Sean K Todd

Phytoplankton form the base of almost all marine food chains, including that of the blue whale, the largest animal known to have ever existed. But far below the penetration of sunlight, a very different and only recently discovered food web relies solely on the chemosynthetic ability of bacteria.

Mammals are certainly represented in ocean life, but which species should be identified as "marine" when considering ocean productivity? The extremely complex marine food webs maintain long-term stability, even as they undergo natural perturbations over time. But when Homo sapiens enters as an apex predator, productivity can deteriorate, and systems can even collapse.

Although the irony is unmistakable, our understanding of marine mammals increased tremendously by having access to carcasses during the years of industrial whaling. Today, we focus on species protection while learning as much as we can via SCUBA, SONAR, tagging, biopsy darts, photo-identification, studying animals in captivity, and examining stranded individuals when available.

With plastic and nylon lines and nets becoming common in the last century, by-catch became an even greater problem for the marine mammals. When the media picked up the story in the mid-1960s, the public became engaged, and the Marine Mammal Protection Act was passed in 1972. But whale entanglement remains a problem, and some argue that even whaling was far less cruel.

Our high-tech use of the ocean for food, transportation, and energy has far-reaching effects, particularly on certain species. Focusing on issues from noise pollution to microplastics, we can mitigate our impact to provide better futures for ourselves as well as for marine life. The work begins with understanding the extent of our true impacts.

Not surprisingly, deep-diving marine mammals have evolved a physiology very different than our own. Adaptations including those related to blood chemistry, the location of stored oxygen, a variable heart rate, and articulated rib cages support the ability to go deep and stay long. But what about rising back up to the surface? How do they avoid getting "the bends"?

If you've ever jumped into frigid water, you quickly realize humans are definitely not adapted to life in the sea. What are we missing? In a word, it's blubber. In fact, blubber is such a successful insulator that marine mammals have evolved internal and external means for getting rid of all that heat, possibly even including planetary migrations.

Although there was a time when we treated the oceans as if they were too vast to feel our impact, we now know the truth: we have contributed to global climate change, ocean acidification, and overfishing. The results are potentially catastrophic - both to marine life and to our own health. But there is a bit of light at the end of this tunnel, and it depends in part on our own daily actions.

Marine mammals did not evolve from marine species. Rather, they evolved from land mammals who found a plethora of "suddenly" open ecological niches when the dinosaurs became extinct. Today's marine mammals might resemble each other because convergent evolution has led to similar adaptation. But best as scientists can tell, they have five separate lineages and no single common ancestor.

Sound travels much better in water than in air. In fact, low-frequency waves, such as those produced by certain whales, can travel through water uninterrupted for hundreds or even thousands of kilometers, allowing the animals to be "in touch" with their group over vast distances. But what happens when human-generated sound gets in the way?