Cumulative germination of seeds ingested by black howler monkeys

Premise of the Study: Primates are important seed dispersers, especially for large-seeded (> 1 cm length) tropical species in continuous and fragmented rainforests. Methods: In three forest fragments within the Montes Azules Biosphere Reserve, southern Mexico, we investigated the effect of howler monkeys´ (Alouatta pigra) gut passage on the germination rate and maximum germination (%) of native large-seeded species. One group of howler monkeys, per fragment, was followed and fresh feces collected. Large seeds were removed to compare their germination success with non-ingested seeds of the same species collected underneath parent plants. Results: Feces contained large seeds from seven tree species and one liana species: Ampelocera hottlei, Castilla elastica, Dialium guianense, Garcinia intermedia, Pourouma bicolor, Spondias mombin, Trophis racemosa and the liana Abuta panamensis. Except for G. intermedia, ingested seeds showed significantly greater germination rates than non-ingested seeds. Non-ingested seeds of D. guianense showed negligible germination. Regarding maximum germination, ingested seeds of D. guianense, P. bicolor, S. mombin, T. racemosa and A. panamensis showed significantly greater germination percentage; while G. intermedia showed significantly lower germination percentage and no effect of seed ingestion was observed on A. hottlei and C. elastica. Conclusions: In general, seed ingestion by howler monkeys confers greater germination rates than non-ingested seeds. Greater germination rates reduce predation probabilities and increases seedling establishment in forest fragments. Primate dispersal services contribute to germination heterogeneity within plant populations of old-growth forest species and to their persistence in forest fragments.