Coevolution of mistletoes and frugivorous birds?*

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Abstract
Small frugivorous birds that feed largely on the fruits of stem‐parasitic mistletoes have independently evolved in various parts of the world. Local populations of mistletoes may be dispersed almost exclusively by these birds. Four attributes of mistletoe dispersal systems may have enhanced the evolution of reciprocal dependence between mistletoes and specialized dispersers:(1) Safe sites for mistletoe seeds (i.e. the young branches of a compatible host) are precisely defined in space and time.(2) The viscidity of mistletoe seeds induces smaller dispersers to deposit seeds in safe sites.(3) Frugivores differ markedly in the efficiency with which they disperse mistletoe seeds to safe sites.(4) Relatively large viscid fruits and adaptive fruiting displays exclude or deter most members of the potential disperser guild.Some birds have anatomical adaptations as a result of dietary specialization on mistletoe fruit, and some mistletoes have fruiting displays that target specialized birds or a narrow disperser spectrum. Coevolution between guilds of mistletoes and specialized dispersers is therefore probable. The uncoupled selective pressures that might have driven their coevolution are the mistletoes’ provision of fruit crops that are unavailable to more generalized frugivores, in return for seed dispersal to the small stems most suitable for infection. As in other mutualistic seed dispersal systems, phylogenetic, ecological and life history factors constrain the evolution of monophyletic interdependence, resulting in varying degrees and patterns of reciprocal specificity between mistletoes and dispersers.
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