The marula (Sclerocarya birrea (A.Rich.) Hochst., Anacardiaceae) has a strongly lignified endocarp or stone which contains several seeds, each of which is within its own locule that is sealed by an individual operculum (Figure 1). The strong casing prevents germination, not by preventing the passage of water to the seeds, but by preventing oxygen from reaching the seeds (von Teichman et al. 1985). It is well known that marula fruits taken from the dung of the African elephant (Loxodonta africana Blumenbach) have more rapid germination than those that have not been eaten by elephants (Dudley 2000, Lewis 1987). This positive impact of elephants on marula germination continues to be considered to be through acid treatment in the digestion system of the elephant (Helm et al. 2011). We hypothesize that the primary mechanism which favours germination is mastication by elephants which physically loosens the opercula, rather than digestive dissolution of the stone. If true, the relevance of this is that only elephants would be the legitimate dispersers of marula seeds, because none of the many other species that are attracted to marula fruits would have jaws powerful enough to loosen the opercula.

We investigated the seed dispersal of Aglaia spectabilis, a large-seeded tree species in a moist evergreen forest of Khao Yai National Park in Thailand. Although one-to-one relationships between frugivores and plants are very unlikely, large-seeded plants having to rely on few large frugivores and therefore on limited disperser assemblages, might be vulnerable to extinction. We assessed both the frugivore assemblages foraging on arillate seeds of Aglaia spectabilis and dispersing them and the seed predator assemblages, thereby covering dispersal as well as the post-dispersal aspects such as seed predation. Our results showed that frugivores dispersing seeds were a rather limited set of four hornbill and one pigeon species, whereas two squirrel species were not dispersers, but dropped the seeds on the ground. Three mammal species were identified as seed predators on the forest floor. Heavy seed predation by mammals together with high seed removal rates, short visiting times and regurgitation of intact seeds by mainly hornbills lead us to the conclusion that hornbills show high effectiveness in dispersal of this tree species.