"Mules" waiting for the next ship to enter the lock. Unlike other large canals such as Welland in Canada where ships transit entirely under their own power, Panama uses mules to guide ships through the locks. There are four in all for each ship that attach to the ship with cables on either side at the bow and the stern of the vessel. They ensure that the ship does not scrape the sides of the lock. They don't actually pull the vessel, which proceeds under its own power.