The correction:

“Empathy and pro-social behavior in rats” by I. B.-A. Bartal et al. (9 December 2011, p. 1427). On p. 1428, the last full paragraph of column 1 was incorrect. The paragraph should be replaced by this corrected text: “All female rats (6/6) and most male rats (17/24) in the trapped condition became door-openers. Female rats in the trapped condition opened the restrainer door at a shorter latency than males on days 7 to 12 (P < 0.01, MMA, Fig. 3A), consistent with suggestions that females are more empathic than males (7, 12, 13). Furthermore, female rats were also more active than males in the trapped condition (P < 0.001, ANOVA) but not in the empty condition (Fig. 3B).”

Odd to me is that they use 24 males and only 6 females. In my work (which is biology), we would usually use the same group size unless we had reason to know better.

For example, there's a new-to-us model that we think might have differences between strains, sexes, and vendors. We ordered in 8 mice of each type to run a pilot.

So they used 30 rats in total?Any numbers on how statistically well-powered this was?

30 rats are plenty if they selected them properly - different gene pools, different breeders etc...

You wouldn't necessarily care about genetic diversity for this sort of experiment. This experiment isn't trying to say "all animals" or "all rat species" exhibit this behavior; the interesting thing is that any animal does.

For me, the more important question is not whether all test subjects were genetically diverse: it's more important that the free rat and the caged rat are dissimilar. The situation would be vastly different if the rat pairs were related, since there is a genetically selfish motivation for caring for those within the same gene pool.

This becomes apparent when you try to co-house rats and mice. Litter mates are much less likely to fight.

Some recent research on the topic:

Oberliessen et al., "Inequity aversion in rats" (2016)

http://www.sciencedirect.com/science/article/pii/S0003347216...

Decety et al., "Empathy as a driver of prosocial behaviour" (2016)

http://rstb.royalsocietypublishing.org/content/371/1686/2015...