“Oh, what a tangled web we weave”... because of small brains?

We have a burning need to explain what big brains do, because humans have such large brains. “Big brains smart!” is a common idea for why species vary in brain size. But testing this is a fiendish problem. How do you compare widely different species with different ecologies?

Is an osprey smarter than a hummingbird, for example? The osprey is smarter at catching fish than a hummingbird could ever be... but a hummingbird would kick the osprey’s tailfeathers at analyzing flower patches.

One approach is to look not at any one behaviour, but some overall suite of behaviours, and see how different species vary in how many behaviours they produce. This might give a sense of how complex the behaviour of one species is versus another.

William Eberhard has been tackling this problem for a few years now, studying orb-weaver spiders. The advantage of using spiders to study behavioural complexity is that you can use the web as a proxy for the behaviour. The web is long lasting structure, so you don’t have to watch and notate the entire sequence of behaviour. And the spiders vary in size by about a factor of 1,000. Although he didn’t measure brain size directly, I think it’s reasonable to expect that one thousand times bigger body size should be reflected in brain size.

Eberhard used Anapisona simoni (pictured) as his key “small species.” This species weighs about one milligram, compared to other, larger orb-weavers that ranged from 30-40 milligrams as adults. But he did some experiments with A. simoni babies, which register at a barely detectable 5 micrograms!

Eberhard looked for variations in the web structure, working on the hypothesis that small brains might leave the smallest spiders less able to vary their webs in response to different conditions.

A picture might help here. If, when a spider is making a web, it “accidentally” lays down a particularly large space (such as those in the right side marked by the arrow and white rectangles), can it then adjust the other spaces around it to try to even out the spacing?

As it happened, larger spiders made these sorts of compensations more often than the small ones. This is consistent with the notion that small spiders are suffering from behavioural limitations because of their small nervous systems (which Eberhard calls the “limitation hypothesis”).

But in fairness to the small spiders, this is only one of several experiments and studies Eberhard reports on. The majority of studies suggest that small spider species are limited in their behavioural repertoire, but other experiments suggest small spiders are not limited in the behaviour.

This makes for an interesting discussion section in this paper. Eberhard interprets the balance of evidence as rejecting the limitation hypothesis. You can almost feel Eberhard arguing with imaginary reviewers and critics and readers as he points out that you can’t just total up “Experiments with results consistent with the limitation hypothesis” versus “Experiments with results not consistent with the limitation hypothesis” and call it a day. Each experiment provides different information.

For instance, Eberhard points to the alternate web forms that his tiny A. simoni spiders make. They are not common, but Eberhard never saw the larger species make them. And there is certainly an appeal to the argument that something that can make two kinds of structures has a more complex behaviour than something that can only make one.

The discussion is long and nuanced. But what is missing in this discussion of these ideas is one critical data set.

Brain size.

I buy that big differences in body size are reflected in brain size in spiders. But we don’t know just how body size and brain size scales in invertebrates, either during growth or across different species, because there are few good measures or databases of invertebrate brain sizes.

And to make matters even worse, we don’t have information about what regions of nervous systems in spiders are involved in making webs. We know that in vertebrates, some regions of the brain may be bigger than predicted by body size or other regions of the brain if those brain regions are important to the animal.

It might be that when we look at the brains of these tiny spiders, some huge proportion of it is devoted to web-building. And it may be that when we look at the brains of big spiders, a small proportion of it is devoted to web-building, making the differences in body size slightly misleading.

In fairness, Eberhard recognizes this limitation. It seemed worth emphasizing here. Any neuroethologist who wanted to start tackling the neural basis of web-building behaviour would no doubt have more than enough challenges for a long and productive career!

Reference

Eberhard W. 2011. Are smaller animals behaviourally limited? Lack of clear constraints in miniature spiders. Animal Behaviour. DOI: 10.1016/j.anbehav.2011.01.016

Spider photo from here; web photo from Eberhard’s Figure 2.