Orangutans bite back

Life as a seed isn’t easy: you need to be tough enough to deter all but the most muscular-jawed predators but not so hard that you can’t germinate.

A new study published this week in Journal of the Royal Society Interface shows just how fine this evolutionary balance between protection and reproduction is.

A team, including Susan Cheyne of Oxford University’s WildCRU, analysed the properties of the seeds of the plant Mezzettia parviflora (Annonaceae) and the effort that seed predators, such as orangutans, have to put into cracking them open.

‘The intricate architecture of the Mezzettia parviflora seed allows its germination while impeding both small predators such as weevils and large ones like orangutans,’ Susan tells us.

‘Orangutans open the seed by biting into the germination bank and cracking the wooden plug, the weaker part of the seed through which the stem of the germinating seedling emerges.’

Field observations by Susan and colleagues of orangutans in Sabangau, Borneo, show that whilst orang-utans consume an average of about 120 seeds per day (up to a maximum of 1001) the jaw strength they have had to evolve to accomplish this task is formidable: the force their jaws deliver is equal to the weight of up to six people bearing down on the seed.

So is all this effort worth it? ‘The seeds contain a small amount of a lipid-rich substance which is very high in energy, so worth the effort to break not only the seed but the hard outer shell of the fruit,’ Susan explains. ‘The toughness of this fruit and seed prevents consumption by other primates, for example gibbons, who lack the jaw strength to open the seeds.’

The research is thought to be the first to show that the mechanical properties of a seed play a central role in stabilising the arms race between seeds evolving armour for protection and the predators evolving a way to open a nutritious snack.

Dr Susan Cheyne is a member of WildCRU, part of Oxford University's Department of Zoology.