The Lions in the Selous have small manes in comparison to the rest of Tanzania's Lions. This could be due to a couple of reasons. The first is inbreeding and the second is due to the climate in the Selous, we are only 70m above sea level and 100km from the coast, so it can get quite warm here!
A big mane would hinder the lions when the temperatures rise later in the year. The Lions mane aids in making it seem bigger, useful in confrontations with other males or Hyenas!