Fruit size and shape: Allometry at different taxonomic levels in bird-dispersed plants

Summary The likelihood that a plant's seeds will be dispersed by fruit-eating birds may depend upon the size and shape of its fruits. Assuming that elongate fruits can be swallowed more easily than spherical fruits of equal volume and that plant fitness is enhanced by seed dispersal by many individuals and species of birds, natural selection should favour increasing fruit elongation with increasing fruit size in bird-dispersed plants. According to this view, this allometric pattern would be adaptive. Alternatively, fruit shape in bird-dispersed plants may be constrained by development or phylogeny. To determine whether there was any evidence to support the adaptive allometry hypothesis, we examined allometric relationships between length and diameter in fruits and seeds in a group of neotropical bird-dispersed plant species. Using the major axis technique, we regressed ln(diameter) on ln(length) for fruits and seeds at various taxonomic levels: (1) within individual trees ofOcotea tenera (Lauraceae) (2) among 19 trees within a population ofO. tenera, as well as among pooled fruits from multiple trees within 20 other species in the Lauraceae, (3) among 25 sympatric species within a plant family (Lauraceae) and (4) among 167 species representing 63 angiosperm families within a plant community in Monteverde, Costa Rica. At most taxonomic levels, a tendency for fruit length to increase more rapidly than fruit diameter among fruits (negative allometry) occurred more frequently than expected by chance. Estimated slopes of the regressions of fruit length on fruit diameter were < 1 within 15 of the 19 individualO. tenera trees, among tree means withinO. tenera, among pooled fruits within 16 of the 20 other species in the Lauraceae, among species means within the Lauraceae and among means of all bird-dispersed species in the lower montane forests of Monteverde. Seed allometry showed similar patterns, although for both fruits and seeds the broad confidence intervals of the slopes estimated by major axis regression overlapped 1 in many cases. Among the 63 Monteverde family means, fruit length and diameter scaled isometrically. Based on measurements of ontogenetic changes in fruit shape in a single species,O. viridifolia, we found no evidence that negative allometry in fruit shape within the Lauraceae was an inevitable consequence of developmental constraints. Instead, increasing elongation of fruits and seeds in certain plant taxa is consistent with adaptation to gape-limited avian seed dispersers. Contrary results from vertebrate-dispersed species from Malawi and Spain may reflect differences between the New and Old World in plant taxa, seed dispersers or evolutionary history.