I really didn't want to get into discussing testis size.

I'm writing a paper on the choice male primates face between caring for the young they already have, and fighting other males for access to mates so they can have more young. There are, of course, other reproductive strategies they could also be investing in, like trying to impress the ladies without fighting, or impressing the ladies by being such good carers. I didn't want to get into all of these, because it just gets to big and complicated, and the relationship between the variables I have is already complicated and interested enough. But my colleague who read the paper for me thinks it is important to include something on investment in sperm competition. Sperm competition occurs when a female mates with more than one male, but only one of them will end up being the father of each baby she produces. The male with the strongest, fastest, healthiest, most numerous sperm will tend to be the father, and as such will tend to pass on his genes for big healthy sperm to the most sons. And to make lots of big healthy sperm, a male needs big testicles. Testicle size turns out to be closely correlated with how much multiple mating the females do, and in highly promiscuous species, males may dedicate a significant portion of their body mass to testes. And this is how I came to be typing search terms like "bush baby testis mass" and "monkey sperm competition" into Google Scholar. You would be amazed what Google is willing to label as a scholarly article.