by Matthew Cobb
The “laws” of biology aren’t like the laws of physics, because they deal with stuff that’s alive, which doesn’t always obey the mathematical rigour of a “law”. And when we think we’ve found something that’s hard and fast, there’s generally an exception. So, for example, over 40,000 species of spider have been described, and they are all carnivorous (even if some occasionally sip nectar or eat pollen). “Spiders are carnivorous” would seem to be an appropriate generalization. But it isn’t a law.
Today’s issue of Current Biology [subscription needed] shows why. It features an amazing discovery – a largely herbivorous jumping spider (Salticid) going by the charming name of Bagheera kiplingi (if you don’t know why that’s charming, then you need to either watch or better still read The Jungle Book). This tiny spider has been found in Mexico and Costa Rica.
The spider eats the juicy orange tips (“Beltian bodies”) of the leaves of the acacia tree, which are protected by ants in a classic mutualist relationship (see picture below). First discovered in 2001 by Eric Olsen (Brandeis), and then – independently – in 2007 by a student, Christopher Meehan (Villanova, the spider uses its acute vision and jumping ability to avoid the ant protectors. Both behavioral observations and chemical analysis show that the spider eats the Beltian bodies. However, it is not strictly herbivorous – it will also nibble the odd ant larva.
The article – with Robert Curry (Villanova) as senior author – describes the biology of B. kiplingi and concludes that, paradoxically, the spider was able to make the shift to herbivory only because there was the previously-existing mutualism between the acacia tree and its Pseudomyrmex ant protectors. The ants protect the Beltian bodies very effectively, and only an agile herbivore – like the spider – could get past their protection (download a video here or watch it at the BBC here). Without the ants, more bovine herbivores would easily out-compete the spider.