It is human nature to look around us and try to make sense of what we see. So, of course, when observers of the natural world long ago realized that there were female crab spiders that came in different colors, they wondered why. It appeared that yellow spiders lived in yellow wildflowers and white spiders lived in white wildflowers. Therefore, the obvious conclusion, color differences must be for camouflage. Yellow spiders resting on yellow flowers are practically invisible – to us.
Crab spiders eat wasps (this is one spider I would not mind having in my backyard). They sit in the center of the wildflower, and wait. Wasps come to the flower to collect nectar, and the perfectly camouflaged spider is waiting there, and happily collects dinner.
It is completely logical and downright sensible. In an homage to Rudyard Kipling, I argue that this is the perfect ‘just-so’ story. So much so, why would any scientist bother to test this?
But, it turns out that this idea has never been tested. That the spiders eat wasps as described above is known. But, is the camouflage important? Do yellow spiders on yellow plants actually do better at capturing wasps than yellow spiders on white plants, or white spiders on yellow plants? Shockingly, they do not, as featured in the journal Science's ScienceShots. The camouflaged spiders are no better than non-camoflauged spiders. Yellow spiders on white, yellow, or purple flowers all have the same success, as do white spiders.
So, why do the spiders come in different colors? They both eat the same food (wasps), so it is not food causing the color change. They cannot actually change colors when they move to different flowers, so it is something they are born with, and they must then choose the right flower in order to be hidden effectively. Perhaps the spiders are camouflaged in order to be protected from their predators, whatever organism is seeking them out for dinner. I am guessing birds. Or, perhaps it is as the authors of this research speculate; not about camouflage at all. The different colors have differing UV protective qualities. The point is – we’ve got no idea.
I recently gave a presentation of my own research to my group of graduate students at our weekly meeting. I was interrupted a few minutes into my presentation by a student who asked “wait a minute, this relationship seems so obvious, don’t we already know this?” Turns out, young grasshopper, that we do not. And, the conclusion was not as it would have seemed to be, on the surface of it all.
That we do not yet know all there is to know about the natural world is reassuring to me. One – job security. Two – the natural world still contains the greatest mysteries of all time. It is enough to keep me entertained for probably the rest of my days.
So, why does the giraffe have a long neck? Giraffes, it turns out, have not evolved long necks in order to gather leaves from tall trees, though those necks are still darn handy in that regard. Long necks are a sexually-selected trait. Meaning, females prefer males with long necks, and have, with their choosiness, driven the course of evolution. Much like the infamous Tule Elk, whose antlers are so large that the largest males cannot lift their heads off the ground. Biology isn’t always logical, but those female Tule Elk sure like large antlers.
Are blue whales the largest animals ever to roam the earth? And, does the fact that they are in the water, and do not have to bear their weight on land, allow them to get so large. There is pretty convincing evidence now that there were dinosaurs much heavier than blue whales, and that they lived on land.
As for how the leopard got its spots, well Rudyard Kipling had this one pretty close to right in his Just So Stories. The hunter in the story observed that the animals, without their stripes and spots, “ought to show up in this dark place like ripe bananas in a smokehouse.” I’m pretty sure he is spot on.