have to compete with other animals for water. Lions can die of thurst as easily as any other animal. In the winter, it is raining, and with the resurgence of the plants, the plant eaters spread out to graze, and the lions move with them. It is "easier" for the lion in the "winter" because it can generally hook up some food, and it doesn't have to search for water. The hot, dry "summer" beats the lion down, just like it hammers all the other animals.