Port Lympne Techniques
Technique of Leaves-with-stem
In all cases, a stem was selected from a bundle previously collected by the gorilla or provided by the experimenter (see Fig.2). Then, any inedible debris was picked out with a pinch grip of the hand not supporting the stem (n=4; that is, four individuals were noted to remove debris), before removal of the tough and often dirty root end of the stem. This was either detached by biting it off (n=5) or by using the two hands symmetrically against each other (n=6), twisting, levering or simply pulling the stem apart. Presumably the mechanical characteristics of individual nettles determine whether hands or teeth are more efficient. After removing the root end, two individuals frequently folded the entire stem with back and forth movements of the hand not holding the stem so that the stem formed a compact, zigzag shape. Foufou did this only occasionally (4 out of 16 stems) whereas Kibi used it for many stems (14 out of 21 processed). Otherwise, the next step was one of adjustment: either within the hand holding the stem (noted in all individuals, n=6), squeezing up leaves into a more compact form, tucking in stray leaves; or using the other hand (n=5), pushing in stray leaves, re-orienting the bundle; or both. The adjusted leafy stem was fed, root-end first, into the mouth by all individuals (n=6), such that only the undersides of leaves faced the mouth. The material was eaten by sausage-feeding, where mouthfuls were detached from the whole handful with shearing bites. However, between mouthfuls more adjustments were often performed, so that the eating process could alternate between adjustment and biting; a single stem could thus result in up to five mouthfuls, although the average was only 1.5.
Because this technique involves processing leaves and stems for consumption in the same handful, it can more closely compared to the technique Karisoke gorillas use to eat leaves of the thistle Carduus nyassanus (see Fig.3 of Byrne, Corp and Byrne 2001b) than with their technique for Laportea alatipes, which normally involves leaf removal as an early step. When Karisoke gorillas deal with Carduus nyassanus, the process results in untidy bundles of stems with leaves attached, arranged so that the spines found on leaf-edges and stem-flanges face away from the mouth, and the whole is then eaten with shear bites. The Port Lympne Leaves-with-stem nettle technique differs from this, in that the tough root end of the nettle stem is first removed, by biting, twisting-, levering- or pulling-off; and the leaf and stem bundle is adjusted, both before and while it is being fed into the mouth, using the same ways of adjusting a bundle as seen in the Leaves-separate technique. The result is a novel technique, which can nevertheless be seen to have a family resemblance to the Karisoke technique of eating thistle leaves.
Technique of Leaves-separate
Stems were procured from naturally growing nettles or from a bundle of stems provided by the experimenter (see Fig.3). Removal of leaves was done in two different ways: either by holding one end of a stem firmly, and stripping up the stem with the other hand held in a loose cone-shape, thus detaching all the major leaves in a single movement; or by picking off the leaves, usually two at a time, while the stem was held in the other hand, accumulating the bundle of leaves by iteration. A typical individual used both ways to achieve the same result on different occasions, or even in the same bout of feeding. Three individuals sometimes stripped leaves off growing stems, without first picking the stems; thus the support of the other hand was not needed. Stripping up the stem was used by all five individuals, and could be iterated repeatedly (n=4; that is, iteration was noted in four individuals), accumulating a larger handful of leaves before continuing. Individual picking was used by only four individuals; it was not clear to us what characteristics of the stems prompted choice of method in those individuals. However leaves were acquired, the next step was always adjustment of the bundle, apparently to produce a compact and dense handful for consumption, with leaf undersides on the outside. This was done is several ways: leaves held in one hand were squeezed up by that hand (n=5), parts that protruded were tucked in by fingers of the same hand (n=4), or pushed in by fingers of the other hand (n=3), and sometimes the whole handful was re-positioned using the other hand, by turning around and re-gripping (n=2). Once adjusted, the handful was finally consumed in one of two ways: placing the whole handful carefully into the open mouth (pop-in; see Supplementary Table; n=5), or feeding it in as more than one mouthful, each made by shearing bites across the ‘sausage’ of processed nettle leaves (n=4). Again, it was not possible to be certain which plant characteristics prompted the choice, but since the silverback male Djala never used sausage-feeding we suspected that this element was adopted by smaller animals when a handful was particularly large. Other than these consistent steps in the process, we noted picking out debris (n=3), either after procurement of a stem or during adjustment of the processed bundle, or both; and one individual, Kishi, twice stripped leaves off the stem with her teeth rather than using her hands.
Although the nettle stem, once the leaves had been picked or stripped from it, was typically discarded, a bare stem was sometimes recovered and eaten by the same or another gorilla. Generally, the entire stem was fed into the mouth and bitten off in stages; however in some cases (6 out of 29 stems eaten) the outer skin was stripped off by pulling the stem through the teeth and eaten. Leaves that had fallen off during processing a bundle of stems were occasionally picked up and then processed in the same way as leaves removed directly from a stem. These data were not used when constructing Leaves-separate flow-charts. Finally, Dishi was once observed picking flowers off nettle stems and eating them.