What an orchid that smells like rotting meat can tell us about carrion flies

Satyrium pumilum Photo by Bernd Haynold licensed by CC BY-SA 3.0

Satyrium pumilum Photo by Bernd Haynold licensed by CC BY-SA 3.0

Orchids are really good at tricking pollinators. Take, for instance, this strange looking orchid from South Africa. Satyrium pumilum is probably obscure to most of us but it is doing fascinating things to ensure its own reproductive success. This orchid both smells and kind of looks like rotting meat, which is how it attracts its pollinators.

It is a bit strange to think of orchids living in arid climates like those found in South Africa but this family is defined by exceptions. That is not to say that Satyrium pumilum is a desert plant. To find this orchid, you must look in special microclimates where water sticks around long enough to support its growth. Populations of S. pumilum are most often found clustered near small streams or hidden under bushes throughout the western half of the greater Cape Floristic Region.

Satyrium pumilum blooms from the beginning of September until late October. As is typical in the orchid family, S. pumilum produces rather intricate flowers. Whereas the sepals are decked out in various shades of green, the interior of the flower is blood red in color. Also, unlike many of its cousins, S. pumilum doesn’t throw its flowers up on a tall stalk for all the world to see. Instead, its flowers open up at ground level and give off an unpleasant smell of rotting meat.

This is where pollinators enter into the picture. It has been found that carrion flies are the preferred pollinator for S. pumilum. By producing flowers at ground level that both look and smell like rotting meat, the plants are primed to attract these flies. The plants are tapping into the flies’ reproductive habits, a biological imperative so strong that they simply do not evolve a means of discriminating a rotting corpse from a flower that smells like one. This is the trick. Flies land on the flower thinking they have found a meal and a place to lay their eggs. They go through the motions as expected and pick up or deposit pollen in the process. Unfortunately for the flies, their offspring are doomed. There is not food to be found in these flowers.

What is most remarkable about the reproductive ecology of S. pumilum is that not just any type of fly will do. It appears that only a specific subset of flies actually visit the flowers and act as effective pollinators. Amazingly, this provides insights into some long-running hypotheses regarding carrion fly ecology.

(A) The habitat of S. pumilum (B) Satyrium pumilum in situ (scale bar = 1 cm). (C–E) Pollination sequence of a S. pumilum flower by a sarcophagid fly in an arena (scale bar for all three photos = 0·5 cm); (C) the fly carrying five pollinaria from ot…

(A) The habitat of S. pumilum (B) Satyrium pumilum in situ (scale bar = 1 cm). (C–E) Pollination sequence of a S. pumilum flower by a sarcophagid fly in an arena (scale bar for all three photos = 0·5 cm); (C) the fly carrying five pollinaria from other S. pumilum flowers enters an unpollinated flower (D) as the fly moves deeper into the flower towards the right-hand spur, it presses an attached pollinium against the stigma, and its thorax against the right-hand viscidium; (E) as it leaves the flower, the fly has deposited two massulae on the stigma (1), and removed a pollinarium (2) – it now carries six pollinaria. [SOURCE]

Apparently there has been a lot of debate in the fly community over why we see so many different species of carrion flies. Rotting meat is rotting meat, right? Probably not, actually. Fly ecologists have comes up with a few hypotheses involving niche segregation among carrion flies to explain their diversity on the landscape. Some believe that flies separate themselves out in time, with different species hatching out and breeding at different times of the year. Others have suggested that carrion flies separate themselves by specializing on carrion at different stages of decay. Still others have suggested that some flies specialize on large pieces of carrion whereas others prefer smaller pieces.

By studying the types of flies visiting the flowers of S. pumilum researchers did find evidence of niche segregation based on carrion size. It turns out that S. pumilum is exclusively pollinated by a group of flies known as sarcophagid carrion flies. These flies were regularly observed with orchid pollen sacs stuck to their backs and plants seemed to only set seed after they had been visited by members of this group. So, what is it about these flowers that makes them so specific to this group of flies?

The answer lies both in their size as well as the amount of scent they produce. It is likely that the quantity of scent compounds produced by S. pumilum most closely mimics that of smaller rotting corpses. The types of flies that visited these blooms were mostly females of species that lay relatively few eggs compared to other carrion flies. It could very well be that the smaller brood size of these flies allows them to effectively utilize smaller bits of carrion than other, more fecund species of fly. To date, this is some of the best evidence in support of the idea that flies avoid competition among different species by segregating out their feeding and reproductive niches.

Rotting meat smells are not uncommon in the plant world. Even within the home range of S. pumilum, there are other plants produce flowers that smell like carrion as well. It would be extremely interesting to look at what kinds of flies visit other carrion flowers and in what numbers. Like I mentioned earlier, reproductive is such a major part of any organisms life that it may simply be too costly for carrion flies to evolve a means of discriminating real and fake breeding sites. It is amazing to think of what we gain from trying to understand the reproductive biology of a small, obscure orchid growing tucked away in arid regions of South Africa.

Photo Credits: [1] [2]

Further Reading: [1]

Are Crickets Dispersing Seeds of Parasitic Plants?

Parasitic plants lead unique lifestyles. Many have foregone photosynthesis entirely by living off fungi or their photosynthetic neighbors. Indeed, there are many anatomical and physiological adaptations that are associated with making a living parasitically. Whether they are full parasites or only partial, one thing that many parasitic plants have in common are tiny, dust-like seeds. Their reduced size and thin seed coats are generally associated with wind dispersal, however, there are always exceptions to the rule. Recent evidence has demonstrated that a handful of parasitic plants have evolved in response to a unique seed dispersal agent - camel crickets.

A research team based out of Japan recently published a paper describing a rather intriguing seed dispersal situation involving three species of parasitic plants (Yoania amagiensis - Orchidaceae, Monotropastrum humile - Ericaceae, and Phacellanthus tubiflorus - Orobanchaceae). These are all small, achlorophyllous herbs that either parasitize trees directly through their roots or they parasitize the mycorrhizal fungi associated with said trees. What's more, each of these species are largely inhabitants of the dense, shaded understory of rich forests.

These sorts of habitats don't lend well to wind dispersal. The closed forest canopy and dense understory really limits wind flow. It would appear that these three plant species have found away around this issue. Each of these plants invest in surprisingly fleshy fruits for their parasitic lifestyle. Also, their seeds aren't as dust-like as many of their relatives. They are actually very fleshy. This is odd considering the thin margins many parasitic plants live on. Any sort of investment in costly tissues must have considerable benefits for the plants if they are to successfully get their genes into the next generation.

Fleshy fruits like this are usually associated with a form of animal dispersal called endozoochory. Anyone that has ever found seed-laden bird poop understands how this process works. Still, simply getting an animal to eat your seeds isn't necessarily enough for successful dispersal. Seeds must survive their trip through the gut and come out the other end relatively in tact for the process to work. That is where a bit of close observation came into play.

After hours of observation, the team found that the usual frugivorous suspects such as birds and small mammals showed little to no interest in the fruits of these parasites. Beetles were observed munching on the fruits a bit but the real attention was given by a group of stumpy-looking nocturnal insects collectively referred to as camel crickets. Again, eating the fruits is but one step in the process of successful seed dispersal. The real question was whether or not the seeds of these parasites survived their time inside either of these insect groups. To answer this question, the team employed feeding trials.

They compared seed viability by offering up fruits to beetles and crickets both in the field and back in the lab. Whereas both groups of insects readily consumed the fruits and seeds, only the crickets appeared to offer the greatest chances of a seed surviving the process. Beetles never pooped out viable seeds. The strong mandibles of the beetles fatally damaged the seeds. This was not the case for the camel crickets. Instead, these nocturnal insects frequently pooped out tens to hundreds of healthy, viable seeds. Considering the distances the crickets can travel as well as their propensity for enjoying similar habitats as the plants, this stacks up to potentially be a beneficial interaction. 

The authors are sure to note that these results do not suggest that camel crickets are the sole seed dispersal agents for these plants. Still, the fact that they are effective at moving large amounts of seeds is tantalizing to say the least. Taken together with other evidence such as the fact that the fruits of these plants often give off a fermented odor, which is known to attract camel crickets, the fleshy nature of their fruits and seeds, and the fact that these plants present ripe seed capsules at or near the soil surface suggests that crickets (and potentially other insects) may very well be important factors in the reproductive ecology of these plants.

Coupled with previous evidence of cricket seed dispersal, it would appear that this sort of relationship between plants and crickets is more widespread than we ever imagined. It is interesting to note that relatives of both the plants in this study and the camel crickets occur in both temperate and tropical habitats around the globe. We very well could be overlooking a considerable component of seed dispersal ecology via crickets. Certainly more work is needed.

Photo Credits: [1]

Further Reading: [1] [2]