Dolphins natural foam earplugs!
Marine mammals can protect their hearing naturally, at least in modest ways. The HIMB research team used suction cup electrodes to study marine mammal brain activity during echolocation. Using false killer whales in captivity, they were able to show that they can adjust their hearing sensitivity when they knew the impending sound would be loud, but they can also increase their hearing sensitivity when they have to find something that is far!
Learn more at: http://www.sciencemag.org/news/2017/12/whales-and-dolphins-can-naturally-muffle-loud-sounds-potentially-protecting-them-sonar