How the cow got its spots

Why do some mammals have spots, stripes, or other sorts of markings on their fur? Usually, this has been chalked up to camouflage or because the patterns might be sexy. But a new idea has been making the round this year: that it helps keep bugs away.

In March, Egri and colleagues published a paper suggesting zebra stripes deterred biting flies. Having tested stripes, the same team are back in a new paper to test spots. In their new paper, the team is working with slightly less exotic animals, but with more practical implications: cows.

If you’ve seen cows in a field, you’ve probably seen them swishing their tails to bat away flies. Flies are not just an annoyance: they can affect cows so much that they seriously harm the cows’ health. The flies bite and suck blood, and can transmit disease this way. Plus, the distraction can be so great that the cows don’t feed enough and lose weight.

To test the idea, the team set out boards painted with varying amounts of spots, but with the same proportion of black and white. They covered the boards with a glue, so that if a fly landed on it, it would be stuck, allowing them to easily measure the attractiveness of the surface. The more small spots, the fewer flies they found on the board. They tried this with the boards in different orientations, but the results kept coming out the same.

The team then moved on to a more realistic, cow model. Covered in glue, naturally – potentially annoying any cow tippers who happened to be in Szokolya, Hungary, where the experiment was carried out. The results were the same: lots of small spots meant few flies captured. Dark brown cows had lots of flies landing on them.

What is going on here? The team suggest that this all happens because of one important fact about the flies: they lay their eggs in water.

How do you get from that to cowhide? Like this:

Flies lay their eggs in small pools of water, so they need to have reliable ways to detect bodies of water.

Light reflected off water is polarized, and many insects, including these flies, can see polarized light. Thus, the flies have a built-in detection system for, and a preference to move to, polarized objects.

The dark fur of cattle polarizes light more than the white fur. (This is the one point in the study where the researchers used actual cows, not just painted models.)

The team goes on to suggest that besides having some agricultural applications, humans might take a few lessons from this research, since flies that bite cattle can just as easily bite humans:

Seems to be working: no flies on her.

References

Blahó M, Egri Á, Bahidszki L, Kriska G, Hegedus R, Åkesson S, Horváth G. 2012. Spottier targets are less attractive to tabanid flies: on the tabanid-repellency of spotty fur patterns. PLOS ONE 7(8): e41138. http://dx. doi.org/10.1371/journal.pone.0041138

Egri Á, Blahó M, Kriska G, Farkas R, Gyurkovszky M, Åkesson S, Horváth G. 2012. Polarotactic tabanids find striped patterns with brightness and/or polarization modulation least attractive: an advantage of zebra stripes. Journal of Experimental Biology 215: 736-745. http::/dx.doi.org/10.1242/​jeb.065540

Cow photo by Brenda Anderson on Flickr; dress photo by VancityAllie on Flickr; both used under a Creative Commons license.