
I have known people who could not even look at pictures of snakes or spiders without reacting with a severe negative mood change. I have known other people who are not normally afraid of snakes, for instance, but who have 'walked on water' when confronted suddenly with a poisonous species. I carried out a scientific study of spiders once and tried my hardest to overcome fear responses to these mostly harmless animals. But there was one big, hairy spider with large eyes and jaws that came leaping out at me whenever I uncovered its burrow and my response was always the same: scream and jump backwards, with heart racing and adrenalin pumping. It was a mood change I simply could not control under those circumstances and it was very unpleasant, leaving me shaking and upset.
I think most of us have that natural fear of large predatory animals as well. My mother grew up in Oregon lumber camps and when she was six or seven, she was stalked by a mountain lion on her way home from school one day. It frightened the life out of her at the time and deservedly so. She also remembers her mother feeding the wild deer and one day having a rutting male attack them both and chase them into the house. The mood went from the joy of contact with nature to one of great fear, very quickly.
One of the effects of living in cities and only seeing wild animals in zoos, is that many people have lost this natural fear. It is great to go to the zoo and enjoy the bears. The effect on one's mood is generally good in this situation. But city people who then journey to places where wild bears still live, often are so eager to be close to wild animals that they put themselves and their children at risk. I suspect most Yellowstone and Yosemite Park Rangers have horror stories to tell. I once saw some people stopped for a black bear family in Canada and the children were playing with the cubs while their parents took pictures! I am sure they were all getting a nice mood high from the experience, but if the mother bear had suddenly felt threatened for her cubs, the mood would have changed rapidly for the worse.
That said, the other side of the picture is the positive effect that pet species and some wild animals have on human moods. Studies have shown that older people whose partners have died survive longer themselves if they have a pet to love and to love them in return. One of the best aspects of having pets is that they love unconditionally and do not judge the people they live with. This is so different from society, which judges us so harshly if we are not young, beautiful and successful. For someone living on their own, old and otherwise ignored, the love of a small dog can make all the difference between depression and contentment, despair and happiness.
Even dentists know the effects of animals on human mood. Many dental surgeries now feature a fish tank in the waiting room and the effects can be quite noticeable. Most people, myself included, come into the surgery filled with anxiety about the upcoming procedure. I have sat in most waiting rooms getting more and more nervous, but when there is a fish tank, I sit and watch the fish and I can feel myself grow calmer, relaxed and ready to accept whatever happens. Most pet fish owners will tell you how their fish affect their moods in positive ways. One friend of mine talks to her fish and plays with them, greets them when she comes home and cries when they die. They are like her dogs she says and they are an important part of her otherwise very stressful life.
Not only pets, but wildlife can have this effect on human moods as well. Swimming with dolphins at Monkey Mia, or strolling through a nice zoo and laughing at the antics of the monkeys, or going for a walk in the park and listening to the birds singing. It can make one's troubles and stresses go away for a few precious minutes. It can help people get perspective back in their lives and realise how small their problems actually are. Contact with nature grounds people, steadies them and gives them strength to go on with the battles of life. It goes way beyond one's 'mood' to one's whole view of life. One only has to watch a mentally or physically disabled person, touching their first dolphin, to see it. The unabashed joy that is not covered up as it often is in 'normal' people is itself a joy to behold and lifts the watcher's spirit as much as the doer.
Without animals in our lives, we would be much poorer emotionally. I could not live without my dogs, birds and horses. My contacts with native wildlife as a rehabilitator lift my spirit each time I am successful. To hold a wild bird up in the air and release it when it had been on death's door, is to experience a mood uplift that is spiritual in nature. It cannot be measured or quantified but it is no less real for that.