Unlike his brother Einion, Dulas has taken his time travelling over the Sahara desert, finally arriving on the north Senegal border on the morning of September 27th. It is impossible to know for sure, of course, but Dulas has most likely gone several days without eating, maybe five days or more.
As soon as he reached the centre of Senegal he stopped for the night on the 27th and then immediately changed course the following morning, heading directly west in the direction of the Gambia River. How did he know to do that? He travelled less than 100 miles the following day after changing direction, he was probably still feeding himself up after his long desert journey.