The Influence of Age on Male Mate-Searching Behaviour in Thornbug Treehoppers

One prediction from life-history theory is that males should increase investment in reproductive effort as they age because the opportunity for future reproductive events declines. However, older males may not be able to increase their reproductive effort if condition declines with age. The effect of age-related changes in condition may be especially important for energetically costly activities such as moving within and between habitat patches while searching for mates. Although such searching is a component of many mating systems, the relationship between age and active mate searching has not been investigated. We investigated whether mate-searching effort increased with age in the thornbug treehopper, Umbonia crassicornis (Hemiptera: Membracidae). In this species, males search for females using a ‘fly-call-walk' strategy consisting of three phases: (1) flying from one plant to another; (2) walking and signalling while on a plant; and (3) close-range courtship of encountered females. We measured several aspects of mate-searching behaviour over the month-long period of a male's reproductive lifetime. Over the relevant period of male sexual activity (19-33 d), male condition remained stable. However, older males (25-33 d) did not search more actively than younger males as predicted; instead, younger males (19 d) had greater plant-to-plant flight activity and found females faster. Within-plant walking rates and courtship duration did not differ among age classes. These results suggest that thornbug males may be investing so heavily in mate searching at younger ages that they are unable to increase investment in searching effort when they get older. As a result, older males are likely to be at a competitive disadvantage when active searching is required to locate sparsely distributed females.