An odd anecdote about giraffes and spinescence: prickles of Senegalia ataxacantha pose deterrent risk
@tessabrunette @tonyrebelo @jeremygilmore @ludwig_muller @alastairpotts @matthewinabinett @wynand_uys @richardgill
At 8 am on 23 August 2000, I wrote the following field-notes, while on a visit to Ithala Game Reserve, in Zululand, South Africa. I have illustrated these notes with photos from iNaturalist.
(Dear reader, please bear in mind that this is the degree of spinescence of which the celastraceous genus Gymnosporia is capable, in southern Africa: https://www.inaturalist.org/observations/66732931 and https://www.inaturalist.org/observations/126035720.)
"A subadult male individual forages remarkably intensively on a 2 m-high shrub of Gymnosporia buxifolia, eating wholesale. This individual plant happens to lack the spines typical of its genus (https://www.inaturalist.org/observations/140289004 and https://www.inaturalist.org/observations/135831966 and https://www.inaturalist.org/observations/113753946 and https://www.inaturalist.org/observations/104460018), allowing it to be damaged efficiently and rapidly by this folivore, in a way that would certainly have been retarded by the normal spines of this species (https://www.inaturalist.org/observations/109746627 and https://www.inaturalist.org/observations/99259658 and https://www.inaturalist.org/observations/115705457 and https://www.inaturalist.org/observations/115075852). The animal eats for more than five minutes from this individual shrub, alternating with an adjacent individual of Hippobromus pauciflorus (https://www.inaturalist.org/observations/61390042 and https://www.inaturalist.org/observations/115157202), which it strips in even more wholesale fashion."
"The only limitation, in the case of G. buxifolia, is the branching pattern (https://www.inaturalist.org/observations/80867976). However, the animal takes bites that are as large as possible, some of which are branches more than 15 cm long, consisting of green leaves plus the green stems bearing them, and others of which are the leaves and shoots stripped from lengths of branch. This includes the ripening capsules of G. buxifolia (https://www.inaturalist.org/observations/78080453), which are turning dull orange. A few minutes after this initial bout of foraging, the same individual of G. g. giraffa returns to the same individual plant, and goes over the same branch systems again, suggesting a determination to make the most of this fortuitous lack of spines."
"I clearly see the action of the tongue in drawing shoots towards the mouth, the tongue stretching to the maximum, curling around the stem, and pulling it to where it can be pressed against the upper lip, in order to detach a bite of food. Accordingly, the bite-size during these bouts ranges from a few leaves to a spray of foliage the size of my outstretched hand, including the stem where this is still green and soft, or leaving the stripped stem behind where this is already brown and hard.'
"Then, an odd thing happens. While foraging on H. pauciflorus, the animal - presumably accidentally - takes a length of stem of what I assume to be Senegalia ataxacantha (https://www.inaturalist.org/taxa/567417-Senegalia-ataxacantha), a deciduous species currently (seasonally) bare, with alternating stems covered in prickles, emergent above the thicket in the form of several wispy branches. The length of the stem taken is probably about 30 cm, with small side-stems, each about six cm long. The animal draws this into its mouth, along with the rest of the food, but then spends about 1.5 minutes struggling to chew and swallow it. The item apparently lodged by velcro-like action in the mouth and throat. The animal shows discomfort and incapacitation by making 'funny faces', shaking the muzzle, contorting the mouth, and even shaking the whole head, for a prolonged period. Even after more than a minute of this behaviour, I can still see the end of the stem at the opening of the mouth. Finally, it manages to swallow the offending section of stem."
"With reference to the main species, G. buxifolia: this individual of G. g. giraffa has probably reduced the foliage of this individual plant by more than half during these consecutive bouts, leaving it rather bare-looking. The animal seems to have an equal appetite for G. buxifolia and H. pauciflorus, the latter evading wholesale stripping by virtue of being obstructed in the thicket. Only 8 m away is a shrubby individual of Gymnosporia ?nemorosa, with small spines and relatively dark green leaves, which has remained untouched by any of the three individuals of G. g. giraffa."
DISCUSSION
This anecdote suggests that Senegalia ataxacantha, in the form of wispy, shooting stems, has a remarkable defensive capacity vs giraffes, the inside of the mouth of which has complex linings (e.g. papillae inside the cheeks).
The effect seems similar to what I myself have experienced with the awns on the whole (unhulled) seeds of Avena sativa (https://www.inaturalist.org/taxa/57156-Avena-sativa), which, after lodging in my throat, could not be moved by my tongue to a chewable position.
According to the literature, the main function of prickles on acacias is to retard - not prevent - foraging by means of snagging and abrasion on the lips (https://pubmed.ncbi.nlm.nih.gov/28311793/ and https://link.springer.com/article/10.1007/BF01036753 and https://www.semanticscholar.org/paper/Effects-of-plant-spinescence-on-large-mammalian-Cooper-Owen%E2%80%90Smith/0694286f9cb71800b8d7f92c183b18543d7f2f7e).
My anecdote suggests that, at least in the case of giraffes with their extremely specialised mouths, the mechanism goes beyond mere retardation - to something like categorical deterrence.
ACKNOWLEDGMENT: Many thanks to Wynand Uys (see comments below) for helping me to identify S. ataxacantha retrospectively.
Σχόλια
http://pza.sanbi.org/dalbergia-armata#:~:text=According%20to%20botanists%2C%20the%20spiky,rub%20themselves%20against%20the%20plant.
I don't know why the young Giraffe gagged on the D. armata shoots. They don't have prickles ot anything I know of that can cause irritation.
Perhaps the side-stems you mention had already hardened into thorns.
As in: https://www.inaturalist.org/observations/80842382
@wynand_uys Many thanks for your helpful response. Can you think of any species that I may perhaps have mistaken for D. armata, i.e. a liane-like plant that projects wispily above thicket-clumps, and possesses curved prickles on thin stems that are likely to be bare at the end of August?
Senegalia schweinfurthii and S. ataxacantha are likely candidates.
Their prickles are particularlly nasty.
@wynand_uys You have solved the problem, How silly of me. Thank you.
https://www.researchgate.net/publication/30848554_Effects_of_spines_and_thorns_on_Australian_arid_zone_herbivores_of_different_body_masses
https://www.tandfonline.com/doi/full/10.1080/0028825X.2015.1130727
During the same visit to Ithala Game Reserve, I observed Strepsiceros strepsiceros foraging on an individual of Gymnosporia that likewise happened to lack spines. This allowed comparison of the method of eating. An adult female individual of S. strepsiceros spent many minutes at a tall (2 m-high) shrub of what I took to be Gymnosporia senegalensis, nibbling off the leaves and shoots. In contrast to G. giraffa, it did not eat any stems, whether green or brown. The main difference was that G. giraffa extended its tongue and stripped the stems with its furry lips, whereas S. strepsiceros performed neither of these actions, instead simply using its bare-edged lips.
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