The large, brown seeds of a South African grass look and smell just like antelope droppings. Thanks to this stinky disguise, the plant attracts dung beetles, who then roll and bury the seeds. While this helps disperse the plant, the nuts provide no benefits for the duped beetle. The work, published in Nature Plants this week, is a rare example of deception by plants for seed dispersal.
While there are lots of examples of flowers mimicking other plants or insects to attract pollinators, until now, researchers weren’t sure if mimicry is used to help the spread of seeds. A team led by Jeremy Midgley from the University of Cape Town investigated the dispersal of the unusual seeds of Ceratocaryum argenteum in the southern cape of South Africa. These 1 centimeter (0.4 inch) long, 0.7 gram (0.02 ounce) nut seeds are much rounder and larger than those of related species, and they give off a pungent smell reminiscent of droppings from African herbivores – even humans can smell this.
At first, the team wondered if the nuts were being hoarded by small mammals, such as the striped field mouse. So the researchers set up motion-activated cameras in the shrublands of the De Hoop Nature Reserve. The rodents, they discovered, ignored the intact seeds (though they might eat them if they were already dehusked). The team ended up filming at least three dung beetles (Epirinus flagellatus, pictured below) rolling the nuts away in their typical head-down fashion.
Similarities between C. argenteum seeds (a-e) and Bontebok feces (g) and dissimilarities with other nut seeds (h-j). Epirinus flagellates (f). J.J. Midgley et al., 2015 Nature Plants
Dung beetles are typically more active during warm, moist periods, so the team then placed 195 of these seeds at several sites after a rainstorm. Within a day, 87 were removed. Using fluorescent threads glued to the seeds, the team was able to recover most of them. In total, 53 had been moved about 20 centimeters (8 inches) away and buried. No dung beetles were discovered with these seeds, however, since they probably figured out that they were duped when they tried to eat or lay eggs in the hard seeds.
Furthermore, when the researchers analyzed the volatile chemicals emitted by the seeds, they found that their chemical concentration and composition were similar to those emitted from the dung of elands (a large antelope) and bonteboks (a small antelope, pictured at the top).
By mimicking antelope dung, Ceratocaryum argenteum seems to be tricking dung beetles into planting their seeds. This species can’t re-sprout after a fire, so they depend on an incineration-proof, buried seed bank.