Reproductive seasonality in tropical satyrine butterflies: strategies for the dry season

Abstract.

• 1 In tropical savanna environments rainfall is often very seasonal, so that much of the year is characterized by a long and unpredictable dry season. Because the timing and availability of rain exerts a major influence on plant growth and production, many species during the dry period exhibit dramatic reduction in leaf quality. Accordingly, and kind of behaviour shown by phytophagous insects that synchronizes larval feeding with food availability will be adaptive.
• 2 The reproductive status of three Mycalesis butterflies was monitored over a 2-year period (1989–90) in north-eastern Queensland, Australia, at a lowland site (Cardwell, 18°16's, 146°02′E) which experiences a pronounced dry season. Females of these species and of five other satyrines (Ypthima, Hypocysta spp.) were also examined less intensively during the dry season in areas throughout northern and central Queensland, north of the tropic of Capricorn.
• 3 These relatively sedentary butterflies exhibit three different strategies for dealing with the unpredictable dry period and associated deterioration of larval food plants (grass). First, five species appear to breed continuously, though for most reproductive activity (mature egg number) declines markedly in the late dry season. Two of these (Hypocysta irius, H.metirius) are restricted to less seasonal and more favourable (wetter) areas but the three others (Ypthima arctous, H.adiante, H.pseudirius) occur widely in the relatively dry savanna, where they may specialize on grass in moister microenvironments. Second, two species (M.terminus, M.sirius) live in predictably moist habitats which are buffered from climatic extremes; they breed for much of the season but reproductive activity declines as the dry season progresses and may cease late in the season. Third, one species (M.perseus) is more opportunistic, breeding for only a limited interval during the favourable (wet) periods; during the long dry season adults contract to moist refugia and remain in reproductive diapause.
• 4 Spending the late dry season as an adult, either in diapause or with mature eggs, may improve the capacity to utilize new growth of grasses at the start of the favourable season, thereby enhancing population growth during good times. It may also provide additional flexibility to counter the temporal uncertainty of the dry season.
• 5 The strategy of residing in more equitable habitats or specializing on predictable foods may be the most restrictive in terms of distribution.