Shrew poo and the origin of species

Pitcher plants have carved up the market for eating animals. Some use sweet perfumes to attract flying insects, some position their pitchers close to the ground where ants roam, others thrive on the poo of shrews. A few even prefer a vegetarian lifestyle, or cannibalism if you like, living off leaf litter falling into their pitchers.

Using plants propagated and growing in Royal Botanic Gardens, Kew (UK), Botanische Gärten Bonn  (Germany), and University of California Botanical Garden at Berkeley (USA), and some growing naturally in Northern Borneo, a team of British and American botanists have mapped the pitcher architectures required for each of these eating strategies onto the evolutionary tree for Nepenthes, the Pitcher Plants.

The full story (apart from the correct nomenclature for the botanic gardens) can be found in the latest issue of the Journal of Evolutionary Biology.

Pitcher plants have various ways to trap their prey. The two main mechanisms are slippery wax crystals on the inner wall of the pitcher and what the authors call the ‘aquaplaning’ rim (or peristome). The latter are good for night insects, when humidity is high and the rim is slippery, but during the day wax crystals would have the competitive edge.

On top of these mechanical modifications, there are odours produced at night, or day, to attract a particular kind of creature. An intriguing variation occurs in species that eat ants. By varying the wetness, and therefore slipperiness, of their teeth they can allow scout ants to escape and recruit their mates back to the pitcher.

If insects and spiders are in short supply, pitcher plants have evolved some novel ways to get their essential nutrients. Nepenthes rajah has recently joined Nepenthes lowii as demonstrated ‘faeces-feeder’. Mammals such as tree shrews and rats, poo into the pitchers while they nibble on the nectar under the pitcher lid. Here the size and shape of the pitcher – toilet-like? – and the shape and angle of the lid all aid in attracting poo.

In the case of Nepenthes rajah is has the best of both worlds, with aquaplaning rim to trap insects when pitchers are too small for any mammal to prop and plop. 

Ulrike Bauer and colleagues argue that the specialisations required to trap different insects may have driven the rapid evolution of the fascinating array of species we see today.

Image: Display of carnivorous plants at Chelsea Flower Show this year - the Nepenthes are in the top left and right corners.