Saturday, 27 October 2012

Weird and Wonderful: Bagheera kiplingi, the vegetarian spider

Jessica Towne

Around 40,000 species of spider have been discovered so far, and all of them were considered to be carnivorous. That is, until one of them was found to survive on an almost entirely vegetarian diet. Bagheera kiplingi, a jumping spider named after the panther in The Jungle Book and his creator, Rudyard Kipling, is found throughout Central America. The spider was named in the late 1800’s from a single dead specimen, and nothing was known of what it might eat.

However, between 2001 and 2008, Christopher Meehan of Villanova University and Eric Olson of Brandeis University independently studied the spider in Mexico and Costa Rica. Their report on the subject was published in Current Biology on 13th October 2009. Until then, the only examples of spiders consuming vegetation were the occasional pollen and nectar eating habits of a few species. They discovered that B. kiplingi lives only on acacia trees, which are also inhabited by ants. These ants have a symbiotic relationship with the trees, as the ants defend the plant from large herbivores, and are rewarded with food and shelter in the form of hollow thorns. The acacias produce nutrient rich nodules on the leaves called Beltian bodies, and these are what both the ants and spiders are after. B. kiplingi was observed “hunting” the Beltian bodies, using the techniques other jumping spiders would use to hunt live prey. Jumping spiders are well known for their acute eyesight, speed, and as their name suggests, jumping abilities. The ants aren’t keen on having the spiders around, so the spiders tend to stay on older leaves where the ants aren’t on patrol. B. kiplingi uses its agility to avoid the ants and grab all the Beltian bodies it can. In fact, the spiders have been observed using several different methods for avoiding the ants, which demonstrates the incredible cognitive abilities that all jumping spiders share. For example, the spiders will hang from a thread of silk until the ants pass, or simply leap away from danger. It has also been suggested that the spiders may also mimic the ants’ appearance and movements, which may be why their remarkable lifestyle has remained a secret for so long.

B. kiplingi is not a strict vegetarian, however. They have been observed preying on ant larvae and other small invertebrates, although in the case of the Mexican population, this only accounts for around 9% of their diet. Meehan’s and Olson’s findings were confirmed by looking at the nitrogen content of the spiders’ bodies, which tends to be higher in carnivores than in herbivores. The ratios of the isotopes N-14 and N-15 were analysed, and it was found that B. kiplingi had 5% less N-15 than other species of jumping spider.

The next mystery to be solved is how the spiders digest the Beltian bodies. Spiders digest their food externally, so solid food must be quickly broken down before it can be eaten. Beltian bodies are about 80% fibre, so how B. kiplingi manages to eat the stuff is unclear. This may mean that the spiders’ digestive systems have become specially adapted to eating vegetation. A final observation by Meehan and Olson is that mutual relationships, such as between the ants and acacias can affect, and be exploited by, other organisms. In this case, the ant-acacia relationship is exploited by the vegetarian jumping spider.

Check out B. kiplingi in action in this video