The article “Punishers Benefit from Third- Party Punishment in Fish” (Science, 8 Jan 2010: 171) describes an experiment meant to simulate behavior of cleaner fish, so named because they eat parasites off of “client” fish but will sometimes take a bite of the client’s mucus instead. (Cleaner fish prefer the mucus over the parasites.) Eight female cleaner fish were provided bits of prawn (preferred food) and fish-flake (less preferred), then a male cleaner fish chased them away. One minute later, the process was repeated.

a. The following data on the amount of prawn eaten by each female in the two rounds is consistent with information in the article. Use Wilcoxon’s signed-rank test to determine whether female fish eat less of their preferred food, on average, after having been chased by a male.

b. The researchers recorded the same information on the male cleaner fish (the chasers), resulting in a signed-rank test statistic value of s₊ = 28. Does this provide evidence that males increase their average preferred food consumption the second time around?

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Female 1st trial 2nd trial 1 2 3 4 5 6 7 8 207 215 .103 .182 282 228 152 293 .164 033 092 003 115 250 056 247