On 25 January billions of people will enter the Chinese Year of the Rat. I really have no idea what that portends, but it gives me the opportunity to write about my favourite fynbos rat– the Vlei Rat.
The Common Vlei Rat (Otomys irroratus) has shaggy hair, a short tail and grows to the size of a small cat. They are so named because they hang out in ‘vleis’ (marshes) streams and swamps. They feed on grass, reeds and other vegetation by biting through the stem near the base and then, holding each end of the stem with a paw, eat short pieces in the middle.
The Vlei Rat doesn’t seem to have a special function in fynbos other than being food for other fynbos species like raptors and snakes. Despite the fact that they are rather large rats, they are totally harmless and very cute.
So if Vlei Rats are not much use, what about other small fynbos rodents?
Let’s look at the Hairy-Footed Gerbil (Gerbilliscus paeba), the Four-Striped Mouse (Rhabdomys pumilio) and the Cape Spiny Mouse (Acomys subspinosus).
It is well established that the striped mouse and spiny mouse are both attracted to the musty smell of ground proteas (e.g. Protea scabra, where the flowers are borne at ground level). The rodents stick their noses into the flower to get the nectar, or to nibble a bit of flower. As they do so, they get dusted in pollen which they then carry to other flowers. In this way, these small rodents are important pollinators of the fynbos.
In a previous article (Village News 4 December 2019) we looked at seed dispersal by ants (myrmecochory). Do we also have seed dispersal by rodents? Indeed, it appears that not all rodents are purely seed eaters and that some species may play an important role in seed dispersal.
The Hairy-Footed Gerbil (but not the spiny mouse) is an important seed-carrier, favouring restio seeds. They bury or cache them for future consumption. But not just any seeds – they prefer seeds without elaiosomes, the fleshy part on the surface of some seeds.
This is interesting if we remember that it’s the nutritious elaiosomes that attract ants, causing them to carry the seeds into their nests. We must conclude therefore that gerbil and ant behaviour is complementary rather than competitive when it comes to seed dispersal. The major evolutionary advantage to the plants, we must conclude, involves protection of their seeds from fire, rather than from predators.
What factors determine which seeds will be eaten and which dispersed by rodents? It’s a bit like Goldilocks and the three bears’ porridge. Some are too small, some are too big and some are just right.
Rodents prefer to consume small seeds with thin hulls. Large seeds and seeds with thick hulls are usually neither eaten nor buried. Medium sized seeds with medium hull thickness are more often buried and thus they are more likely to be protected from fire and live to germinate at the right time after the fire.
I have no idea what the evolutionary advantage is of selecting medium-sized seeds, which would obviously stabilise this particular seed trait in a population. One can guess that puny seeds are not vigorous enough to convey advantage in a tough world, so they may as well get eaten. And large seeds are just too heavy for the little carriers.
So average is seemingly the right size for success in the fynbos world.
About the Author
Whale Coast Conservation passionately lives by its slogan “Caring for your environment”.
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