Seed dispersal that’s hard to swallow

If you eat a fruit, such as an apple (Malus domestica), that contains seeds, you have a dilemma. Do you swallow the seeds, or spit them out? If you swallow them you may be concerned that they’ll germinate inside your body and you’ll end up with an apple tree growing out of your head.

If you spit them out, you risk being told off by those around you because – in polite company, such as a band of brotherly/sisterly botanists – spitting is considered rude. These are matters of delicacy and etiquette for heavily farmed fruits whose future is largely assured by farmers who tend such crops. But when it comes to plants in the wild, spitting out seeds, for all its unattractiveness, can be a matter of life or death for some species. Especially if the plant is Morocco’s Argania spinosa (the argan tree), a medium-sized, thorny, evergreen tree, 8-10 m high.

In an open-access study, Miguel Delibes et al. report that domestic goats (Capra hircus) eat the fruit of the argan tree, but spit out the seeds. Depending upon how far from the parent the seeds are spat, and how suitable are the conditions there to support germination, seedling establishment and growth, this behaviour may help to secure a new generation of the trees. And that’s important because the argan tree is not only ecologically and economically important in southern Morocco, but argan forests also serve as an effective barrier for the Saharan Desert [a natural example of the sub-Saharan Great Green Wall], and provide local people with wood, fodder for livestock, cooking oil, medicine, and cosmetic materials.

Spread of the seed further from the tree is no doubt assisted by the tendency of the goats to climb the trees to take fruit directly from the branches and ‘spit’ from on high. Now, those in the know about such matters, may wonder why the goats don’t swallow the seeds and allow them to be removed from their digestive tract amongst their faeces – in the process known technically as endozoochory. The answer has nothing to do with any goat fear of argan trees growing out of their heads, but is rather more straightforward. The seeds are too large (on average, 22 mm long and 15 mm wide), to permit their being swallowed and ‘voided with/in the faeces’ safely. Like the argan trees, goats are also valuable and to lose any through choking to death on an argan nut is not good for the farmer – or the goat! Entitled “Tree-climbing goats disperse seeds during rumination”, Delibes et al’s study is definitely something to think about.

But, there is also a down-side to goats. Feral goats have been blamed for destroying 1000 native trees planted by children in the Te Peka Lookout and Reserve in Taumarunui (North Island, New Zealand). And such capricious behaviour doesn’t stop there; apparently, the wild goat population has built up to the point where it also poses a traffic hazard. No kidding!

[Ed. – a plant example of such a seed-spitting dispersal is Ecballium elaterium. Known as the squirting cucumber, it is so-called because its seeds are squirted out from the fruit at speeds of up to 95 km/hour, and at distances of up to 6 metres from the parent plant.]