Species Saturday

Species Saturday #17 : Why orang-utans like big (big) bananas!; Pisang-pisang besar.

Mark Harrison, OuTrop’s Managing Director, has spent more hours with orang-utans than the average academic! Having enjoyed many hours on the Sabangau OuTrop research grid, here he tells us about orang-utan’s healthy appetite for an OuTrop-famous species – Pisang-pisang besar!…

The pisang-pisang besar seedling grows out of it’s protective
casing which remains on the top of the shoot during germination.

“Everybody knows that many primates, including the orang-utan, like eating bananas in captivity. Most people could relate to that – bananas are both sweet and nutritious. Unfortunately for Sabangau’s orang-utans, the rainforest fruits typically available to them (and other primate species in the area) are typically much less nutritious. This means that orang-utans have to spend the bulk of their day feeding: around two thirds(!) of their time awake each day is spent busily munching away. See some OuTrop footage on orangu-tans munching here!From the hundreds of known and potential foods available to orang-utans in Sabangau, the most eaten is a fruit called “pisang-pisang besar” in the Indonesian language (Family Annonaceae: Mezzetia leptopoda / parviflora). Consumption of this fruit comprises a mean of around 15% of time spent eating. In English, this Indonesian name literally translates as “banana-banana big”, but unfortunately for the orang-utans, this refers to the curvy shape of the trees branches, rather than the nutritional value of its fruits! As well as having an amusing translation, pisang-pisang besar fruits and their seeds are also scientifically very interesting. The fruit’s pulp is rather fibrous, bitter and not particularly nutritious, and its seed is an incredibly hard stone that we mere humans can only open with repeated hammer blows.So why do orang-utans eat so much of it?..
The answer lies in the lipid (fat) rich embryo that is found inside this stone: these lipids make the embryo one of the nutritious fruit foods available to Sabangau’s orang-utans. Although our attempts at breaking into these stones with hammers against the railway running through camp are laughable, orang-utans have immensely strong jaws and teeth, which allow them to open the stones with remarkable ease. Indeed, in what may be a World Record longest feeding bout for any orang-utan anywhere (any challengers to this title, let us know!), one of our flanged male orang-utans was once observed cracking open and eating these seeds continually for over 14 hours! How he avoided jaw cramp/failure we don’t know, but he then managed to get up and eat more fruits from the same tree for 2 hours first thing the next morning!

Orang-utan feeding on this species represents one example of seed “predation”, where seeds are destroyed by the consumer, rather than dispersed. This is clearly undesirable for the plant and pisang-pisang besar protects itself (and its precious embryo) well from most other potential consumers through this immensely hard seed shell. It’s just the tough orangu-tans that get through this protection – gibbons and kelasi are not up to the challenge! This hard shell presents a challenge for the seed, however: how can the embryo germinate and break out of the seed when protected by such an impenetrable shell?Research by OuTrop scientists and colleagues (Lucas et al., 2011, see OuTrop paper here!) has demonstrated that the mechanical properties of this seed are actually exquisitely balanced between these conflicting demands of avoiding predation and allowing germination. This is achieved through a complex heterogenous micro-structure (meaning the structure is not continually the same). This structure means that the seed is resistance to damage during germination. The shell cracks via an increase in internal turgor pressure (i.e. pressure from internal contents pushing against the shell wall) during germination, but the seed is not damaged. Both complicated and clever!Work in our Sabangau Seedling Nursery has also shown that, unfortunately, this species is not the most suited for peat-swamp forest restoration efforts, with a mortality rate of over 65%. Seemingly intact seeds of this species have been seen in burnt areas of peat-swamp forest when all other vegetation has disappeared, but it is unknown whether these seeds are able to germinate and survive. So it looks like this species will be continuing to rely on the odd dropped seed by orang-utans and accidental swallowings by larger forest-floor animals to do its seed dispersal work for it!”Thanks Mark! Who knew orang-utans had such demands on their jaws! Find out how you can contribute to OuTrop and conservation in Borneo here!
Dr Mark Harrison, Managing Director
Lucas P. W., Gaskins J., Lowrey T., Harrison M. E.,Morrogh-Bernard H., Cheyne S. and Begley M. (2011) Evolutionary optimization ofmaterial properties of a tropical seed. Journalof the Royal Society Interface. 9:34-42.