Body is singular also talking about self (my Body ) or another person (Their Body) Bodies is plural talking about ( all Bodies ) a group (Their Bodies ) It could Have also said, in the first sentence : It really should have said Bodies not Body (Dogs move their bodies to show us how they feel. Hope this helps, , Rich

It really should be “bodies” in both cases. The reason they have chosen body in the first example is that the use of the word “they” can also denote a singular, gender neutral replacement for “it”, making it comfortable for the writer to use the singular form “body”. It’s a quirk of the language that it is not easily considered an error. I’d recommend using the plural “bodies” if you were to write these sentences yourself.