Curious case of Cape coprobotany

Now, and please be honest, if you saw an article entitled, “Faecal mimicry by seeds ensures dispersal by dung beetles” you’d want to read it, right?

Now, and please be honest, if you saw an article entitled, “Faecal mimicry by seeds ensures dispersal by dung beetles” you’d want to read it, right? Well, in case you haven’t seen that item, I’ll do my best to summarise the study for you, and place it in a wider context. (By the way, I do hope you’re not just about to start/in the middle of/just finished breakfast/lunch/dinner/supper.)

Scarabaeus laticollis
Scarabaeus laticollis. Photo by Rafael Brix / Wkipedia.

As fairly stationary organisms plants have a major problem if they are to realise any ambitions to expand their territory, occupy new areas and capture more life-sustaining resources. The best hope seed plants have of seeking out new spaces in which to establish, grow and prosper is as a seed, a dispersal unit that is potentially highly mobile. But that’s not a lot of good unless the seed, ideally surrounded by nutritionally ‘distracting’ fruity material – i.e. as a fruit – as in angiosperms (flowering plants), is attractive to an animal partner that is willing to take the perfectly-packaged propagule from the parent plant and remove it to a new home. And plants have recognised that as, over the millions of years they’ve pondered this perennial perennation problem and perfected creatively-designed evolutionary solutions to achieve that eminently desirable outcome. Accordingly, we are used to all manner of animals swallowing the propagules, digesting the fruity tissues and then ‘passing’ the undigested seed out of their bodies surrounded by a modicum of nutritious fertiliser (aka faeces) which can supplement seed-derived nourishment during germination and early seedling establishment.

It’s an elegant solution that’s not to be dismissed lightly, and works for many plant species. But it’s not the only coprological connection that plants have hit upon in their quest to cast their seed far-and-wide. Cue, the curious case of the plant, Ceratocaryum argenteum (in the Restionaceae) from the southern Cape region of South Africa, and the dung-beetle, Epirinus flagellatus.

Seeds of the Ceratocaryum have an odour that is similar to that of the dung – so Jeremy Midgley et al. tell us – of large mammalian herbivores in the area, particularly eland (Taurotragus oryx) and bontebok (Damaliscus pygargus ssp. pygargus), and are similar in size, shape and brownish colouration of the latter’s dung. Seeds of the plant are rolled away by the dung beetle and buried (which rolling-away is facilitated by the seed’s circular outline).

Deceived by their aroma/shape/size/colour, it appears that the beetles mistake the seeds for dung which they would normally eat as a food source or else use to deposit their eggs within. However, they’re unable to use the seeds in this way because of their hard seed coats. Consequently, the seeds remain unharmed and buried, some way from the parent plant. And it’s surely not coincidental that C. argenteum, which cannot resprout after fire (a natural hazard in its habitat), but is dependent upon post-fire seedling establishment from an incineration-proof, buried seed-bank – such as that provided courtesy of the ever-so helpful dung beetle.

All of which procreatively beneficial service is provided with no discernible reward for the hapless duped beetle. Which leads us to this month’s searching question. If you had to choose, which/what would you rather be? As clever as a Ceratocaryum that takes advantage of the gastronomically gullible? Or as daft as a dung-beetle that displays this great phyto-centric altruistic behaviour? Choose wisely.

[Ed. – plants don’t have it all their own way in the excruciatingly exciting ‘excrement ecology’ stakes. Take for example the case of maize and Fall army worms (FAW, Spodoptera frugiperda) and the work of Swayamjit Ray et al. During their feeding activities upon the plant FAW caterpillars deposit frass (excrement) on the leaves. Over time, proteins within the frass – which material is in part derived from the nutritional onslaught of the worms on the plant – cause activation of the maize’s pathogen-defence genes (and concomitant de-activation of herbivory-defence genes). This in turn led to ‘increased herbivore performance’ – i.e. the caterpillars grew more – because they fed on better quality plant material that didn’t have the elevated levels of compounds that make it distasteful to the insects. A ‘bonus’ was reduced growth of the fungus that causes southern leaf blight disease in corn.]