Hilltopping

One of the more interesting types of insect behavior is something called “hilltopping.” While hiking along a ridge in Tucson Mountain Park on Friday, March 5, I witnessed this firsthand.

Hilltopping describes the mating strategy of male insects of some species that will fly to the highest point in their immediate landscape. This may be a mountain, a butte, or even a small hill, provided it is sufficiently elevated above the surrounding terrain.

Once at the summit, the insects pursue one of several courses of action. Males of some species will defend a small territory such as a perch on a boulder, shrub or tree that affords the best view for spotting approaching females of the species. Competing males will be driven off. Butterflies in the family Lycaenidae, such as the Great Purple Hairstreak, Atlides halesus, and the Gray Hairstreak, Strymon melinus, both pictured below, defend perches.

Another method employed by males is “lekking,” in which several males congregate in an area where they display to females. Females then choose what they perceive as the fittest male specimen and mate with him. A “lek” is the term for the “stage” on which the males “perform.”

Still another form of hilltopping is when a male “patrols” a route along the summit of a hill or ridge, hoping to intercept a passing female. Many butterflies in the family Pieridae exhibit this behavior, also known as “scramble competition polygeny.” Males don’t have territories, but will actively compete when two of them spy the same female. The battle may consist of spectacular vertical flights, the two males spiraling around each other until one ceases to ascend further, conceding defeat. On my own hike, I noticed several males of the Desert Orangetip, Anthocharis cethura patrolling along the ridge, together with what I believe were Sleepy Orange butterflies, Eurema nicippe. Also seen at the summit of the ridge was a male West Coast Lady, Vanessa annabella, that repeatedly returned to the same patch of ground.

Besides butterflies, major hilltopping insects include several species of Hymenoptera (bees, wasps, ants) and Diptera (true flies). Flesh flies in the family Sarcophagidae, such as the one pictured below, were present on my afternoon sojourn.

To see the most variety of species, I would recommend spending all day at the very top of a likely hill, ridge, or mountain. The fauna will change as the hours pass and conditions change. Here in the desert at least, some species hilltop for only a few hours in the morning, then disappear. Other species are present all day, or may be most active in the afternoon.

Especially at this time of year, early spring, hilltops may be the only places to find an abundance of insects. Not only will you see hilltopping species, but other species that come to prey on them. I’m sure that is the only reason that dragonfly was up there, perching on a cactus of all things.

NOTE: I highly recommend reading the works of Dr. John Alcock for more about hilltopping insects. His books include Sonoran Desert Spring and Sonoran Desert Summer, both being collections of outstanding literary essays.