At the turn of the century there were 20 species of bottle-nosed
dolphins, twenty years ago there were three and now there is one. Nothing
has gone away or gone extinct, it's just that perceptions and definitions
First we will talk about brain size, which is measurable, but is just a variable. It's thought to be correlated to intelligence but it's not a direct sign. However, no one has a real definition of how it connects.
Brain size is pretty much comparable to humans- ours is 5% of body weight and it takes 20% of our metabolic output. Evolutionarily, it's got to be paying its way! This is why he doesn't believe that common saying that people only use 10% of their brain. So basically, we have a large, complicated, costly organ. How do we explain it?
How do we measure brain size? We have to have humans come out on top, or course, since we're so great. This means that we can't use absolute size because of elephants, sperm whales, etc. We also can't use a brain size to body size ratio because smaller animals like shrews have bigger ratios. So they use this other method in which they take the log of both numbers and look at who is above the line and who is below the line. In this way people have come up with an encephalization quotient. You look at how big the brain is and how big an average animal of that size is and plug in the numbers.
There's also another way to do it which is a little simpler. You take the body size in kg and the brain size in g and divide them. There are four different body size classes. In each class, there is a dolphin-like animal. In each class except the largest, there is also an ape. In each, at least one of the dolphins in the class has a bigger ratio than the ape (excepting the human who is far and away about the others).
So it's a costly organ, as he said before. Why has it evolved only in a few taxa? Basically just in dolphins and apes. A lot of speculation has gone on about human brain size as well; it has exploded in the last 2-5 million years. A lot of theories have come up to explain what caused the really fast development of brain size. They have all been very specific, for instance having to do with hand-eye coordination, tool use, or walking bipedally. However, it would be nice to have a general theory that explains large brain size in all taxa including dolphins who have no hands, no or very little tools, and don't walk bipedally.
This may have given rise to the common myth that dolphins have x-ray vision and can see inside of things but they really can't. It's probably just that if something has air in it, they can tell that, and they can tell the difference between solid and hollow.
So a lot of their brain is probably devoted to echolocation and all this stuff. However, looking back at the chart of brain size to body size, we note that even though all of the dolphins echo-locate just fine, their brain size varies a lot- so the ones with the much larger brains must have a lot left over if you only need as much as the smaller-sized brains have for echo-location.
There are two classes of general purpose hypotheses- The ecological model hypothesis (also known as the patchy resource hypothesis), and the social complexity modelling hypothesis.
One criticism- people like to talk about encephalization (increase of brain size with body size) but it's well known that animals who eat cellulose (grass, leaves, sticks, bark) have a tough time digesting this and they have long guts to digest it. So folivores may be somaticized because of the need for bigger guts- so for a given brain size, folivores might have bigger bodies because they need the space. In other words, maybe it's not that the folivores have smaller brain for their body size, but just that they have larger bodies for their brain size. Anyone who deals with this hypothesis has to take this into account.
Another word of warning about any of these hypotheses- there's a difference between the origin and the maintenance of a trait. The reasons for which cognitive skills and large brains evolved may be different from the things they're being used for today. For example, our brains probably didn't evolve to do calculus, but we use them for that anyway. So it is possible that a species that evolved a large brain to find fruit now has a lot of social skills that they use the brain for, or vice versa. It might have evolved to get through a complex social life but now they use to it find fruit better.
This theory is interesting because both chimps and bottle-nosed dolphins live in complex social systems. In chimps, females disperse while males live with related males and form alliances with each other against individuals in their own groups and against neighboring communities. A group of males from one community often fights males from another community. Bottle-nosed dolphins have a remarkably similar social system.
Dolphins have huge testes for their body size, which is indicative of sperm competition, which indicates that the females mate with several males. Researchers have not seen mating too much, but they've seen females consorting with up to thirteen different males in the season during which they conceived. Females first give birth at about 12 years old. They have a long period of infancy; a kid might still be suckling at like 5 or 6 years old. Mothers invest heavily in the kids, who are socialized during infancy and juvenilehood.
Dolphins have a fission-fusion social system; they live in a larger community but break into smaller parties for day-to-day life, just like chimps. However, they don't just have random associations; females have one or more other females that they're often found with but not always, while males have one of two other males that they are almost always found with. Female associations might be several females travelling together. Male alliances, however, are usually just one or two other guys. The male alliances cooperate to coerce females in estrous to stay with them.
The males work together to herd the female. Often they are seen directing aggression at her to get her to stay with them; biting, hitting, or threatening her. Sometimes she stays with them for weeks, but it doesn't always work, 'cause during the breeding season, females hang out with a few different alliances. The female often tries to escape, which is how they know it's coercion. Sometimes the males give chase, and sometimes they let her go.
There are two levels of male alliances- pairs/triplets often join together with other pairs/triplets to steal females from other alliances. They'll charge into other alliances and grab the female. Sometimes she cooperates with them and sometimes she doesn't. Females also form alliances with each other, and he thinks that maybe they're helping each other to get away from the males 'cause they usually form these coalitions when one is in estrous.
There's also the anecdote about two vervets having a fight. Right in the middle of the fracas, one yelled "leopard!" and the fight broke up.
A Warning: It's very easy for us to attribute motivation, personality, and intentionality to things. We name our hurricanes, for goodness sakes! We have to be careful we're not doing this by accident when we're looking at smart animals.