Brain of the Ichneumon wasp too small for intelligent behaviour.
Development of wasp brain defies Haller's Law.
Master's student Emma van der Woude measured brain volumes of about ninety Ichneumon wasps - large and small - of the Trichogramma evanescens species. With a length of about 0.3 millimetres, this parasite of butterfly eggs is one of the smallest known insects. Van der Woude's MSc research resulted in an amazing discovery. The brains of the smallest wasps seem in fact to be smaller than the minimum size purportedly required for intelligent behaviour. And yet, intelligent behaviour is what the wasps display. A unique observation, according to the researchers.
But it got even stranger. The brains of the wasps seem to grow proportionately with their bodies. A wasp which is double the size of another has a brain which is also twice as big. This is contrary to what we know. In other animals, the brain grows more slowly than the rest of the body. An elephant which is half as big as its parents has more than half of their brain mass. In the same way, a domestic cat has relatively more brain mass than a tiger.
This principle is known as Haller's Law, named after the Swiss scholar who mentioned it in passing in an anatomy book in 1762. However, the Ichneumon wasp has broken this law. 'Its brain is really frighteningly small,' says Van der Woude. And yet it displays intelligent behaviour. It has the ability to learn how to link smells to the presence of butterfly eggs. 'It therefore seems to have broken all the laws.'
Van der Woude will continue to study the relationship between brain size and intelligence as a PhD student. Perhaps the smallest wasps do in fact take longer to learn, have a lower 'fitness' or have fewer offspring.
Another option exists. It seems unlikely, 'but would be really cool,' says Van der Woude. Perhaps the insect has a unique shrinking tactic. 'It could, for example, have very small brain cells,' she says, 'with just as many neurons as other wasps, but with these located closer to one another.'
But for the time being, this is just speculation. It is known, however, that other minibeasts on which research has been carried out do stick to the letter of Haller's Law. Some of them have brains that - being too big for the head - even extend into the body.