Insect Territoriality and its Evolution: Population Studies of Desert Grasshoppers on Creosote Bushes

Abstract

However, territorial defence, a component of site-dependent behaviour is extremely rare in grasshoppers (Acridoidea). The only species now known to defend territories (among more than 150 species studied -Jacobs 1953; Otte 1970) isLigurotettix coquilletti McNeill, a gomphocerine grasshopper living on creosote bushes (Larrea divaricata Cav.) in the Sonoran Desert. Elucidation of factors selecting for territoriality is difficult because other species living on creosote bushes in both North and South America fail to display any territorial defence. Both Ligurotettix and another gomphocerine species, Bootettix argentatus Br., advertise their whereabouts acoustically by stridulating, and both feed on creosote leaves. Bootettix resembles creosote leaves in colour and remains on the foliage day and night, while Ligurotettix resembles the grey stems, resides on stems during the daytime and feeds on the foliage at night. Several Bootettix males commonly inhabit the same bush or even the same branch, whereas Ligurotettix males tend vigorously to defend bushes against other males and may even avoid occupation of adjacent bushes. Such behavioural differences are evidently responsible for the considerable differences in -population density between the two species. Goniatron planum Bruner, a close relative of Ligurotettix, lives on southern blackbrush (Flourensia cernua DC) in the Chihuahuan Desert (Tinkham 1948). This species also does not defend territories but appears to have characteristics which could easily lead to territorial defence, given a change in bush size. In Argentina several species of the proscopiid genus Astroma are host-specific on several Larrea species. They also display no territorial defence system and may reach densities in excess of 15 males per bush. Territorial defence in insects appears to be more common in groups which are capable of recognizing and returning to unique points in their landscape, e.g., Odonata (Jacobs 1955) and certain Hymenoptera (Lin 1963; Wilson 1971). But insects which remain in small, clearly bounded, and defensible places may also be territorial even though they are incapable of distinguishing among similar sites. Our objectives here are to present results of observations and experiments on populations of Larrea-inhabiting grasshoppers occurring in North and South American deserts and to discuss general features of territoriality in relation to distribution and density in insects. Marking experiments and manipulations of population densities were carried out mainly in Ligurotettix coquilletti, but observations on other species allow us to illustrate