Oh god - it *is* an Amazon thing, isn't it!!
I figured it was that 'flock-bonding' thing - similar to contact calling to make sure everyone is OK, even though they are out of sight. I guess it might also be the reverse of the saftey response that keeps them quiet when everyone else is quiet: in the jungle you don't want to draw attention to yourself by being the one making all the noise - but if everyone is shouting, you can shout too...
Or maybe it's just some kind of choir instinct?
I know that we've had to start saving episodes of 'Glee' just because Isis loves to hollar away to the shopw tunes with the rest of the gang (or maybe that's just her having gay dads?
)
But laughing is always a killer: if either of us laugh it will *always* trigger off Isis having a good laugh too; unfortunately, she has my cackle down so well that it just makes me laugh harder - which makes her laugh more too...!