Jeremy Midgley, Joseph White, Steve Johnson and Gary Bronner

Plant dupes dung beetles

It looks like a pellet of buck dung, it smells like one too, but the centimetre-long seed is not dung, and is one of the few proven instances of sensory deception for seed dispersal.

“The smell is incredible,” says Jeremy Midgley, a professor in biological sciences at the University of Cape Town. “From the smell, we should have known there was something going on.”

The relatively large seeds of long-grass-like Ceratocaryum argenteum have such a pungent odour that they fool dung beetles into thinking that they are pellets of dung. The findings of the research were published in journal Nature Plants earlier this month.

Below is a multimedia clip made by Nature:

 

“As the seeds are hard and offer no reward to the dung beetles, this is a remarkable example of deception in plant seed dispersal,” the authors write in the paper.

Midgley was initially convinced that a rodent species was removing the seeds and eating or burying them. During a field trip to the De Hoop Nature Reserve in the southern Cape, Midgley, UCT small mammal expert Dr Gary Bronner and masters student Joseph White set up motion-sensitive cameras to see what was happening to the seeds.

“We used motion-sensing trail cameras to observe small mammal interactions with the nuts under field conditions, and it seemed that they were either disinterested or even repelled by the seeds,” White says. While a field mouse species ate the insides of the seeds if they happened to be cracked, they did not hoard them.

Instead they saw that dung beetles removed the hard-shelled seeds and buried them, thinking that they were dung. The advantages for the plant are that its seeds do not get incinerated in the fire-prone area and that they get dispersed, but the dung beetle “can’t eat them or lay its eggs inside them”, Midgley says.

The team included University of KwaZulu-Natal professor Steve Johnson, who undertook a chemical analysis of the seeds. He found that the volatiles [the chemicals which create the aroma] were three hundred times higher in the C. argenteum seeds than in seeds of other plants in the family (Restionaceae). “The volatile composition of emissions from C. argenteum seeds is similar to that of the dung of large mammalian herbivores, particularly eland and bontebok,” the authors write in their paper.

But because the seeds are only around for two to three weeks of the year, the dung beetles have not adapted to recognise the different between dung and the C. argenteum seeds, Midgley says. “There isn’t enough time for beetles to get smarter about what is dung and what isn’t.”

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