Interactions between hawkmoths and flowering plants in East Africa: polyphagy and evolutionary specialization in an ecological context

Abstract

Hawkmoths (Lepidoptera, Sphingidae) are considered important pollinators in tropical regions, but the frequency and degree of reciprocal specialization of interactions between hawkmoths and flowers remain poorly understood. Detailed observations at two sites in Kenya over a two-year period indicate that adult hawkmoths are routinely polyphagous and opportunistic, regardless of their proboscis length. About 700 individuals of 13 hawkmoth species were observed visiting a wide range of plant species at the study sites, including 25 taxa that appear to be specifically adapted for pollination by hawkmoths. We estimate that 277 plant species in Kenya (c. 4.61% of the total angiosperm flora) are adapted for pollination by hawkmoths. Floral tube lengths of these plants have a bimodal distribution, reflecting the existence of two hawkmoth guilds differing in tongue length. Hawkmoths exhibited strongly crepuscular foraging patterns with activity confined to a 20-min period at dusk and, in some cases, a similar period just before dawn. Corolla tube length appears to act as a mechanical filter as the longest-tubed plants were visited by the fewest hawkmoth species and these were exclusively from the long-tongued guild. Tube length showed a strong positive relationship with nectar volume, even after phylogenetic correction, which implies that plants with long corolla tubes are under selection to offer relatively large amounts of nectar to entice visits by polyphagous long-tongued hawkmoths. Our study shows that diffusely co-evolved pollination systems involving long-tongued hawkmoths are clearly asymmetrical, with plants exhibiting a high degree of floral specialization, while hawkmoths exhibit polyphagous behaviour. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 110, 199–213.