According to a recent study published in Current Biology, the diversity in size among modern cetaceans, such as whales and dolphins, emerged much later in their evolution than previously believed. The study, which utilized the largest dataset of its kind, involved analyzing body length measurements of 345 species, including 89 living species and 256 fossil lineages.
Contrary to expectations, the research found that cetacean size remained relatively consistent for over 20 million years after they entered the ocean. Despite the existence of whales of varying sizes during this period, they were all evolving towards an optimal length of approximately 12.5 meters, similar to that of a humpback whale.
Co-author Dr. Gustavo Burin noted that previous studies on cetacean size evolution had not combined measurements from living and fossil species to this extent, making their findings particularly comprehensive.
The study’s inclusion of fossil species proved instrumental in shedding light on the complex patterns of cetacean evolution. It challenged the notion that evolutionary trends in body size are straightforward, emphasizing the importance of considering extinct species in such analyses. The research identified two major shifts in cetacean evolution, with the early focus being on larger sizes, followed by a second shift approximately 30 million years ago, which saw the emergence of fast-moving, agile predators, such as modern dolphins. The most significant size changes occurred in specific branches of the cetacean family much closer to the present, including a 175% increase in the average size of baleen whale ancestors. The scientists visualized these changes as adaptive landscapes, where advantageous characteristics are represented as peaks. The height of these peaks signifies the degree of advantage they provide in a particular environment.
The research team has made an interesting discovery: when studying both living and fossil cetaceans together, the previously observed body size peaks disappeared. Instead, they found that the adaptive landscape was relatively flat, with very few peaks.
The earliest cetaceans were small creatures roughly the size of goats. They lived near rivers and lakes, spending time both on land and in the water. However, over time, their descendants became increasingly adapted to aquatic life until they eventually abandoned land entirely.
These types of species, which transition from land to water, are referred to as secondarily aquatic. Along with cetaceans, they include animals like crocodiles and penguins. Moving into the water presents unique challenges for these species, and one way they overcome these challenges is by growing in size.
According to Dr. Travis Park, one of the co-authors of the study, “Many secondarily aquatic lineages tend to increase in size shortly after transitioning to water as it often provides an advantage. For example, small species lose heat rapidly underwater, so growing in size can help regulate body temperature.”
“Returning to the water may also release evolutionary constraints like gravity, allowing for the development of much larger body sizes. It’s hard to say which factor is the most significant since body size is influenced by a variety of different factors that all work together.”
The size of the minke whale, the smallest baleen whale, has reportedly declined in recent times. This has led some researchers to speculate that the species may have adapted to target smaller and more agile patches of krill at night when other whales are less active.
Interestingly, over 30% of the skull measurements used in the study were obtained from specimens in the Natural History Museum. Additionally, data on the total length of 1,659 individual whales was sourced from the NHM cetacean strandings database, highlighting the crucial role played by museum collections in facilitating research.
Moving forward, the team of scientists responsible for this latest research plan to apply a similar approach to studying other semi-aquatic animals, starting with seals. Their goal is to investigate how these groups have evolved over time and how competition between them may have influenced their development.
Source: Natural History Museum