Marsupial dispersal of mistletoe seeds in the temperate forest of southern South America

AMICO, GC; AIZEN, MA

San Pedro, San Pablo, Brasil

Simposio; 3rd International Symposium-Workshop on Frugivores and Seed Dispersal; 2000

Mistletoes are shrubby stem-parasitic plants for which dispersal represents a critical link in their life cycle.  Birds have been described so far as the only true animal dispersers of the two most species-rich mistletoe families, Loranthaceae and Viscaceae.  Efficient bird dispersers eat mistletoe fruits, and transport, defecate, and deposit the sticky mistletoe seeds on living branches of an appropriate host.  In the Lake District of southern Argentina, however, we found that the arboreal marsupial Dromiciops australis is the exclusive disperser of Tristerix corymbosus (Loranthaceae), a mistletoe that in our study area produces green fruits.  This animal is a small, nocturnal mammal of Gondwanan origin, presently restricted to the temperate forest of austral South America.  First, we never observed avian consumption of T. corymbosus fruits, a fact corroborated by the absence of mistletoe seeds in the stomach content of ca. 200 frugivorous birds.  Second, fruits are removed at night and seeds defecated in patterns that may rarely be attributed to bird behaviour.  Third, 84 % of a total of 57 live-captured D. australis defecated seeds of this mistletoe before removing them from the traps.  We did not find evidence of other mammals dispersing T corymbosus seeds in the forest.  D. australis is also a highly efficient mistletoe disperser.  This marsupial was observed to swallow the fruits whole after peeling the exocarp off with the aid of their forehands, defecating >95 % of the ingested seeds undamaged.  We also estimated that about 90 % of all defecated seeds by D. australis were deposited on the branches of its preferred host.  Most importantly, the passage of seeds through the animal’s gut is critical for seed germination and development of the adhesive disc.  We estimated that this peculiar mistletoe-marsupial mutualism might be >60 million years old.  Our finding has implications for the origin of animal seed-dispersal in one of the most diverse groups of angiosperms.