Monday 29 August 2016

Ceratocaryum argenteum: A plant producing dung-mimicing seeds.

Ceratocaryum argenteum is a large Rush-like Grass native to Cape region in South Africa. It produces lage, hard seeds which Dung Beetles have been observed burrying, despite the fact that they do not appear to be a suitable food source for either the Beetles or their larvae, which has led to suggestions that the seeds somehow mimic the dung of a large Antelope, fooling the Beetles into burrying the seeds.

In a paper published in the South African Journal of Science on 26 July 2016, Jeremy Midgley and Joseph White of the Department of Biological Sciences at the University of Cape Town describe the results of an investigation into the relationship between the seeds of Ceratocaryum argenteum and the Dung Beetles.

Midgley and White placed piles of Ceratocaryum argenteum seeds beside a sand road in the De Hoop Nature Reserve in early in the morning after heavy rain events in February 2016. They found that two species of Dung Beetle, Epirinus flagellatus and Scarabaeus spretus, arrived at the seed stations within a few minutes of placing the seeds, and immediately began rolling the seeds away and burrying them. One specimen of Scarabaeus spretus even flew into a bag containing the seeds before they could be placed. Flesh Flies, Sarcophagidae, and Lesser Dung Flies, Sphaeroceridae, were also attracted to the seeds.

(a) Epirinus flagellatus rolling a Ceratocaryum argenteum seed; (b) Scarabaeus spretus rolling a seed (the arrow indicates a Sphaerocerid Lesser Dung Fly); (c) the large hole made by Scarabaeus spretus for burying several seeds (the arrow indicates the location of the Dung Beetle); and (d) a female Sarcophagid Fly on a seed. Midgley & White (2016).

Two Antelope species present in the De Hoop Nature Reserve, the Eland, Taurotragus oryx, and the Bontebok, Damaliscus pygargus. The two species have different diets, with the Bontenok favouring Grasses and the Eland being a browser with a mixed diet. 

This is a useful difference, as many tropical Grases use a different photosynthetic method to most other plants (which enables them to use less water in the process), as a side affect of which they tend to fix more of the lighter carbon isotope Carbon 12 (12C) and less of the heavier isotope Carbon 13 (13C), which results in the  tissues of animals which eat these Grasses having a distinct isotope signature; useful to archaologists trying to understand the diets of ancient populations and ecologists trying to understand the diets of animals. 

The larvae of Dung Beetles feed on undigested plant tissues within dung balls burried by their parents. This means that a Dung Beetle should have a carbon isotope signature which matches the dung of the animal on which it fed. Midgley and White analysed dung from both the Antelope species as well as a number of individuals of both Dung Beetle. Both Beetles had a carbon isotope signature closely matching that of the Eland dung, suggesting that this is their natural diet. Since other large Dung Beetles are present in the De Hoop Nature Reserve which did not pay any attention to the Ceratocaryum argenteum seeds, Midgely & White suggest that the seeds must specifically be mimicing the Eland dung, presumably be emiting chemical signatures (scents) detectable by the Beetles.

See also... clavijeroi & Onthophagus martinpierai: Two new species of Scarab Beetles from the Trans-Mexican Volcanic Belt.                                                          Mexico has an essentially tropical climate, but is... cujuchi: A new species of Scarab Beetle from Rodent burrows in Bolivia. Tuco-tucos, Ctenomys spp., are small South American Rodents which live their entire lives underground within underground burrow systems, never venturing to the surface. Studies of other Rodents with similar... new species of Scarab Beetle from Hardap Province, Namibia.                                    There are currently eight species of Scarab Beetles in the genus Scapanoclypeus, which is known from Namibia and western South Africa. They are... 
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