Who is pollinating Pr humiflora
Trish Fleming and Sue Nicholson, University of Pretoria
In 1978 John Rourke (from the National Botanical Institute) and Del Wiens (from the USA) published their idea that some Protea species did not rely on the birds and the bees, and that mammals (of the non-flying variety) could be pollinators too! In studies carried out at Jonaskop in the western Riviersonderend Mountains in the late 1970s, they demonstrated that small mammals carried Protea pollen on their noses, presumably dusted there when they visited flowers for nectar. The source of the pollen was the Patent-leaf Sugarbush Pr humiflora, pinkish grey bushes about 1m high, growing in dense stands on the rocky slopes.
Pr humiflora and its relatives lack the large, obvious, colourful inflorescences that are held high in the air to attract bird or insect pollinators. Instead, their inflorescences are cryptic and dull-coloured, often dark brown or peachy maroon, and hidden under foliage, on or near the ground. Another feature of these flowers is their sweet nectar, which is between 30 and 60% sugar by weight and rich in sucrose, sometimes having the texture of syrup. This sweet nectar probably contributes to the musky smell of the inflorescences. Tony Rebelo has dubbed Pr humiflora, amplexicaulis, subulifolia, decurrens and cordata the "Rodent Sugarbushes" (although not all their pollinators are of the large-incisored variety see below), while John Rourke classifies them in the section "Hypocephalae" (meaning under-head).
Our recent study, also in the Jonaskop area, was designed to determine how important the nectar and pollen of Pr humiflora are to the animals that visit these flowers. To keep certain botanists happy, we also asked how important the mammals were for the plants. This was done by examining seed set in inflorescences at various heights on Pr humi bushes: it declined with increasing height above ground (presumably since the mice dont climb too high). We also placed wire cages around other inflorescences, preventing animals larger than insects from foraging on the inflorescences. And seed set was reduced by half! That makes the larger pollinators, presumably mammals, pretty important for these plants.
To find out which mammals were visiting these inflorescences was a little more work. We set up permanent trapping grids on two slopes where Pr humi is abundant. At each site we had two grids, and removed all the inflorescences from bushes on one of them. The other grid served as a control, allowing us to determine how the numbers of mammals change with season, without removing the flower resources. Then we trapped every month, from winter when Pr humi was flowering, through the mammal breeding seasons to summer.
The most common mammal in our traps was the Namaqua Rock Mouse (Aethomys namaquensis). There were substantial numbers of Protea pollen grains on the noses of these guys, so they are definitely visiting Pr humi. Unfortunately, Aethomys also have an appetite for the flowers themselves, and given their less than adequate tongue for the delicate business of nectar-lapping, these animals probably resort to chewing the flower styles to the base in order to gain access to nectar. Such destruction is hardly aiding the plants reproductive cause. Interestingly, Aethomys were oblivious to our removing inflorescences, and were probably more interested in their own social problems (mates and housing in particular).
The smallest mammals in our traps were Cape Spiny Mice (Acomys subspinosus). These little mice are not common, and were only found at one of our sites, but here their numbers were significantly reduced after removal of inflorescences. Spiny Mice appeared in traps near the most profusely flowering bushes. During winter, these mice fed almost exclusively in Sugarbush inflorescences, with an average of 30% of the volume of their scats being Protea pollen grains - for some individuals this figure reached 80%. We have no idea how much nectar they consume, since nectar is so easily digested (unlike pollen), but we guess that it would be a significant amount. Interestingly, Acomys appears to be an opportunistic breeder, and was breeding during flowering of Pr humi, presumably utilising the extra resources provided by the flowers.
The most surprising candidate for pollination of Pr humi was the Cape Rock Elephant Shrew (Elephantulus edwardii). Why surprising? Well, this isnt even a rodent; elephant shrews are insectivorous and belong to a totally different order, the Macroscelididae. And yet these shrews were hopping around the Fynbos with large Protea pollen loads on their delicate little trunks (noses, that is). Because of less fastidious grooming, they have more pollen on their faces and less in their faeces than Aethomys and Acomys. They may even be better pollinators, since they dont eat plant material. In fact, they may not even be after the nectar: we suspect that they are foraging in inflorescences for ants and other insects. They also have massive home ranges, far greater than those of the rodents, benefiting the plants by increasing genetic flow via pollen transfer.
So much for who IS pollinating Pr humi. We also caught a few other species that did not appear to be strong candidates for pollinators in the 2000 flowering season. Low numbers of Striped Mice (Rhabdomys pumilio) were trapped (although they were around half of Rourkes and Wiens captures), and most were caught after flowering. The Vlei Rat Otomys we captured had few pollen grains on noses or in scats, and are more likely to eat the flowers than pollinate them.
If you have seen Pr humi, then you will surely recall the harsh and inhospitable conditions it lives in, preferring the hot, dry, north slopes of mountains. The next time you are there, think about the mammals that live there too, and what a treat those nectar-rich inflorescences must be. Although the plants have a patchy range and a limited flowering season, for the mammals, the carbohydrate-laden nectar and protein-rich pollen make these cryptic, hidden inflorescences well worth the search.
Readers may recall that a few years ago the Cape Spiny Mouse made our
cover of PAN 26 as the first (and still only) known cacher or hoarder of Conebush seeds.
Back PAN 52