Co-evolved plant–animal interactions play a vital role in ecosystem functionality and the persistence of biodiversity. Through natural selection, plants have evolved a variety of dispersal units and mechanisms to overcome mobility issues, the most common of which is to utilize animal agents to facilitate zoochory – dispersal by animals – including endozoochory (inside the gut), epizoochory (outside on fur and feathers) and hoarding (where the squirrel puts his nuts). For example, up to 90% of the plant species in tropical forests disperse seeds via animal agents. Many dry-fruited plants produce appendages (e.g. hooks and hairs) on their reproductive structures to facilitate epizoochory, i.e. external dispersal by animals. Birds and monkeys often act as agents of epizoochory, by carrying infructescences to remote perches to peck or chew on the pulp. The predominant plant dispersal strategy is, however, to produce diaspores that elicit endozoochory, i.e. the consumption and subsequent reappearance of viable seeds by embedding seeds in fleshy pulp. Lush foliage can also attract herbivores to small-fruited herbaceous plants, to promote endozoochory.
A recent paper in Annals of Botany examines why the Chinese raisin tree, Hovenia dulcis, produces enlarged, fleshy peduncles with outer, dry drupes and investigates how this dry-fruited plant disperses its seed. In contrast to previous studies reporting that fleshy appendages entice bird epizoochory, field observations combined with experimental manipulations confirm that H. dulcis peduncles are adapted primarily to achieve mammal endozoochory, with viable seeds being found in the faeces of species such as bears and martens. The germination rate of egested seeds is higher than that of unconsumed seeds, demonstrating a mutualistic association similar in function to production by the plant of fleshy pulp or foliage.
Peduncles elicit large-mammal endozoochory in a dry-fruited plant. Annals of Botany (2013) 112 (1): 85-93. doi: 10.1093/aob/mct096
Plants have evolved a variety of seed dispersal mechanisms to overcome lack of mobility. Many species embed seeds in fleshy pulp to elicit endozoochory, i.e. disseminating seed through the animal gut. In contrast to well-studied fleshy fruited plants, dry-fruited plants may exploit this dispersal mutualism by producing fleshy appendages as a nutritional reward to entice animals to swallow their diaspores, but this has been little studied. In this study, it is hypothesized that these accessory fruits represent. Field observations (focal tree watches, faecal surveys and fruiting phenology) with experimental manipulations (examination of seed germination and feeding trials) were conducted over 2 years in a native population of the raisin tree, Hovenia dulcis, which produces enlarged, twisted brown peduncles with external black seeds, in central China. Birds were not observed to swallow seeds or carry infructescences away during 190 h of focal tree watches. However, H. dulcis seeds were detected in 247 faecal samples, representative of two herbivore and four carnivore mammalian species. Feeding trials revealed that peduncles attracted mammals to consume the entire infructescence, thereby facilitating effective seed dispersal. The germination rate of egested seeds proved higher than that of unconsumed seeds. It was also noted that this mutualism was most vulnerable in degraded forest. Hovenia dulcis peduncle sets are confirmed to adapt primarily to mammalian endozoochory, a mutualistic association similar in function to fleshy pulp or foliage. This demonstrates that plant organ systems can be adapted to unique mutualisms that utilize animal dispersal agents. Such an ecological role has until now been attributed only to bird epizoochory. Future studies should consider more widely the putative role of peduncle sets and mammalian endozoochory as a dispersal mechanism, particularly for those plants that possess relatively large accessory fruits.