Humans are diurnal. We sleep at night and are active during the day. (That isn’t to say that I feel particularly diurnal most mornings, given that my alarm has to make it through a few snooze cycles to wake me up and coffee is the only thing keeping me from napping under my desk at work.) Most mammals, though, don’t share our ostensible predilection for daylight; only 20% of mammal species are diurnal like us. Of our mammalian relatives, nearly 70% are nocturnal. The rest are crepuscular (active at dawn and dusk) or cathemeral (active during both day and night).
Mammalogists like myself often think nocturnality is a particularly mammalian thing because—let’s be honest here—nearly all of the coolest nocturnal vertebrates are mammals. How can you compete with the likes of tarsiers, vampire bats, leopards, and—strangest of them all—the aye-aye? I’ll throw the ornithologists a bone and acknowledge the enduring awesomeness of owls, but they are the odd birds out in a group that’s mostly diurnal.
We have long assumed that nocturnality was one of the key traits that allowed mammals to succeed after the mass extinction that killed off the dinosaurs and ushered in the Cenozoic—the “age of mammals.” And we thought their ability to hunt and be active during the night was a relatively uncommon behavior at the time, only recently evolved, and constrained mostly to Mammalia. Well, it turns out things aren’t so simple when you peel back the evolutionary history of the much larger group that contains living mammals and their extinct ancestors and cousins.
Mammals are part of an evolutionary group called Synapsida, which includes a spectacular diversity of utterly bizarre extinct creatures unlike anything alive today. This includes the well-known, dorsal-sail-wielding Dimetrodon (which is emphatically not a dinosaur. It’s more closely related to your Aunt Mildred than to a Tyrannosaurus rex), the long-jawed predator Pristerognathus (pictured), or the maybe-kind-of-cute, tusked Diictodon.
Paleontologists had considered most of these extinct oddballs diurnal pretty much by default, but that assumption remained untested until the publication of a recent study by paleontologists Kenneth Angielczyk and Lars Schmitz. The key to their research was a small bone found in nearly all terrestrial vertebrates, including reptiles and birds: the scleral ring. But before you start probing your own orbit for a little ring of bones, you should probably know that no extant species of mammal retains this feature. It was lost somewhere along the branch leading to living mammals, perhaps fused to other bones in our cranium.
In living birds and reptiles, the size and shape of scleral rings correlate with a propensity for nocturnality. Nocturnal lizards, for example, have large scleral ring diameters compared to the overall dimensions of their orbits. Fossil evidence tells us that nearly all synapsids had scleral rings, so it should be possible to build a statistical model that can predict the degree to which a fossil species was nocturnal by first comparing the scleral rings and day-night activity patterns of living species.
So Angielczyk and Schmitz had to rely on lizards to provide the data they needed for living species. They compiled a “training dataset,” so called because the associations between morphology and behavior identified in extant species informs the statistical model used to classify the fossil species into different behavioral groups. Once their lizard training dataset was assembled, they measured a bunch of synapsid scleral rings in different extinct species and applied their statistical model to those data to classify fossils according to nocturnal or diurnal behavior (or a mixture of both).
When considered in the context of what we know about the relationships among different synapsid lineages (including modern mammals), the results were surprising. Fossil synapsids had scleral ring morphologies that covered the whole spectrum of light tolerance, from scotopic (low-light specializing, assumed to be nocturnal) to photopic (high-light specializing, assumed to be diurnal), and many with intermediate morphologies (mesopic). These extinct species were clearly not just diurnal, as had been assumed before.
Because they tied their analysis to a phylogenetic tree, the researchers were able to make some inferences about the number of times nocturnality may have evolved over the entire evolutionary history of synapsids. When their categorizations were mapped onto the tree, scotopic traits (those that are linked to low-light, nocturnal activity) appeared in various different groups. This suggests that the trait is somewhat flexible over evolutionary time. Some lineages lose it, while others gain it independently.
I think studies like this, which use statistical models based on living species to make inferences about how extinct species lived, are inherently fascinating. It paints a picture of ancient behaviors and ecologies in a rigorous manner (i.e., without resorting to evolutionary “story-telling”). It’s especially exciting to see these types of methods applied to groups of extinct animals that don’t get much attention despite being amazingly diverse. Future research efforts could expand this study to a larger sampling of synapsids, and, eventually, even older evolutionary groups.
Nocturnality appears to have been present among the earliest synapsids, and certainly long before the rise of modern mammals. In fact, some of the oldest known synapsid fossils have scleral rings and orbit dimensions similar to extant nocturnal species, suggesting that nocturnality might extend even further back in the vertebrate evolutionary tree, perhaps to the ancestor that includes all living amniotes (e.g., birds, reptiles, and mammals). If that’s true, Earth’s nighttime landscapes hundreds of millions of years ago may have been teeming with far more nocturnal species than paleontologists had ever considered.
Tom Giarla is an evolutionary biologist working as a postdoc at Louisiana State University. He studies the diversification of tropical mammals. This is his first post at Nothing in Biology Makes Sense, but he’ll be back throughout the year posting about macroevolution in vertebrates, especially mammals. For information about his research, check out his website.
Angielczyk, K. D., and L. Schmitz. 2014. Nocturnality in synapsids predates the origin of mammals by over 100 million years. Proceedings of the Royal Society B: Biological Sciences 281: 20141642. doi: 10.1098/rspb.2014.1642
Bennie, J. J., J. P. Duffy, R. Inger, and K. J. Gaston. 2014. Biogeography of time partitioning in mammals. Proceedings of the National Academy of Sciences 111: 13727-13732. doi: 10.1073/pnas.1216063110