The End Of The Dinosaurs Marked The End Of Most Nocturnal Mammals

We wake up from a night’s sleep and go about our day until it becomes dark again and we head off to bed. That is the life of most humans and many mammals. We are diurnal because of this behavior. It is the behavior seen in many plants and animals because it is a convenient cycle that has proved successful over the many years since its existence.

It is easier to see during the daylight and harder for predators to hide and sneak up on preys. Navigating, migrating, and doing anything that requires sight is considerably easier during the daylight, especially since most animals do not have streetlights, flashlights, and easy light sources. Of course, there are animals that do become active at night, called nocturnal behavior, because through evolution they have been successful. They evolved means to survive in the night. Since most species are diurnal, the nocturnal species do not have as much competition during the night as the daylight ones. Recently, scientists have been looking into the evolution of diurnal behavior in mammals and why it became so popular amongst the many species that do it.

Nocturnal Animals In The Present

There are many nocturnal animals that we are familiar with and we even keep some as pets. The most popular nocturnal animal might be owls, which is distinguished by its large eyes. Their eyes are large because it captures more light in the night so that they can see their preys better. They are avid hunters that use the night to their advantage as they are very fast and stealthy.

Their large eyes mean that they cannot move it like we can. To compensate, they have evolved to have swiveling heads capable of moving up to 270°, which also makes them better hunters because they do not have to move their bodies around to see things. On the opposite end of the spectrum from owls are rodents, which are both pets and pests. There are some rodent species that are nocturnal and they have developed eyes that are more sensitive to light. Apoica flavissima, a species of wasp, is also nocturnal. Their nocturnal behavior seems to have evolved out of a need to preserve the hive. The hive needs to maintain a stable temperature of 27.3℃ to survive and they generally insulate their nests with things like plant matter. They also become very inactive during the day to minimize any activity in the hive that might generate heat and raise the temperature because it is already hot outside from the sunlight. The night cools everything down and allows them to be active without disruption to the stable temperature. They do become active in the day if there are any threats to the hive.

All tarsier species are nocturnal and have eyes adapted to that lifestyle. Image from Wikipedia

There are also animals that are capable of moving between nocturnal and diurnal behaviors. Many of them are obligated towards one or another but, like the wasps, are capable of shifting activity due to environmental cues. There are many humans who have adopted a nocturnal lifestyle because of things like jobs and other circumstances. Likewise, there are animals that have to be active during the time they are usually resting in response to environmental cues like danger.

The Switch From Nocturnal To Daytime

A commonly held and supported theory is that the common ancestors of mammals were nocturnal and, somewhere along the way, this switched to diurnal. Most animal today have some sort of nocturnal behavior that is thought to have originated from the pest when our ancestors were nocturnal. This includes humans because we can become nocturnal, albeit temporarily, in response to environmental situations. It is generally understood that the common ancestors were under threat from much bigger and very dangerous predators that were active during the day. It was far safer for them the hide and rest rather than try to compete for resources with these predators and risk dying, which would threaten species survival.

Researchers from the University College London wanted to understand when the switch occurred. They analyzed the behavior of over 2,000 existing species in order to create a dataset to reconstruct the activity patterns of the mammalian common ancestor. They found that the appearance of diurnal coincided with the death of the dinosaurs and other larger predators. Essentially, the biggest threat to the mammalian ancestors was eliminated and they could be active in the day without peril of death and less competition for resources. They estimate that the diurnal behavior began 66 million years ago, though more research is needed to make that figure more accurate. They found that ancestors of simian primates, like gibbons and gorillas, were the first to begin the change from nocturnal to diurnal. This did not happen overnight and took several million years to embed itself within mammals. This finding supports the fact that simian primates have visual powers similar to those of avian and reptile species, which did not go through a phase of nocturnal to diurnal.

The night nest of a gorilla in a tree. Image from Wikipedia.

One of the shortfalls of this study is that it used existing species only to discern the timeline. Fossils were not used because there are not enough of them to give an accurate picture of this particular behavior. Also, it is extremely difficult to learn about behavior from fossils. That is one of the issues that researchers face when trying to understand what happened millions of years ago. We have limited fossils and fossils are limited in what they can tell us. We have to continue researching ways to improve our analysis capabilities by improving the technologies we use as well as the methods we employ.

One of the important things to keep in mind is that not every species became a fossil and we have lost some data. While we may never understand the entire situation of dinosaurs, we can continue to build an idea of them that comes close to reflecting the truth.

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