How does intellect, rather than physical strength, enable the boys to survive on the island?
In my opinion, it is intellect, rather than physical strength, that allows the boys to survive. I think it is always this way with human beings -- after all, we are not very strong or fast or anything when compared with animals.
The boys use their intellects to do things like building shelters. They also use their intellects to make spears (no animal can do that). Finally, the use their intellects to make fire, which allows them to cook when they finally do kill a pig.
So, their intellects allow them to stay warm and to find food. Their physical strength really doesn't do much for them. (And, after all, they don't have much strength because they're so young.)
On the island the boys use intellect to survive. At the age the boys are, 6-12 years, they would not have a great deal of brawn to begin with. I agree with the first editors comments.
Survival for humans is dependent on intellect. The boys would have had to have some prior knowledge in order to make the spears, plan a fire, etc. Those things do not occur when someone has no intellect.
Simon uses his intellect to think out the issue of the beast. It is unfortunate that he appeared through the brush at an inopportune moment. Ralph uses his intellect to find a place to hide from Jack's tribe. It took intellect for Roger to know how to use a lever to launch a rock off the top of the mound in order to get Ralph out of the hiding place.
Maurice and Piggy used intellect to try and calm the littleuns.