How did the rat’s tail plant get its tail?

The rat’s tail makes a perfect perch for a femal sunbird to sit on while feeding. (Photo: Bruce Anderson)

To make the most of its relationship with animals, a type of iris found on the Cape West Coast called the rat’s tail has developed some funny looks. So says botanists from Stellenbosch University and the University of  Toronto in Canada.

The rat’s tail (Babiana ringens) is endemic to the sandy areas within the Cape Floristic Region and is especially common on the West Coast, where its bright red tubular flowers are conspicuous in late winter to late spring. Its flowers are clustered together on an inflorescence stem, mere centimeters off the ground.

Some plants cluster their flowers at the top of an inflorescence stem, sometimes even leaving the lower parts of it bare. This ensures that the apical flowers are positioned on a level where they will be easily noticed by pollinators, and can aid with seed dispersal as well.

However, the rat’s tail is the only plant that botanists know of where the arrangement of flowers happens the other way round. The top part of the inflorescence stem is quite bare or naked, whilst the flowers grow at the bottom of it.

“Ground-level flowers are rare in general, and particularly among bird-pollinated species, because feeding on the ground makes them vulnerable to predators,” explains Ms Caroli de Waal, a doctoral student in Botany at Stellenbosch University.

Close-up of a rat’s tail (Babiana ringens). (Photo: Bruce Anderson)

“We needed to find an alternative explanation for the design looks of the rat’s tail,” she added.

Ms de Waal is the lead author of a new study on the rat’s tail published in the influential publication American Journal of Botany, which she wrote together with her supervisor, Dr Bruce Anderson of the Department of Botany and Zoology at Stellenbosch University and Prof. Spencer Barrett of the University of Toronto (Canada).

Ms De Waal, a born and bred Free State resident, first worked on the Cape’s flower diversity and rat’s tail in particular when she studied for her MSc degree under the guidance of Prof Barrett in Toronto.

Previous research by the scientists from Stellenbosch University and the University of Toronto revealed that the naked flowering stem provides a convenient perch for its main pollinator, the malachite sunbird (Nectarinia famosa). From its perch, the bird turns upside down to access nectar in the tubular flowers and therefore facilitates cross-pollination.

New findings show that the particular position of the flowers growing on the ground actually discourages plant-eating animals such as the Cape grysbok from nibbling on them. As a result, these plants are able to produce more seeds, providing them an evolutionary advantage.

“To be eaten, or not to be eaten, seems to be at the heart of the matter,” Ms de Waal sums up the findings.

In nature, it is uncommon to find species with flowers growing so close to the ground, especially if they are to be pollinated by birds.

“There is always the risk that birds could be caught by a predator while feeding so close to the ground, rather than when they perch higher on a plant,” says Ms de Waal, who along with Dr Anderson and Prof Barrett published a previous paper describing the unique rat’s tail and its relationship with the malachite sunbird.

Their latest findings are based on comparisons between the rat’s tail with its closest relative, the “strandlelie” (Babiana hirsuta), a common and conspicuous species along the West Coast.

Ms Caroli de Waal. (Photo: Spencer Bartlett)

“We noticed that in populations of B. hirsuta, many plants were damaged by plant eating mammals, with the upper portions of the stems completely eaten off,” Ms de Waal notes. “We then started to wonder whether herbivory might have contributed to the two unique features of the rat’s tails: ground-level flowering and the naked inflorescence axis.”

Although B. ringens and B hirsuta are closely related and even share the same pollinators, they differ significantly in the way their flowers are positioned on the plant.

The researchers set off to test whether the evolution of the flowering display of B.ringens was influenced by the preference of antelope to eat the top flowers on a stem, whilst leaving flowers growing near the bottom alone.

They surmised that the two unique features of B. ringens probably evolved from a B. hirsuta-like ancestor in which the production of side branches was suppressed.

Therefore, they conducted a comparative study to test the effect of grazing antelope on the design of flowering displays. The researchers used three B. hirsuta populations near Mamre and Hopefield, and three B. ringens populations around Velddrif, Elands Bay and Lamberts Bay.

Much higher levels of herbivore damage were found in B. hirsuta populations, with more than half of all flowers being eaten off. Moreover, they found that antelope mostly grazed the top parts of a flowering plant, leaving the parts growing at the bottom to continue flowering. In comparison, herbivory in B. ringens was much lower and the ground-level flowers were never eaten.

They also conducted a field experiment near Velddrif to test their hypothesis and for this they manipulated the flower position along the stem. The researchers also used cages placed around some the plants to keep the antelope from grazing on them.

“We saw that the plants with flowers at the tips of stems were eaten by browsing antelope, whereas those with flowers at ground-level were consistently ignored,” Ms de Waal explains.

“Significantly, the plants that were manipulated to have only ground-level flowers like B. ringens produced more seeds,” she said. “This means that plants that manage to escape damage by herbivores also have higher reproductive success, and this may be the reason why B. ringens evolved its unusual ground-level flowers.”

“The overwhelming preference of herbivores to graze on only apical flowers found at the end of the stem indicates why plants like B. ringens, with basal flowers, could have evolved while plants with only apical flowers would be maladapted,” says Dr Anderson.

“Our results indicate that in addition to pollinators, herbivores can also be important selective agents on inflorescence architecture,” he adds. “This is significant because most scientists attribute variation in floral display to selection by pollinators, whilst often forgetting the importance of herbivores.”

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Caroli de Waal, Spencer C.H. Barrett, and Bruce Anderson. 2012. The effect of mammalian herbivory on inflorescence architecture in ornithophilous Babiana (Iridaceae): Implications for the evolution of a bird perch. American Journal of Botany 99(6): 1096-1103. DOI: 10.3732/ajb.1100295

The full article in the link mentioned is available for no charge for 30 days following the date of this summary at http://www.amjbot.org/content/99/5/806.full.pdf+html.

• The Botanical Society of America (www.botany.org) is a non-profit membership society with a mission to promote botany, the field of basic science dealing with the study and inquiry into the form, function, development, diversity, reproduction, evolution, and uses of plants and their interactions within the biosphere. It has published the American Journal of Botany (www.amjbot.org) for nearly 100 years. In 2009, the Special Libraries Association named the American Journal of Botany one of the Top 10 Most Influential Journals of the Century in the field of Biology and Medicine.