Plants have a big problem; they’re rooted in place. It isn’t easy to ensure that a plants offspring will successfully move into the wider world, if the parent isn’t able to give a helping hand. If a seed falls adjacent its mother, once sprouted – the seedling is more likely to be herbivorised by insects making the short journey from next-door. Mature plants are more likely to have accumulated diseases, and the soil may be a thriving reservoir for a variety of pathogens with the potential to threaten seedling health. Furthermore, limited dispersal distance means that parents and off-spring may be in direct competition for the same limited resources.
Plants get around this problem in a variety of ways. Some plants like the dandelion produce thousands of fruits (each containing a seed), and release them on tiny silken parachutes. Some of these fruits will travel great distances, some hopefully finding a safe place to germinate, grow and produce offspring of their own. Other plants put a little more effort into producing seeds, hiding them in sweet and enticing fruit. When mammals or birds consume the fruit, seeds will be swallowed and passed – moving them a considerable distance from the parent plant. Some seeds grab free rides by with tiny hooks that latch on to animals fur, or even onto hikers socks. Some seeds ‘explode’ from pods, and others are dispersed by cautiously-spitting rodents. In short, plants are expert seed dispersers and achieve success in weird and wonderful ways. New research from South Africa, shines light on another innovative strategy used by plants used to solve the problem of seed dispersal. The solution? Produce seeds that mimic dung.
Ceratocaryum argenteum is an endemic species of plant occurring in shrublands on deep sand. It produces unusual seeds that are large, and spherical with a rough brown seed coat. Unlike other close relatives, this species has no elaiosome (an oily reward that promotes ant-mediated dispersal), but perhaps most peculiarly – it has a distinctively dung-like smell.
As closely related species rely on ants to disperse seed, the lack of an elaiosome is curious. Some large-seeded species within the region are helped along by scatter-hoarding mammals, but these species were not encountered in the study area. However, one small mammal – a species named the striped field mouse (Rhabdomys pumilio) was common on the site.
A curious research team hoped to find whether striped field mice were caching, consuming, or moving the seeds of C. argenteum. They set out placing small batches of seeds of in plain view of camera traps. To make the seeds easier to spot, the team attached small lengths of fluorescent thread to each. While the team captured many striped field mice on camera, the mice showed very little interest in consuming or moving the seeds. However, the team did spot something unexpected: tiny dung beetles rolling and burying the seeds.
The shape, and smell of the C. argenteum seeds are so convincingly dung-like, that even the foremost experts (Epirinus flagellatus) were fooled. To better understand the magnitude of the role dung beetles could play in seed dispersal, the researchers ran another experiment placing small numbers of C. argenteum seeds at 31 feeding stations following a heavy rain (when E flagellatus is known to be most active). Beetles rolled the seed short distances, and buried 24% in the sandy soil within 24-hours. The team found no instances of eggs being laid on the seed, suggesting beetles are fooled by the convincing mimic until they attempt to lay their eggs. However, by the time the beetle is ready to lay an egg, the seed has been moved some distance from the parent plant, and is happily buried.
This is a remarkable adaptation, illustrating a perfect example of how a plant can ‘trick’ another species into performing a beneficial service. This is well-documented in the case of pollination where many plants smell like a rotting carcass to attract pollinating flies. Some orchids even mimic female bees – in a valient attempt to woo potential males and receive some pollination action.
In this particular case, seeds of C. argenteum are likely to germinate after being rolled away from the parent plant and buried snugly in the soil. By the same combination of mechanisms, the seeds of C. argenteum are likely to be unappetising to rodents – remaining intact and viable.
Unfortunately, as a result of the plants cunning deception – the dung beetle is now stuck with a problem of its own. She will go home hungry, while also having lost the opportunity to lay her egg. However, with a bit of luck, the next bit of dung shouldn’t be too far away. And with a little bit of luck for the plant – the dung beetle will have a short memory.
Midgley, J.J., White, J.D., Johnson, S.D., Bronner, G.N. 2015. Nature Plants. 1: 15141.