Red-tailed guenons (Cercopithecus ascanius) and Strychnos mitis: Evidence for plant benefits beyond seed dispersal

Abstract

I report data collected on red-tailed guenon (Cercopithecus ascanius schmidti) fruit processing behaviors between June 1993 and April 1994. Red-tailed guenons consumed the fruit of Strychnos mitis in 542 of 2,930 fruit-eating events (FEEs). The monkeys spat out cleaned seeds of Strychnos mitis in a majority of these records (477/542; 88%); seeds were occasionally swallowed whole, but only when pulp was unripe (69/542; 12%). In 83% of the FEEs on Strychnos, the red-tailed guenons spat out seeds within 10 m of the removal site; they typically stayed in the same tree while processing fruit, and in 56% of the FEEs, they moved <1 m before spitting seeds. I monitored spat seeds to evaluate the impact of monkey fruit processing on seed fate. Results indicate that 83% of seeds spat out by the red-tailed guenons germinated, while only 12% of unprocessed seeds survived to germination (p < 0.01). Of the processed seeds that germinated, 60% survived to germination and seedling establishment, while only 5% of unprocessed seeds survived to seedling establishment (p < 0.01). Unprocessed seeds were also more likely to be attacked by seed predators (p < 0.01) and fungus (p < 0.01). Although there is generally high mortality in seeds/seedlings, mature trees of Strychnos mitis are found in groves of adults, under which dense populations of seedlings and saplings can occur. These data suggest that Strychnos mitis does not conform to expectations of the Janzen-Connell model of seed escape from parent trees. Instead, I suggest that by removing pulp, a process that results in a reduction of fungal pathogen attack, red-tailed guenons positively effect the seed survivorship of Strychnos mitis. Although this effect has been observed in pulp-cleaning ant species, it is a hitherto undescribed effect of primates on their fruit resources.