Scientists recently discovered that there is a trade-off in intelligence as well--with great intelligence comes fewer offspring. At least in the case of guppies.
Alexander Kotrschal, a postdoctoral fellow at Uppsala University, and his colleagues bred two lines of guppies. Next, he dissected the fish and measured their relative brain size, then selectively bred those with the biggest and the smallest-brained fish to create two lines. After two generations the team had two groups of guppies with 10% difference in brain size in females and an 8% difference in males.
They then taught the fish to count by training them to look for food where a card had either two or four symbols. Kotrschal reported that the large-brained fish seemed to get it, but not the small-brained fish.
That primates have fewer offspring compared to other mammals and humans have fewer still, may be a consequence of the bigger brains, Kotrschal suggested.
It makes perfect sense because large brains are hard to maintain. Although the human brain accounts for less than 2% of a person's weight, it consumes 20% of the body's energy. A creature with larger brain could rely on its intelligence to survive, whereas a smaller-brained creature would have to rely on physical strength, or reproduce abundantly to ensure the survival of its species.
So if you have to choose between having high intelligence or abundant offspring, which would you choose?