cdiver2:
Do they kill for pleasure or other reasons Yes some species kill to be able to breed with a female, is this for pleasure or to continue there genes.
Ah, now we're getting into true sociobiology. Much of that deals entirely with behaviors that link in some way to reproduction. A lot of human emotions dealing with aggressive emotion are also so cued, especially those involving males. The lay-folk would call it killing for "sport", but the primate biologist might call it a dominance ritual, harem protection, competitive exclusion, fitness posturing, etc...
So really, "killing for fun" is a cop-out of terminology, used in any context including people. There's
always supposed to be a functional behavioral reason, otherwise why select for it?
When critters display complex behaviors that are more similar to that of humans (i.e. dolphin aggression), anthropomorphy can kick in and frequently muddle things. We often use simpler terms that make little sense to animal behaviorists. To them, EVERY behavior has an intrinsic function.
Dolphins kill another spices of dolphin, why that spices? why not anything that lives? If they are killing for pleasure then what doe's it matter what they kill.
I just think there is more to it than what seems to be the obvious.
Well, dolphins do in fact kill a great deal of critters for non-feeding purposes. I've seen them retrieve sand dollars off the bottom and fling them like frisbees out of the air. They'll chomp on crabs, starfish, cucumbers, and other slow-moving animals just to taste, play with, or use in mating rituals. They don't have any sense of "higher duty" to preserve this sort of life.
With the smaller porpoises, there are several prominent theories. The most popular one (at the moment) has to do with infanticide. Male bottlenose will kill babies of their own kind to free up a female. Local porpoise loosley resemble baby dolphins. Dolphins are smart enough to easily differentiate a porpoise from a baby dolphin, but the rough similarities may be just "enough" cuing to tick male dolphins off and start them into a killing spree.
There are similarities in such behavior with chimps and humans. Chimps seem to prefer killing monkeys of different species, but still similar enough in appearance to look like chimps. And throughout mankind's history, we've killed ethnic groups that don't conform. A lot of anthropologists believe that neo-humans like the Cro-magnons and Neanderthals might have helped kill off other hominid lines, which would greatly explain why
Homo sapiens has been the only living species the last several thousand years. Layfolk might call such behavior racism, but an animal behaviorist would choose defining terms that implied an intrinsic function (i.e. habitat expansion, genetic purity).
It may very well be that bottlenosed dolphins are smart enough to have figured out "racism", just like the higher primates did. Porpoises are smaller and less organized than other dolphin species that bottlenose cohabitate with, so it may just be they're an "easy mark". They can't put up much fight.