Passage: A Greek coalition made up of Aetolians, Boeotians, Athenians, Phocians, and other Greeks north of Corinth took up quarters at the narrow pass of Thermopylae, on the east coast of central Greece. During the initial assault, Brennus' forces suffered heavy losses. Hence he decided to send a large force under Acichorius against Aetolia. The Aetolian detachment, as Brennus hoped, left Thermopylae to defend their homes. The Aetolians joined the defence en masse – the old and women joining the fight. Realizing that the Gallic sword was dangerous only at close quarters, the Aetolians resorted to skirmishing tactics. According to Pausanias, only half the number that had set out for Aetolia returned.Eventually, Brennus found a way around the pass at Thermopylae, but by then the Greeks had escaped by sea.

Question: who suffered a heavy loss?
Brennus' forces