Hilltopping is a strategy used by many species of butterfly to find mates! It is also used by some other insects, but butterflies are the most notable hilltoppers.
Males fly up to the top of a hill. They are usually the older and more mature males. They compete with each other to get the best territory. They sometimes stay at the top of the hill for a long time. The young males tend to stay at the base of the hill, where the most important activity is searching for food.
When a female needs a mate, she also flies to the top of a hill. Hopefully, she will have several males to choose from. The “best” male, from a genetic perspective, is likely to the one with a territory at the very top. Once she has mated, the female flies away. Usually, the plants on the hilltops are not the food plants for the caterpillars, and the hilltops are not suitable places for the female to to lay eggs.
The hills used for hilltopping are sometimes just a little bit higher than the surrounding countryside.
For butterfly species which are not common, and which are unlikely to encounter a potential mate by flying around randomly, hilltopping represents a cunning and efficient reproductive strategy. This has two consequences. (1) Hills have conservation value, and we should try to keep them intact, and it is a good idea to maintain corridors of natural vegetation to the surrounding areas. (2) Hilltops are good places to go and look for rare butterflies!
For butterfly species which are common and abundant, hilltopping makes no sense! As population density increases, the larger is the probability that a female encounters a male without needing to visit a hilltop! The best strategy for males is then to remain close to the food plants where the females lay their eggs.
A well-written short article on hilltopping is available here.
Although this is an oldish paper (long before PDFs, but you can find it here, in the Biodiversity Heritage Library), it is eminently readable, and remains an accurate account. Scott JA 1968. Hilltopping as a mating mechanism to aid the survival of low density species. Journal of Research on the Lepidoptera 7: 191-204.
If you are comfortable transferring human emotions to animals, hilltopping can be summed up at “Love in high places“.