How plants dupe dung beetles into burying their seeds

A Cape Restio (Ceratocaryum argenteum) produces large, hard nuts that smell and look remarkably like dung, and are buried by dung beetles. These nuts provide no food for dung beetles or their larvae – a classic example of biological deception, and possibly one of the best examples of faecal mimicry for seed dispersal anywhere in the world. They were recently described by biologists from UCT and the University of KwaZulu-Natal (UKZN), in a paper in Nature Plants.

Deception is a very interesting biological phenomenon, as it involves a co-evolutionary arms race between one species (the deceiver, or mimic) that benefits from resembling another species (the dupe, or model), with no advantages for – and sometimes even to the evolutionary disadvantage of – the latter. Some of the most striking examples of deception in plants are those that deceive insects into pollinating flowers without any reward. Some orchids, for example, produce colourful flowers that contain no nectar to reward pollinating insects. These plants rely on sensory exploitation (insects are attracted to colour in general), and in some extreme cases mimic other rewarding plants that occur in the same place, thereby duping insects into pollinating their nectarless flowers.

Deception for seed dispersal, however, is far less common. Some plants produce hard red or black seeds (such as the so-called lucky beans) that look like berries; but these do not seem to fool birds, and are hardly ever eaten or dispersed. Also, such seeds are often poisonous, and their bright colours act more as warning colouration than as an attraction to fruit-eating birds. Dung beetles being duped into dispersing 'dung-like' Ceratocaryum nuts may therefore be the best example globally of faecal mimicry for seed dispersal.

The seed of the Cape restio (left) is remarkably similar in size, shape and colour to the dung of the bontebok (right).

The scent of Ceratocaryum nuts is very strong. “I have nine-month-old seeds in a paper bag in my office that are still very pungent,” says Jeremy Midgley, a professor in the Biological Sciences Department at UCT, who discovered the deception. Steve Johnson, a professor at UKZN who did the chemical analyses for this study, was amazed at both the complexity of the scents emitted by Ceratocaryum seeds, and their similarities to antelope dung. “It still remains to be seen exactly which chemical is the most attractive to the dung beetles," says Johnson.

“I have long had an interest in seed burial by certain Cape rodent species, and was convinced that the enormous size of Ceratocaryum seeds would make them attractive to rodents – either to eat immediately, or to bury,” says Midgley. Together with MSc student Joseph White and small-mammal expert Dr Gary Bronner, both in UCT's Department of Biological Sciences, he began investigating whether free-ranging small mammals were interested in Ceratocaryum nuts.

“We used motion-sensing trail cameras to observe small mammal interactions with the nuts under field conditions, and it seemed that they were disinterested or even repelled by the seeds. When small mammals did crack seeds open, it was clear they were interested in the nutritious inner parts of the seeds,” says White.

Watch camera-trap footage of the small mammal R pumilio dehusking Ceratocaryum nuts.

The most surprising result from their field experiments was the discovery of dung beetles dispersing Ceratocaryum nuts. “Through both camera trapping and direct observation, we saw dung beetles being attracted to the nuts, rolling them away and then burying them, by pulling them down from below,” comments Bronner. “Previously, we had observed the same behaviour by another dung beetle species in the Cederberg, where Ceratocaryum plants do not occur; suggesting that this phenomenon may be quite general and widespread in fynbos.”

Watch camera-trap footage of a dung beetle burying a Ceratocaryum nut.

“I wonder what would happen if we put these nuts out in the savanna?” ponders Midgley. “Would they fool savanna dung beetles?” 

Dung beetles do inadvertently disperse some seeds – for instance, those already in the dung of fruit-eating mammals, which the beetles bury to nourish their offspring. But this is not deception, as the beetles gain a reward. With Ceratocaryum nuts, however, dung beetles are duped into dispersing and burying nuts with no reward, but with an energy cost. This type of dispersal is probably quite rare, because it depends on the right ratio of dung to dung beetles. “Too much dung, and the nuts will not be buried – because beetles have too much of a choice; too little dung, and there will be a similar lack of burial, owing to too few dung beetles. We still have much to learn about the dynamics of such faecal mimicry,” concludes Midgley. 

Images by Joseph White.

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