Thursday, July 14, 2011

The Horseshoe Crab Diaries Part Four: In Which Paul Tries to Explain Why They Have So Many Legs to a Preschooler

So it's summer and during the summer my job has me working with a lot of preschoolers. I love them: they're hilarious. But in terms of science education it can be a really tough audience. I've been thinking about it in this way: preschoolers ask some of the greatest science questions but more often than not they are not developmentally/cognitively capable of understanding even a watered-down version of the true answer.

So the other day I was at an academic summer camp talking with some preschoolers about my favorite animal and one of them asked me: "why does it have so many legs!?"

I know! Why does it need all those legs? Wouldn't six or even just four be enough? Probably it would be. This is more likely than not an example of randomness in the body plan of animals. Take humans as an example. Why do we have four limbs? Because our mammalian ancestors had four legs. Why did they have four legs? Because the lobed-fin fish that gave rise to land vertebrates had four fins that transitioned to limbs as they developed adaptations that allowed them to crawl out of the water. And why did the first lobed-fin fishes have four fins? Probably due to a random mutation in the hox genes (the genes that help to control embryonic development of bodies and limbs).

The same is true of a horseshoe crab: it has ten legs because...well because it's ancestors probably did. Or because there was a random mutation in the lineage's hox genes.

That's a pretty abstract concept even for an adult. So here's what I said: "They have a lot of legs because the group of animals they belong to has a lot of legs. Crabs, insects, spiders, they all have lots of legs and so do horseshoe crabs. That's really all I can say about it!"

I'm pretty sure my answer made no sense to the little guys. I know they can do similar and different but the concept of "relatedness" among different kinds of animals I think is a little out of reach. And like I said: it's frustrating that I've never been asked this question by an adult with the background knowledge and cognitive ability to understand the answer.

Any similar stories out there? What was your biggest success helping young children understand tough science concepts?


  1. I love this story! I can just picture it all going down!

  2. Those preschoolers keep us on our toes, don't they?
    When I taught them, I would return the question to them. "Why do you think they have so many legs?" That would start a little brainstorming on the usefulness (or not ) of having many legs and the naming of other animals with many legs. In the end, as you say it , that's just the way horseshoe crabs are.

    I do not read your blog regularly, but when I do I find myself reading several postings in a row because they are so full of information and reflection on the diversity of our natural world. Also they are so well written. Thank you for the MN.