In 1982, Daniel H. Janzen and Paul S. Martin advanced the hypothesis that a number of plant species we see in present-day forests shows fruits and seed dispersal adaptations not consistent with their interactions with extant frugivores. Thus, only when we consider the extensive frugviory shown by the extinct Pleistocene megafauna (horses, toxodons, gomphoteres, mastodons, macrauchenias, etc.) we can understand how these species evolved the fruit traits we see nowadays. How then, did these tree and shrub species persist in the absence of the animal mutualists they required for population persistence? The core of the hypothesis expects these anachronic dispersal systems to be best explained by interactions with extinct animals, showing impaired dispersal resulting in altered seed dispersal dynamics.
Janzen and Martin defined seed dispersal anachronisms as those dispersal syndromes with fruit traits and phenological patterns best explained by interactions with extinct animals and offered some striking examples of Neotropical fruits with anachronic traits. These ‘‘unfit’’ species share fruit traits and phenological patterns that are at least in part not expected from their interactions with the extant frugivore community, but logically explained if we consider the extinction or local absence of the main frugivores.
Key traits of megafaunal fruits include 1) overbuilt design, with large seeds protected mechanically by thick and hard endocarp and indehiscence, with nutrient-rich pulp and external similarity to fruits eaten by extant large African/Asian mammals; 2) phenological segregation of ripening times throughout the year; 3) fruits falling to the ground upon ripening; 4) fruits unattractive or not very attractive to arboreal or flying frugivores; 5) a large proportion of the fruit crop rots on the tree without being consumed; 6) frugivores include a large coterie of seed predators that might act sporadically as legitimate dispersers; 7) fallen fruits are avidly eaten by introduced horses, pigs, and cattle; and 8) natural habitats of the plant species today are alluvial bottoms on gentle slopes, usually along forest edges with grassland.
It is clear that functional dispersal for many of these species operates in present-day neotropical communities by means of diplochorous and alternative seed dispersal systems involving other agents such as scatter-hoarding rodents, tapirs, cattle, some primates and even bats, as well as haphazard (runoff) and human-mediated dispersal. However, the loss of seed dispersal by extremely large mammals may imply marked shifts in the patterns and consequences of seed dispersal for these plant species. Ongoing and future research should unveil the signals of the “ghosts of past mutualisms”.
Guimarães Jr., P.R., Galetti, M. & Jordano, P. (2008) Seed dispersal anachronisms: rethinking the fruits extinct megafauna ate. PLoS ONE, 3, e1745.
Janzen, D. & Martin, P.S. (1982) Neotropical anachronisms: the fruits the gomphotheres ate. Science, 215, 19–27.
Martin, P.S. & Klein, R.G. (1984) Quaternary Extinctions: a Prehistoric Revolution. University of Arizona Press, Tucson, AZ.