Why are yawns contagious in the first place? (i.e. human to human)
Read about it, mammals will yawn when other non-mammals yawn.
if a see a snake (yawning or otherwise) i'll definitely not yawn.
puking, screaming or running would be more likely.
IIRC, it wasn't that contagious.
Or, restated, they confirmed it was contagious.
Also, primates are naturally selected to act mutually. Whoever didn't do so lost the tribe and died.
That's just high me thinking though.
Occam’s razor.
Not mentioned in the Wikipedia article, but in a referenced article [2]: "Another puzzling phenomenon is that some male animals, men included, yawn in association with penile erection"