So,the Kilo Five Triology states that ODST sent Sangheili skulls as thropys to their friends and relatives.But only Sangheili skulls?Or also skulls from other defeated enemys like Grunts,Jackals,the occaisional rebel?
And when do you think did this "tradition"started?
Although we don't know one way or another, it's possible that only Sangheili skulls were collected, perhaps due to the accomplishment of killing one. Though I'm sure there are exceptions.

I'm sure this tradition started pretty early. Maybe after the first Sangheili was dissected by humans?