Shinare
A gene therapy was done to cause the meadow vole's receptors...
That's reeeeeeeaaaaaaaallly cool. Do you perchance have a link to this study? I'd love to read it!
We are just beginning to delve into the world of animal emotions. I think it's safe to assume creatures such as the sea sponge probably do not feel much, emotionally, seeing as they lack brains or any real social contact. Emotions exist, I think, to facilitate interaction with other individuals.
Why do we choose monogamous relationships (mostly)? Because we feel romantic love for another individual? I can't think of any other reason. Societal pressures, maybe, but those must come from somewhere. Neurophysiology aside, why should we assume, then, that any other creature has a different reason for monogamy?
Recently, I've heard a lot about grief. There was a video somewhere, not long ago--I might try to hunt it down and include it here--of a gorilla who speaks sign language being told that someone who visited regularly had died. She didn't see this death. It hadn't been long enough for her to assume something had happened, or to miss experiencing time with this person, but when her handler told her, via sign language, that this person had died, she made the sign for 'tears' with no prompt from others.
When elephants come upon an elephant skull in the desert, they gather, rub their trunks around the bones, and then bury them before moving on. I can't see any evolutionary benefit for this. It seems to me that when they do this, they are grieving.
Two dogs live together for a number of years. When one dies, the other becomes extremely lethargic, loses interest in things like chasing balls, and often stops eating. This is pretty common. Why do they do this? Depression-like behavior seems like a very bad instinct to have, if mere survival is your only goal.
There was even a study on birds--bluejays, I think--following the observation that when a dead jay is found on the ground, other jays in the area will gather and begin cawing. Some blue wood arranged to look somewhat like the form of a dead jay did not induce this response, but seeing the body of a fellow bluejay did. This could perhaps be an evolutionary trait; one can see how alerting other members of the species of potential hazards in the area could come in handy. But inasmuch as emotions are a reaction to external stimuli, I'd say that's something. It certainly shouldn't be ruled out.
Now, as several others have stated, emotional range will depend on the animal. Ants, for example, are extremely social creatures but they exhibit no real attachment to their fellow ants. If one ant starts to show signs of a fungal infection, it will quickly be carried far away from the anthill to die where the spores cannot spread through the colony. I've never been inside an ant colony, but I seriously doubt they follow this up with a moment of silence for their fallen comrades. The ants in my kitchen certainly don't exhibit any visible behavioral changes when I start raining toxic death upon them.
Dogs with abandonment issues experience sometimes quite severe bouts of anxiety when their friends leave. Purely anecdotal, but I heard there is a dog in Italy who, when his owner passed, continued to find his way to the church every Sunday morning. Maybe it was just habit. Maybe not.
I don't expect my beta fish cared what happened to me. I never saw any indication of love or hate or anger from my rabbits either. But I know my dog sighs when someone says the "w" word but doesn't fetch her leash, I know she wags her tail when someone says her name in a happy tone and she drops her gaze when someone shouts it, and I know she smiles at me when I wake up in the morning. That's proof enough for me.