Diet of the feral camel in Australia, part 2

The most recent wild ancestor of the dromedary (Camelus dromedarius) lived in what is now Dubai (https://www.pnas.org/content/113/24/6707 and https://www.pnas.org/content/pnas/suppl/2016/05/04/1519508113.DCSupplemental/pnas.1519508113.sapp.pdf) on a remote coastal plain which is hard to relate to the directions of the compass but forms a 'horn' of the Arabian subcontinent in the same way that the Horn of Africa relates to the African continent.

Given this origin, it might be informative to refer the favourite food-plants of the dromedary, in its feral state, in Australia to the flora of Arabia. In particular, which of the genera preferred in Australia are also indigenous to Arabia, albeit as different species?

A principle to bear in mind is that, in general, the leaves and shoots of stem-spinescent or hedge-forming plants are more palatable and nutritious than those of plants lacking these structural defences. Leaf-spinescent plants (other than thistles, https://www.farmonline.com.au/story/4243900/scotch-thistles-secret-enemy/#:~:text=%E2%80%9CThey%20love%20Scotch%20thistle%20in,and%20camels%20are%20the%20answer. and https://www.armstrongorganics.com/camels) tend to follow the opposite trend. Their foliage tends to be nutrient-poor, epitomised by the distinctively Australian hummock grasses (Triodia, see https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1469-185X.2007.00017.x).

In comparing Australia with Arabia, we can start with the amaranths and other halophytes (https://en.wikipedia.org/wiki/Halophyte).

Atriplex, a cosmopolitan genus of sodic-adapted shrubs and herbaceous plants, is locally conspicuous in semi-arid Australia (https://www.daf.qld.gov.au/business-priorities/agriculture/plants/crops-pastures/pastures/saltbush#:~:text=Saltbushes%20are%20members%20of%20a,staple%20diet%20of%20many%20animals.), and inconspicuous in Arabia (https://www.cabi.org/ISC/abstract/20013152218).

Wherever it occurs, Atriplex is a fairly nutritious plant for herbivores (http://researchhub.buan.ac.bw/bitstream/handle/123456789/284/10.1.1.585.8784.pdf?sequence=1 and https://www.tandfonline.com/doi/abs/10.2989/10220110309485822). However, its food-value tends to be limited because its concentraction of sodium is not necessarily matched by other useful elements.

Probably more important in the diet of the dromedary in Arabia is the related genus Caroxylon, which does not occur in Australia and is associated with nutrient-rich soils rather than merely sodic ones. The branching stems and diminutive leaves (e.g. see https://en.wikipedia.org/wiki/Salsola_vermiculata and https://en.wikipedia.org/wiki/Salsola_imbricata) provide antiherbivore defences lacking in Atriplex and most of the other amaranths in Australia. This suggests that Caroxylon is more palatable than Atriplex to the dromedary.

One way to view the various halophytic amaranths (Atriplex, Chenopodium, Enchylaena, Maireana, Sclerolaena, Tecticornia) eaten by the feral dromedary in Australia is as nutrient-poor counterparts - which were relatively exempt from herbivory in Australia before the arrival of domestic livestock - to Caroxylon.

The amaranth Ptilotus (https://en.wikipedia.org/wiki/Ptilotus) is non-halophytic but conforms to this pattern in being common but not nutritious enough to be under pressure from herbivores. This is true notwithstanding the phosphorus-richness of certain species of Ptilotus (https://www.publish.csiro.au/bt/BT19188).

Zygophyllaceae are unrelated to amaranths phylogenetically, but show a similar relationship to sodicity and herbivory. Although Zygophyllum occurs in Arabia and in Australia, the closely related genus Tetraena - absent from Australia - has a growth-form convergent with Caroxylon in its adaptation for intense herbivory (https://en.wikipedia.org/wiki/Tetraena_alba and https://en.wikipedia.org/wiki/Tetraena_qatarensis).

The zygophylls also contain Tribulus, which is not halophytic. This is a favourite food of the dromedary (https://arkbiodiv.com/2019/05/05/camels-flower-%D8%B2%D9%87%D8%B1-tribulus-is-the-precious-flora-of-the-arabian-desert/) and indigenous to both Australia and Arabia, although the commonest species has probably been introduced anthropogenically (https://en.wikipedia.org/wiki/Tribulus).

Portulaca is yet another genus indigenous to both Australia and Arabia, and likely to be a favourite food-plant of the dromedary in both regions.

Acacias occur in both Australia and Arabia, but in the form of different genera. Whereas the species eaten by the feral dromedary in Australia belong to the genus Acacia and have phyllodes instead of leaves, those likely to have been eaten in Arabia belong to the genus Vachellia, have bipinnately compound leaves, and are extremely spinescent (https://en.wikipedia.org/wiki/Vachellia_tortilis and https://en.wikipedia.org/wiki/Vachellia_flava).

Carissa and Capparis nowhere dominate the vegetation, but exemplify preferred species shared between Australia and Arabia. Not only is the dromedary likely to eat these plants (which are variably spinescent depending on pressure from herbivory) in both regions, but the same species is indigenous in both cases (https://en.wikipedia.org/wiki/Carissa_spinarum and https://en.wikipedia.org/wiki/Caper).

The feral dromedary in Australia coexists in places with kangaroos. However, there is little competition for food, for several reasons.

Firstly, all species of kangaroos prefer grasses, which are generally not preferred by the feral dromedary. Secondly, kangaroos do not generally eat acacias, eucalypts or eremophilas. Thirdly, the dromedary can reach twice - and perhaps threefold (https://www.naturepl.com/stock-photo-bactrian-camel-camelus-bactrianus--three-beneath-tree-one-standing-on-nature-image01643198.html) - as high as kangaroos can. And fourthly, much of the range of the feral dromedary is in the Australian Empty Quarter, where all kangaroos were scarce at the time of European arrival and remain scarce today.

It is noteworthy that the feral camel accepts eucalypts and proteas to a greater extent than hummock grasses.

I can summarise as follows.

Although the vegetation differs greatly between semi-arid Australia and Arabia, the dromedary is mobile and versatile enough to find equivalent, and in many cases closely related, plants to eat in both regions. Among comparable genera of favourite food plants, Australian species tend to be less spinescent or hedged than Arabian species, which may allow the dromedary to forage more efficiently - but not necessarily more sustainably - in its adopted than in its original habitat.

Posted by milewski milewski, October 10, 2021 22:44

Comments

Posted by milewski about 2 months ago (Flag)

@rhytiphora According to https://dpir.nt.gov.au/__data/assets/pdf_file/0004/233455/tn116.pdf, the feral camel in Australia does eat 44 species, in 24 genera, of grasses. However, its preference score for 84% of these species is only 2, i.e. second-least preferred. The only species scoring 3 are in the genera Cenchrus (introduced, not indigenous), Dactyloctenium, Digitaria, Eragrostis (only one of the seven spp. available), and Triraphis. The hummock grass Triodia schinzii was minimally preferred, scoring only 1. This leaves only one species of grass preferred by the feral camel in Australia: Tripogon loliiformis (https://upload.wikimedia.org/wikipedia/commons/b/b6/Tripogon_habit.jpg), which scored 5 on the scale of 1-7.

Posted by milewski about 2 months ago (Flag)

Here is a potentially favourite food of the dromedary in Arabia, with no counterpart in Australia: https://en.wikipedia.org/wiki/Leptadenia_pyrotechnics.

Posted by milewski about 2 months ago (Flag)

Although Capparis occurs in semi-arid Australia, it is a mere token of the importance of the same family, the Brassicaceae, in the original habitat of the dromedary. In Arabia the perennial brassicas are represented by Capparis, Cadaba, Maerua and Boscia, all of which are remarkably nutrient-rich although with a tendency for excessive calcium relative to phosphorus. Even more extreme in this way is the related Salvadora (https://en.wikipedia.org/wiki/Salvadoraceae), which was also probably an important food for the ancestral dromedary.

Posted by milewski about 2 months ago (Flag)

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