Biotic interactions modify the effects of oxygen on insect gigantism

Size is one of the most significant and fascinating characteristics of all organisms. Its significance derives from the strong relationships, especially in animals, between body size and many physiological traits and ecological characteristics (1, 2). What makes size fascinating is its extremes. They not only compel wonder at deviation from the average for a given group (3), but also provide insight into the mechanisms underlying size variation (4). Among the insects, the giant forms of the late Paleozoic have perhaps captured most attention. Many species from this period were large by modern standards; an extreme case is a member of the aerial predator group, the Protodonata, with a wingspan of ca. 71 cm (5). Modern dragonflies have wingspans typically one-tenth of the size (6). How such gigantism could have arisen and why these large species subsequently disappeared have long been staples of biology (5, 7). Although several explanations for the rise and fall of insect giants have been proposed, the physiological limits to performance or fitness at a given size and the improvements (constraints) brought by higher (lower) atmospheric oxygen concentration (partial pressures) have garnered most attention (7, 8). In PNAS, the work by Clapham and Karr (9) uses a massive compilation of fossil data to show that biotic interactions, especially predation by birds, are plausibly responsible for a decoupling of size from oxygen concentration from the Early Cretaceous onward. Their findings bring a robust and fresh ecological perspective to the discussion of insect size variation. Unlike many other ectotherms, in insects, gas exchange takes place through a system of tubes, the tracheae, that lead directly to the cells if not the mitochondria, where oxygen acts as an electron receptor during respiration (10). The exceptionally high power demands …