10.09.2018 – Mammals without borders

Both our boats had to cover a vast search area during our tours in order to find cetaceans today. Amidst our usual quest on Monday mornings to find dolphins we could snorkel with, we had the pleasure of encountering Sperm whales (Physeter macrocephalus), Short-finned pilot whales (Globicephala macrorhynchus) as well as a swift swimming Sei whale (Balaenoptera borealis) and her calf. Our traditional boat encountered an energetic school of Striped dolphins (Stenella coeruleoalba) while the late afternoon tour on the Stenella had sightings that included Tropical whales (Balaenoptera edeni), Short-finned pilot whales and Bottlenose dolphins (Tursiops truncatus).

Apart from their genetic makeup, whales and dolphins have an array of characteristics in common although several vary amongst species and families. One feature that we tend to forget amidst our admiration of their complex social behaviour, their communication and their sheer size is how well these mammals are adapted to marine life. The fact that an air-breathing creature can survive so magnificently in the open ocean is truly special. If we want to understand why and how these mammals adapted so well to life in our oceans, it definitely helps to consider the evolution of these unique beings.

To keep this brief and understandable, it’s important to keep one thing in mind; life on earth as we know it began in the ocean. The ancestors of cetaceans are assumed to have been terrestrial mammals, known as even-toed ungulates, that returned back to the ocean around 50 million years ago in the Indian subcontinent. The evolutionary adaptions that gradually took place in these creatures causing their taxonomic divergeance from terrestrial mammals, gradually occurred over a time span of about 15 million years. Despite this long process that involved several significant physical changes, cetaceans still share certain characteristic features with terrestrial mammals. They breathe air using lungs, have 5 finger bones in their flippers, they nurse their young and have retained several skeletal elements that are still seen in land animals.

Their aquatic lifestyle, however, did call for several modifications. It caused echolocation to evolve in toothed whales (Odontoceti), filter-feeding using keratin plates in baleen whales (Mysteceti) and allowed the gradual development of the streamlined body shape in all cetaceans along with their laterally compressed fluke and their formidable horizontal tail fin. The horizontally arranged tail fin is again a reflection of their ancestry; since the vertebral column of whales evolved from that of land animals it is designed to move up and down, rather than back and forth as it does in bony fish. Whales and dolphins undulate their vertebral column as they swim, which also led to their bodies eventually becoming streamlined.

When I explain this to guests one particular question tends to crop up; why? Why would air-breathing mammals return to live in the ocean, why go through all the trouble of adapting to an ecosystem that doesnt favor animals that breathe air? Well one theory may have been that these land mammals were foraging near or in marine habitats and evolved to gain certain advantages during hunting. Apart from the possible initial reasons as to why whales evolved; living in the ocean does have its advantages. The absence of geographic barriers allows the animals to migrate and move around freely. Baleen whales have more or less fixed migration routes which are often adapted according to the physical changes occurring in the worlds oceans. Dolphins are opportunists that will happily change location if it brought them any foraging or mating benefits. Terrestrial animals often do not have such liberties and this may be one reason why the phenotype of so many marine animals has remained relatively unchanged for millions of years and why, despite anthropogenic pressures on their environment, these animals are still successful today.

By Paula Thake

Sightings of the day

Ribeira Brava

13:30 Striped dolphins

Stenella

09:00 Sei whales, Short-finned pilot whales, Sperm whales

15:30 Bottlenose dolphins, Short-finned pilot whales, Tropical whales