Scientific Article for Students

Introduction Did you know that cetaceans (whales, dolphins, and porpoises) are mammals? About 50 million years ago, their ancestors lived on land. They later evolved to live in marine environments. Such an extreme habitat transition required many adaptations. One remarkable example is how cetaceans sleep: one half of their brain rests, while the other half remains active! This unihemispheric sleep ensures cetaceans are always watching out for predators and food. Other adaptations helped cetaceans hunt their prey underwater. For example, reduced oxygen demands allowed them to dive for longer and control their blood pressure at great depths. Thick layers of fat also helped insulate them from cold waters. There are many molecules that control physiological traits like sleep, blood pressure, and temperature control. Among them, neuropeptides are especially important. Neuropeptides are chemical messengers. They relay signals from one nerve cell to another, triggering a physiological change. They can also have more than one role in the body. Luckily, neuropeptides are found in all mammals. We wanted to know how these neuropeptides changed as mammals evolved. This information can help us understand how mammals adapted to living in the ocean. Methods Step 1: Identifying important neuropeptides We identified 12 neuropeptides. Previous research has shown they may be related to important traits in cetaceans. Step 2: Obtaining DNA sequences for each Neuropeptides are microscopic, so it is hard to detect differences between them. An easier way to compare neuropeptides is to look at their DNA sequences. DNA is like an instruction manual. A specific sequence can build a specific molecule (in this case, a neuropeptide). We got the DNA sequences for our neuropeptides from online databases. We did this for 202 different mammal species. This included 41 species of cetaceans. Step 3: Comparing the ability to create neuropeptides We used a computer program to “build” the neuropeptides based on their DNA sequences. The computer program gave each DNA sequence a score from 0 to 5. A score of 0 meant that the neuropeptide would work. (Think of it as having zero problems!). A score between 2 and 5 meant that the neuropeptide would not work. Then we compared the scores for each of the 12 neuropeptides in cetaceans and land mammals. Results It turns out cetaceans have lost the ability to make many of the neuropeptides found in land mammals. We found that many cetacean neuropeptides had scores higher than 2. Cetaceans also had fewer DNA sequences that made functional neuropeptides than any other group of mammals. For example, neuropeptide B helps mammals sleep and suppresses hunger. Most cetaceans do not have the correct DNA sequence to build neuropeptide B (Figure 1). There were some DNA sequences that created neuropeptides that worked. For example, cetaceans still produce neuropeptide Y. This controls blood pressure, fat storage, and sleep. Discussion Why would cetaceans have lost the ability to make many of the neuropeptides that land mammals still have? Over time, DNA can collect mutations. Sometimes these mutations create new traits that are beneficial. Sometimes they don’t do anything. But at other times they can mess up the instructions the DNA carries. This means that whatever the DNA sequence makes, it won’t work properly. This is probably what happened to these neuropeptides. The ancestors of modern cetaceans probably didn’t need a lot of those neuropeptides in their new marine environment. So when the DNA mutated, it didn’t matter. They may have even benefited by losing the ability to make specific neuropeptides. For example, they may have lost the ability to make neuropeptide B (which suppresses hunger and induces sleep). This allowed cetaceans to eat lots of food and keep one half of their brain awake. Luckily, most neuropeptides have more than one role. So cetaceans didn’t completely lose the ability to sleep or regulate their appetite. The evolution of cetaceans from land mammals is complex. But it is clear that the loss of working neuropeptides played a role. Their loss allowed early cetaceans to thrive in a marine habitat. Conclusion Understanding how cetaceans have adapted to the marine world could help us understand how other mammals have evolved in their new environments. After all, transitioning from land to water is not the only major habitat change mammals have undergone. For example, bats transitioned from climbing trees to flying. Unfortunately, many animals and plants are facing habitat loss. Find out about local plants and animals in your area and how you can help protect them!