Oestrus? synchronized cycles, clutches every few years, festivals and orgies. Would the females be sexually receptive the rest of the time?

But then... even if the females in one place are in sync... males could try to go somewhere else to try their luck if the local females are not on heat.

Being invited to a friendly clan's orgy could be a way of cementing alliances. And thinking that some of the young of an allied clan may be your own could make you less likely to fight against them in future.

But does this reduce the female control of blood lines?

F: "You can't have babies with us when we are not on heat and we control the bloodlines (somehow)".

M: "OK. I'm just off to visit our neighbours for, umm, reasons."

: That would work.

And, combined with sperm selection, gives females control over bloodlines that would be hard for males to circumvent".