I don’t always talk about whales on this blog, but when I do, I prefer to talk about their size or echolocation. This time it’s about size. We all know many whales are really, really big. What we hadn’t quite understood yet is when they got big, or why.
A photo of several baleen whales surfacing as they engulf tiny krill. Source unknown.
Thankfully, a new [article] ran an analysis to figure out these two missing pieces. The authors used a dataset of 13 living and 63 extinct whales, including DNA (for the living ones) to create an evolutionary tree with time estimates for each branching point (called branch lengths). Using this tree and a dataset of body sizes, the authors used a model-testing approach to assess how the timing of whale evolution took place.
What’s a model testing approach? A model-testing approach is when we use computers to test the fit of different models to the data we give it. In this case, the authors gave the computer the tree and the body size data. The authors picked several different models for the computer to test. One of the models was based on random events driving evolution. One of them was based on evolution with a trend towards one trait. Some of the models were combinations of the others, where the model changes at a certain point during evolution. Once the computer is done testing each model, it produces a few statistical values that tell us how well each model fit the data.
A representation of the authors hard at work.
For this study, the authors wanted to know which model of evolution best fit the evolution of giant sizes in whales. The computer’s analysis showed that body size evolved randomly until around 3 million years ago when there was a shift to evolution with a trend of becoming very large. Even though filter feeding using baleen had been present in whales since 25 million years ago, it wasn’t until recently that they became very large. Around 3 million years ago, during the Plio-Pleistocene, wind-driven upwelling (where wind patterns on the surface bring up nutrients from the ocean floor) started getting stronger. This concentration of small food items is probably what lead to whales getting gigantic.
How upwelling works. Higher winds on the surface push the warmer surface water away from the coast. Colder, nutrient-filled water from the bottom gets pulled up. From NOAA.
Climate Change. It’s a topic that’s been in the media recently for a number of reasons, namely because we’re experiencing it now. We know that because we have climate records going back to 800,000 years (possibly [1.5 million years]) from ice cores. The gases trapped in the ice are made up of the atmosphere that was around when the ice formed. The atoms that make up the gas, like oxygen, carbon, and hydrogen, have different values of positive and negative particles (different isotopes) that relate directly to the temperature the planet was at the time. That’s how we know what temperature the planet used to be and how we can plot the data and form predictions about what the future holds (spoiler alert: it’s gonna get hot). For more on that, watch the video at the bottom.
This graph shows carbon dioxide levels over time. The higher the level of carbon dioxide, the hotter the temperature. From NASA. Credit goes to Vostok ice core data/J.R. Petit et al.; NOAA Mauna Loa CO2 record.
A [paper] was published this week that analyzed these same isotopes from the past. The Earth has gotten hotter in the past. One of these times, called the Paleocene-Eocene Thermal Maximum (PETM, 56 million years ago) lasted for 200,000 years and caused the Earth to rise 5°-8°C over 10,000 years. (Note: we’ve risen [0.7°C over 100 years, which is about 10 times faster] than the natural warming cycles the planet has experienced.)
Lots of information in this graph. Here’s the breakdown. Time, in millions of years, is on the bottom (the x-axis) with present day on the right at 0 million years ago. The abbreviations above the time are for the names of the period (Pal = Paleocene, Eo = Eocene). Along the right, the y-axis shows the amount of a particular isotope of oxygen that tells us temperature. The green line in the graph is the amount of that isotope over time, and gives us a sense of temperature. The higher the line is the hotter the temperature, and the lower the line is, the cooler the temperature. Right between the Paleocene and the Eocene, you can see the spike that indicates a sudden hot temperature. You can also see the cooling trend that lead to the ice ages (“Rapid Glacial Cycles”).
This new study examined a time after the PETM (about 2 million years later, at 53.7 million years ago), called the Eocene Thermal Maximum 2 (ETM2). The record of this time is pretty complete in the Big Horn Basin of Wyoming. The authors analyzed the temperature before, during, and after the ETM2 using the isotopes contained in the soil and in the teeth of an early horse, Arenahippus pernix, and a couple of other mammal species. They also calculated the body size of these animals using the size of the first molar. Molar size corresponds well to overall body size in mammals, so we can use the molar size to estimate body size when we only have teeth.
Arenahippus pernix at the Swedish Museum of Natural History. From Wikipedia.
The authors found that as temperature increased, the size of the horse shrank (from 7.7 kg to 6.6 kg). As temperature fell again after the ETM2, body size increased (from 6.6 kg to 7.9 kg). One of the reasons for this shrinkage is that it’s easier to cool off a smaller body than it is to cool off a larger body. If the environment is warming up, then being able to cool off faster is an advantage. Also, there may have been fewer nutrients available if droughts were happening, so the horses may not have been able to grow to their full size. The last reason could be related to how much rain was available. Less rain means less plants and less food for herbivores. Whatever the reason or combination of reasons, what we do know is that climate change, like what we’re seeing now, will affect mammals in ways we are still discovering.
Figure 3 (A and C) from the paper. The first two columns show the carbon isotope levels in the soil through time. The further the points move to the left, the hotter the climate was. The right column shows molar size in Arenahippus. Points to the left are smaller molars and points to the right are larger molars. The molars get smaller just as temperature is the hottest, and then grow again when the climate cools down.
Video on atmospheric carbon dioxide from the Earth System Research Laboratory at the National Oceanic and Atmospheric Administration. More sources can be made available on request.
This week, an [article] was published in which the authors try to answer the question: “How many times did whales become big?” In biology, there are a couple of ‘rules’ about how large body size is evolved. Cope’s Rule says that over evolutionary time, lineages of animals tend to get larger. Now, biological rules are not scientific laws (laws are backed by lots of scientific experiments and are considered to be true, like the law of gravity). Biological rules can, and do, have many exceptions, but can sometimes be useful in addressing evolutionary questions.
An example of Cope’s Rule using horses. When horses first evolved, they were small, and got larger as their lineage continued to evolve.
Now, does the evolution of large body size in whales follow Cope’s Rule? To answer this, the authors compiled a list of all known stem baleen whales (that’s all the baleen whales that are not within the living baleen whale group), their body sizes, geographic locations, and geologic ages. Because the relationships of baleen whales are not well-known, they used three different phylogenies (diagrams of evolutionary trees) to test out how body size changed in this group over time.
A living blue whale with a human for scale. They’re tremendous animals.
By using the body sizes of the known fossils, paleontologists can use a method called Ancestral State Reconstruction to make an educated guess about what size the ancestral form was. They found that having a small body size was more likely for the ancestral form, and that large size was independently evolved multiple times in baleen whales. However, some recently extinct forms also evolved small forms. So Cope’s Rule is not enough to explain body size evolution in whales.
A drawing of the early whale Herpetocetus morrowi, with a human for scale. From Adli et al. 2014.
A larger early whale, Llanocetus, with a human for scale.