listen to a conversation between a student and her marine biology professor. Sorry to interrupt. Professor Lang, looks like you\'re cramming for a test or something. Oh, I subscribe to three marine biology journals. Advances in my field come fast and furious. I have to stay one step ahead of my students. So what\'s on your mind, Sarah, I wanted you to know that letting me take the makeup midterm exam in your office yesterday really meant a lot. As I mentioned, both of my parents had to go overseas for conferences last week. That almost never happens. So I had to go back home on Thursday and Friday to look after my little brother. Glad it worked out. So what did you think of the midterm? I felt pretty confident, except for one question, maybe because I missed your Thursday lecture. Oh, I thought we\'d arrange for you to attend that class session remotely. The audiovisual system in the lecture halls allows students to do this, as long as their professor gives them the access code. I didn\'t forget to email it to you. Did I? No, I got your email and was planning to attend class through my laptop, but my brother\'s teacher called that morning. He fell on the playground and landed on his elbow. Oh, no yeah, it wasn\'t serious. But by the time I got back from the doctor\'s office, class was over. That\'s a shame. Sarah, I haven\'t granted your exam yet, but I\'m curious, which question Did you have trouble with? I wasn\'t sure about the marine mammals you asked about, like the blue whale, why natural selection led it to grow to such a massive size, much bigger than the largest dinosaurs were. I wrote that whales grow big because, well, because they can water so liberating. Weights just not an issue if you don\'t have to support yourself on legs like land mammals do. Yes, that\'s what intuition tells us. But, and we did cover this in the class you missed, water actually has the opposite effect. Water doesn\'t allow mammals to grow big. Marine mammals have to be big in order to survive. Why？ body heat. If the mammals were very small, they\'d lose too much body heat too fast. They couldn\'t ingest enough calories to keep warm. Bigger mammal bodies retain more body heat, but there\'s a limit on the upper end of the size spectrum too. If the blue whale got any bigger, it wouldn\'t be able to consume enough calories to support its massive body. Fish don\'t have this problem, by the way, their metabolism differs significantly from that of mammals.