MU biologist Rex Cocroft studies communication, something crucial to life at many levels, as it occurs within a cell, between cells, and between organisms within social groups. "Once we reach the level of communication between individuals," waxes Cocroft, "not only is there the fascinating intellectual challenge of studying communication, but there is also this tremendous aesthetic appeal…. The signals themselves are often beautiful—the songs of whales, the colors of butterfly wings, the scents of flowers." His first calling was that of a musician, so it's perhaps no surprise that Cocroft was drawn to this aspect of biology, and no accident that he enjoys being at MU. "I love it here [in Missouri] in the late summer," he says, "when the katydids and the cicadas are out and there's this din of calling insects."
These are insects that we as humans can hear. But actually the most common form of insect acoustic communication—plant-borne vibrational communication—is inaudible to us. Oscillating a part of the insect's body, those vibrations travel through plant leaves and stems, to be detected by the "vibrational ears" (actually legs) of another insect on the same plant. As Cocroft notes, "it's amazing to place a microphone on the plant…to hear these sounds." The myriad tones sweep up and down in complicated patterns: "There's a whole telephone network of signaling going on out there that we're just not aware of."
Beyond its innate beauty, such communication is crucially significant for the biology of organisms. In fact, the development of the signals seems to have much to do with the evolution of the species itself. By focusing on a particular kind of plant-feeding insect as an evolutionary model, Cocroft's research seeks to understand its songs as they influence species formation. His study animals are tree-hoppers, in particular a group of 12 species known as the Enchenopa binotata complex that are Missouri residents. Tree-hoppers are intimately adapted to the host plant on which they live; "their whole life-cycle is timed to the progression of their plant," explains Cocroft admiringly. In the fall the female lays eggs in the bark; later, in the spring, the tree's sap rising triggers their maturation and hatching just as the leaves begin to emerge, providing new growth for the nymphs to eat.
The immediate local environment supports the entire spectrum of these insects' activities. As Cocroft puts it, "not only do they live on the plant, feed on the plant, mate on the plant, and lay their eggs on the plant, but they communicate through the plant stems and leaves themselves. The plant is everything to them." As a matter of fact, the host plant also appears to be key to these insects' diversification, revealing the process of species formation. Consider the consequences of a "host shift" – that is, a relatively rare event during which insects that are specialized to live on one kind of plant manage to colonize a different kind of plant. When this happens, the insect populations on the old and new plants don't remain a single cohesive gene pool; they diverge from one another. The species of tree-hoppers with which Cocroft works could become a model for the study of how communication acts as a trigger for the process of speciation.
In addition, being resident on a particular plant has consequences for the evolution of mating signals. If a group of insects colonizes a new plant, a major wrench is thrown into their communication system, demanding adjustments and leading to change. For example, tree-hoppers engage in a system of courtship that starts from long-range. The male produces a mating call via vibrational signals and the female answers back. The male then replies in turn and the pair engage in a duet. Next, the male locates the female and they engage in a closer-range courtship. As it turns out, the female tree-hopper cares a great deal about the sounds produced by potential mates; in fact, she's very selective. She lets the males do a lot of signaling over a long period of time (from two to twenty-six hours), an activity that is physically demanding for the males and seems to function as a sort of endurance test. "It may be that the last one standing and still signaling [is] the one that mates," Cocroft explains.
As for social activities, these insects are at present poorly understood. "People often say that tree-hoppers are beautiful but boring," Cocroft observes, that they "just sit on a plant their whole life." Yet these insects prove to be very social, living in groups and demonstrating intricate interactive behavior, including a fascinating communication system. Beyond the once-in-a-lifetime courtship rituals, they also engage in other kinds of social interchange. Consider, for example, the female thornbug, Umbonia crassicornis, who lays only one clutch of eggs (about 100 of them) and stays with her family while the eggs hatch, continuing to protect the immature nymphs that are vulnerable to predators as they sit on a stem and feed for weeks. As for their communication, "these groups of immatures are more analogous to a nervous system, where each individual is constantly monitoring the environment. When it detects some change, like the approach of a predator, it signals. Then, like a wave at an MU basketball game, it spreads through the group—a big vibrational shout every second or so." In response, the mother charges to locate the predator and try to scare it away, signaling afterwards to reassure her young. Clearly the communication strategies of tree-hoppers are intimately related to their lifestyle as plant-feeders.
Another species of tree-hoppers, these located in Panama, talk not about predators or mating but about the plant itself. This species lives in family groups of 40-50, feeding on the new growth when it is most tender and vulnerable. When the leaf matures, the family discusses that situation and decides to find another leaf. What eventuates is a sort of group decision-making process in which a few scouts go out to find another leaf, calling to the rest with vibrational signals transmitted through the plant – something to the effect of "Get your fresh leaves over here!" These signals "galvanize the behavior of the group," which bit by bit coalesces on the new spot. Vibrational communication thus helps solve some of the practical problems of living on a plant. As Cocroft explains, "this very cooperative form of communication allows them to more efficiently exploit their host plant."
After learning a bit about insect communication during the interview, I was inspired to step outside and marvel at this hidden world—listening to its music. Cocroft adds that a side benefit of studying vibrational insect sounds is the opportunity to appreciate the sheer wonder and diversity of the calls: there are so many species communicating this way, and so few of them studied, "that you could literally go into your backyard and hear some neat signal that no human has ever heard before." Furthermore, because insects are the most important feeders of green plants, "they are incredibly important ecologically and we ought to be keeping track of what they're doing, understand why there are so many, understand more about their relationship with host plants and how they influence each other's evolution."