Spitting Seeds From the Cud: A Review of an Endozoochory Exclusive to Ruminants

Abstract

Given their strong masticatory system and the powerful microbial digestion inside their complex guts, mammalian ruminants have been frequently considered seed predators rather than seed dispersers. A number of studies, however, have observed that ruminants are able to transport many viable seeds long distances, either attached to the hair or hooves (i.e., epizoochory) or inside their body after ingesting them (i.e., endozoochory). However, very few studies have investigated a modality of endozoochory exclusive to ruminants: the spitting of usually large-sized seeds while chewing the cud. A systematic review of the published information about this type of endozoochory shows a marked scarcity of studies. Nonetheless, at least 48 plant species belonging to 21 families are dispersed by ruminants in this manner. Most of these plants are shrubs and trees, have fleshy or dry fruits with large-sized seeds, and are seldom dispersed via defecation. Many cases have been observed in tropical areas, where more frugivorous ruminant species occur, but other records are from temperate and dry areas, covering thus all continents except Antarctica. Twenty-one species of ruminants from 18 genera have been reported as endozoochore spitters. They involve domestic and wild species belonging to the families Tragulidae, Cervidae, and Bovidae. This suggests that almost any ruminant species could potentially eat fruits and regurgitate large hard seeds during rumination. Likely, this seed dispersal mechanism has been neglected due to the difficulty of observing rumination behavior and locating spat seeds. Further research on the potential of wild and domestic ruminant species as long-distance seed dispersers through spitting seeds from the cud appears particularly important given their increasing pervasiveness and abundance worldwide.

Multiple External Seed Dispersers Challenge the Megafaunal Syndrome Anachronism and the Surrogate Ecological Function of Livestock

“…Under normal browsing conditions on pastures, leaves, and twigs, rumen vomiting in livestock is a rare pathologic process associated with the accidental ingestion of unusually large items such as stones and artifacts causing blockages and injuries to the digestive system (Van Soest, 1994). This agrees with our results showing that palm seed regurgitation from the cud while ruminating was apparently provoked by the ingestion of large numbers of palm fruits, for which seed defecation appears anatomically impossible for smaller livestock (sheep, goat) or unnecessary due to nutritional and physiological costs for larger livestock (Van Soest, 1994;Feer, 1995;Clauss et al, 2009;Delibes et al, 2019). Livestock ruminants chew their cud and ferment their food for variable periods of time (Van Soest, 1994), thus variably limiting seed survival through the upper gut passage (Janzen, 1982;Bodmer and Ward, 2006;Schwarm et al, 2009).…”

“…The presence of seeds was also recorded at outdoor resting sites used during daylight (cows) or in corrals used at night (sheep, goat) depending on livestock operations in each study area. In addition to feces, we also recorded the presence of seeds in regurgitations, especially cud-chewing regurgitations produced during rumination (Delibes et al, 2019). Dispersal distances were determined as detailed above for wild animals when seeds were found at random, but not when they were searched for specifically at resting sites.…”

“…This assumes endozoochory as the main process driving trade-offs between body size and dispersal ability of contemporary dispersers (Fleming and Kress, 2013;Jordano, 2014) and extinct megafauna (Guimarães et al, 2008;Pires et al, 2018). These assumptions underestimated potential selective pressures imposed by frugivores moving large fleshy fruits to consume the pulp without ingesting or predating the seeds (Stevenson et al, 2005;Baños-Villalba et al, 2017;Delibes et al, 2019), or to predate and disperse seeds while ingesting or discarding the pulp (Jansen et al, 2012). This is striking because stomatochory allows the movement of fruits and seeds that are much larger than those dispersed by endozoochory (Lambert, 1999;Stevenson et al, 2005;Blanco et al, 2016;Castañeda et al, 2018;Fuzessy et al, 2018), thus promoting selective forces other than gape or gut size in the evolution of large-sized fleshy fruit and their seeds.…”

“…In this context, assessing the role of livestock as seed dispersers of oversized fruits was proposed as a feasible approach to test the megafaunal hypothesis (Janzen, 1982;Janzen and Martin, 1982;Howe, 1985). To date, however, there is still little information on seed dispersal by particular livestock species like cows, horses, sheep, goats, and pigs, on their external or internal seed dispersal mechanisms, as well as on their impact on seed germination and recruitment of particular large-fruited plants (Janzen, 1982;Boone et al, 2015;Baños-Villalba et al, 2017;Delibes et al, 2019). The ways in which different livestock species exploit large fruits and disperse their seeds can be especially informative for understanding their current role in plant population dynamics, and to assess the past role of extinct megafauna in the evolutionary ecology of their presumed food plants.…”

“…This includes the determination of whether seeds are defecated (endozoochory), cleaned by stripping the pulp from the fruit in the mouth and then ejected (stomatochory), regurgitated after a variable period in the upper digestive system (endozoochory) or dispersed by a combination of these methods. This remains generally unknown for most livestock-large fruit interactions (Delibes et al, 2019).…”