In 1975, McKey hypothesized the existence of two seed dispersal syndromes. One centered on specialist avian frugivores that rely primarily on fruit for their nutrients and the other on opportunistic avian frugivores, including all partially frugivorous bird species. Plants with large seeds would rely on specialist frugivores for seed dispersal because specialists are often large and can presumably handle larger seeds. Large seeds would be embedded in nutritious pulp to attract the fruit-dependent dispersers to ensure that carrying such large seeds would be energetically cost-effective for the birds. Plants dependent on dietary generalists would have small fruits with small seeds, to maximize the number of potential disperser species. Because those generalists would not depend upon fruit to meet their nutritional needs and because the seeds would be small, the pulp provided could be far less nutritious and still attract dispersers. I examined McKey’s hypotheses by comparing the pulp rewards offered by fruits with seeds of different sizes. Measures of pulp quality used were grams of protein and metabolizable energy/pulp/dispersal unit.
I also examined an alternative, although not mutually exclusive, hypothesis that nutritionally rich pulp is primarily a packaging adaptation that allows a plant to maximize the amount of nutrient reward offered to a disperser while minimizing the volume and weight of that reward, so as to maximize the number of species capable of dispersing seeds of a particular size, especially large ones. For this analysis, I compared pulp weight to seed weight and pulp width to seed width among plants with seeds of varying size and reward.
Bird-dispersed fruits of ca. 100 species were collected from lowland tropical forests in Peru and Mexico and weighed and measured, as were their component parts. Water and nutrient contents were analyzed, and energy and protein content/pulp/dispersal unit were calculated.
Results/Conclusions
Results indicate that energy and protein content/pulp/dispersal unit increase with seed weight. However, amounts per gram wet weight of seed do not, but rather decrease as seed weight increases. Pulp width also decreases as seed weight increases along axes of the dispersal unit most critical for swallowing by a disperser. This suggests that producing a resource-dense pulp can minimize the size of the pulp without decreasing its nutritional content thereby maximizing the number of potential disperser species, particularly for large seeds.