Original Article

Heredity (1990) 65, 401–406; doi:10.1038/hdy.1990.110

Predator induced colour polymorphism in Danaus plexippus L. (Lepidoptera: Nymphalidae) in Hawaii

John Stimson1 and Mark Berman1

1Zoology Department, University of Hawaii, Honolulu, H.I. 96822, USA

Received 27 March 1990.

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Abstract

On the island of Oahu, Hawaii, the Monarch butterfly, Danaus plexippus (Lepidoptera: Nymphalidae) has two distinct and genetically determined wing-colour morphs, white and orange. The white morph has increased in frequency in the last 20 years, perhaps because of the predation on the orange form by the introduced birds Pycnonotus jacosus and Pycnonotus cafer, bulbuls. These birds are the only insect-eaters whose distributions coincide with that of white monarchs in Hawaii. The birds are not deterred by the cardiac glycosides in the monarchs' tissues. The orange form may be at greater risk of predation because it stands out against the foliage and infloresences of the milkweeds where the butterflies oviposit and feed in large numbers, and where predation commonly occurs. Several observations are consistent with the hypothesis that whites are more cryptic. A larger proportion of white butterflies are captured by netting at a particular time than eclose during the same period. Secondly, monarch wings found on the ground at the study sites and presumably broken off by the predators were almost all orange, again suggesting that the risk of predation is lower for white individuals. Lastly, the three other major Hawaiian islands which have been surveyed have orange monarchs but lack both bulbuls and white monarchs. These results suggest bulbuls exert a strong selection against the orange morph and are probably the cause of the increase in frequency of the white morph.

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