On the dispersal of seeds by mammals
H. N. Ridley, first Director of the Singapore Botanic Gardens, published this ecological study in 1894 in the Journal of the Straits Branch of the Royal Asiatic Society, drawing on extensive field observations across the Malay Peninsula to argue that mammals—though secondary to birds—constitute a significant and evolutionarily formative agent of seed dispersal in tropical forests. His central thesis is that the morphological characteristics of mammal-dispersed fruits (dull colouration, large size, smooth or slippery surfaces) represent adaptive responses to the specific foraging behaviours of monkeys, bears, squirrels, civets, and bats, and that these interactions have shaped the flora of the region as profoundly as insect pollination has shaped flowers.
Summary
Ridley opens by challenging the assumption that long-distance seed dispersal is universally beneficial, arguing that for unisexual species common in East Indian jungles (notably Euphorbiaceae, Sapindaceae, and Myristicaceae), carrying a seed too far from its parent population renders fertilization impossible. He extends this logic to plants dependent on specialized pollinating insects and to the risk of deposition in unsuitable habitats. This framing establishes the ecological problem that the rest of the paper addresses: how do mammals, constrained by their slower movement and limited visual acuity in dense canopy, serve as effective dispersal agents without carrying seeds beyond the viable range of the parent population?
The body of the article proceeds taxonomically through the mammalian orders of the Malay Peninsula, with Ridley’s own field observations providing the empirical backbone. He documents the foraging behaviour of the Kra macaque (Macacus cynomolgus) on Willughbeia, Dialium, Nephelium, and Eugenia; the bear’s (Helarctos malayanus) destruction of Durio fruit; the squirrel’s interaction with oak and chestnut acorns; and the civet’s role in coffee dispersal. A recurring analytical thread is the contrast between mammal-dispersed fruits (plain green, large, inconspicuous) and bird-dispersed fruits (scarlet, orange, small), which Ridley attributes to the different sensory modalities and flight altitudes of the two groups. His observations of Willughbeia in open sandy country near the Pahang River—where the fruit has evolved bright apricot-yellow colouration and reduced size to attract birds in the absence of monkeys—provide a particularly neat natural experiment supporting this distinction.
The article’s most extended analytical passage concerns the dispersal of oaks and chestnuts by squirrels. Ridley demonstrates that the smooth polished surface of Quercus lucida acorns, and the fine silky coating of Q. encleisocarpa and Q. Cantleyi, function as anti-handling adaptations: squirrels cannot grip the fruit firmly enough to consume it, so the seed drops and rolls away. He acknowledges this is an “expensive” strategy—many seeds are destroyed—but argues that the enormous crop size compensates. He closes with a brief treatment of adhesive fruits (rare in the Peninsula due to limited open country) and a sweeping evolutionary conclusion linking the evolution of grasses to graminivorous mammals.
Key Findings
- Ridley observed germinating Rambutan (Nephelium) seeds in the excreta of Sakai encampments in Pahang, confirming that human digestion does not destroy the seed and that the Malays considered swallowing the seed “the most wholesome and pleasant way” of eating the fruit (p. 16).
- The Musang (Viverra malaccensis) was recorded passing intact seeds of coffee, Gnetum scandens, Caryota Cumingii, and Mimusops elengi in its excreta; planters deliberately collected these seeds for cultivation on the belief they produced stronger plants (p. 19).
- Quercus encleisocarpa and Q. Cantleyi possess acorns coated in a fine silk with a “greasy feel” that makes them extremely difficult for squirrels to hold; Ridley observed Sciurus bicolor repeatedly failing to grip Q. lucida acorns, which rolled away from the cup when the squirrel bit the rim (pp. 25–26).
- Scoparia dulcis, a South American introduction, was traced by Ridley up the Pahang and Tembeling Rivers as far as the water buffalo (Bubalus arnee) ranged; on sandbanks where buffaloes had not gone, the plant was absent (p. 22).
- In dense jungle localities where monkeys are scarce, the ground beneath fruiting Willughbeia trees becomes “carpeted with seedlings” within days of fruit fall, yet nearly all perish within a few weeks from overcrowding and absence of light (pp. 14, 17).
- The article concludes with a tabulated list of approximately forty plant species with their identified mammalian dispersers, spanning genera from Adinandra to Zalacca (pp. 27–29).
Conclusion
Ridley’s definitive takeaway is evolutionary and hierarchical: just as the appearance of pollen-seeking insects drove the evolution of floral colour and form, and frugivorous birds drove the evolution of fruit colouring and sweetness, the evolution of mammals produced its own distinct suite of fruit modifications—dull colour, large size, smooth or slippery surfaces, and in some cases chemical deterrents against premature consumption. He further speculates that the development of grasses and other herbaceous plants bore a close relation to the evolution of graminivorous mammals, a claim he presents as a hypothesis for future investigation rather than a demonstrated fact.
Context
- Ridley held the position of first Director of the Singapore Botanic Gardens (1888–1913), and this paper draws directly on his fieldwork across the Straits Settlements and the Malay states, including observations in the Tahan Valley (Pahang), Mount Ophir (Singapore), and the Botanic Gardens itself.
- The paper is situated within the late-Victorian naturalist tradition of descriptive ecology, predating formal population ecology; its evolutionary arguments are framed in Darwinian terms but lack the quantitative apparatus of later dispersal studies.