Bats Under the Bypass

If you are in the Davis or West Sacramento area in the late summer or early fall, and have an evening to spare, go and find a spot where you can sit beside the Yolo Bypass Causeway. This is where highway I-80 crosses over the Yolo Bypass.

Streams of Mexican Free-tailed Bats over the Yolo Bypass

Just as the sun begins to set, you will see an amazing sight. Columns of bat will flood out from under the bypass and stream across the sky in sinuous ribbons. About a quarter of a million Mexican Free-tailed Bats (Tadarida brasiliensis) live under the bypass this time of year, and every night they pour out and spread across the surrounding area to find small flying insects to eat.

These bats are incredible! They can fly about about 100 miles per hour, making them among the fastest mammals in the world! Remember that Cheetahs are the fastest land-mammal, but bats have them beat by a healthy margin. These bats can fly as high as a mile above the ground, and can forage out distances of several miles from their night roost before returning around dawn to sleep. Using their sonar they can detect and pinpoint the exact position of little insects flying through the air and then capture those insects on the wing, at speed!

My wife, daughter, and I joined some friends and went out for an evening visit to see the bats about a month ago. I was a spectacular evening in the Yolo Bypass Wildlife Area. We saw lots of Swainson’s Hawks; herons, egrets, and ibis galore; some of the biggest Western Saddlebags (which is a species of dragonfly) I have ever seen; and then we got to the causeway.

When we arrived, the sun was still a touch above the horizon, so we had some time to stand around the dirt road that runs parallel to I-80 and chat and watch the sunset. We got a very nice surprise when an adult Peregrine Falcon flew past and landed in the top of a tree a little ways to the west of us. I was so excited to see this bird that, in turning around for a better look, I clumsily stepped on my wife toes (sorry sweetheart)!

Mexican Free-tailed Bats as they leave from under the Yolo Causeway.

As the light began to fade, we started seeing little movements under the causeway. The first bats were starting to move. Interestingly, the bats do not wake up, take flight, and simply fly out from under the causeway wherever they happen to be. Instead, they wake up, take flight, and then fly directly under the causeway for a few hundred yards before turning a sharp left, and lifting up into the open sky. I have no idea why they decide to do this, but volunteers at the Wildlife Area know it is gong to happen so consistently, that they can tell you exactly which tree the bats will fly out near.

The numbers of bats moving under the causeway built and built until there were bats streaming along between the support pillars. Then they made that left, and out in to open they came! A snaking stream of bats began raising and twisting into the sky! Thousands and thousands of bats following one another out from where they had been sleeping to look for food. As we watched the seemingly endless flow of bats, we got a very cool surprise. That Peregrine Falcon that we had seen earlier came back. It started strafing through the flow of bats. It was hunting bats!

I have seen this behavior of raptors hunting bats as they leave their night roost on video before, and it is pretty spectacular to see on a screen. Seeing it in real life was thrilling! After a couple of passes, the Peregrine made a quick move to one side, and suddenly it had a bat in one talon! It flew off and out of sight carrying it’s dinnertime snack.

The rest of the bats were generally nonplussed by the Peregrine attack, and keep streaming and streaming into the coming night.

Finally, the last bat that was going to leave had departed, and the darkness was getting deep enough that we would not have been able to see the bats fl by even if they were there, so we piled back into our cars and headed for home.

All in all, a terrific way to spend and evening!

 

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Information is Important

Information is important. With information each of us as individuals, and our society as a whole, can learn about the world. With information, we can all make decisions that make sense. With information, we can all discuss ideas.

Without information none of that is possible. Without information, we are, at best, at the mercy of our current, limited knowledge, and our base instincts. Without information we are, at worst, at the mercy of the limited knowledge and instincts of someone else.

This is why the gag order, and insistence that all reports and data be pre-screened before release to the public, issued by the President to the EPA are so concerning to me, and I think should be so concerning everyone else. This is exactly the kind of action that limits access to, and spread of, information. It will only hamper all of our abilities to operate as rational, critically thinking individuals. It is the kind of action that is put in place to control what we, as citizens, know and when we know it. This is censorship and it has no place in science or a free society.

#thisisnotnormal

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Powered Flight in Vertebrates

Powered flight, the ability to propel oneself through the air against the force of gravity, requires a great deal. From specialized bones to specialized skin, and everything in between, the demands of flight penetrate all aspects of an animals’ life. And yet, despite how mechanically difficult powered flight is to achieve, it has evolved three different times in vertebrate evolution, once in birds, once in bats, and once in the now extinct pterosaurs. One of the things that make the three different evolutions of flight especially interesting is that the three different groups accomplished the feat is such different ways.

A pterosaur wing has a large upper arm bone, two smaller forearm bones, a few wrist bones, and then four fingers. The first three are small fingers that have small claws at their tips and are free to move and grasp objects. The fourth finger is extremely long, extending all the way to the tip of the wing. The bones of this fourth finger are longer and thicker than the bones of the other fingers, and this is because this fourth finger supports the entire wing membrane. The wing membrane attaches all along the rear surface of the fourth finger, the rear surface of the arm, and then to the side of the trunk of the animal and even to the leading edge of the hind legs. That means that, when a pterosaur is in flight, the whole weight of the animal is being supported on once finger of each hand! Some pterosaurs got to an estimated 550 lbs (although most were about 25 lbs) with a 10 or 11 meter wingspan, so those are some strong fourth fingers!

A bat wing is the same upper arm bones of the pterosaur, but the hand is very different. Instead of supporting al the body weight of the animal on just one finger, all five of the fingers of a bat are elongated (the thumb is the only finger that is small and clawed and free to grasp). In between each of these fingers, and extending from the pinky finger to the body are membranes that the bat can stretch or fold as needed by moving the bones in its fingers, hands, and arm. Since all the fingers are sharing the load, each one is proportionately much finer and thinner than the fourth finger of a pterosaur. Additionally, bats have thin muscles that cover the surfaces of the membranes just beneath the skin. By tensing or relaxing these muscles, the bat has very fine control over the sharp and tension of the wing membranes, and this turns out to be very important in bat flight. It is not known if pterosaurs had similar muscles on the membranes of their wings, but it seems likely that they did.

A bird wing also has the same basic upper arm bones seen in the other groups, but instead of elongating bones and making them delicate and distinct, birds go the opposite direction. Most of the bones in the hand and fingers of a bird are fused together. This makes a structure that is short, thick, and strong. Out of this support structure extends feathers. Not a membrane made of skin like a pterosaur or a bat, but a completely different evolutionary innovation. Feathers, and flight feathers in particular, are strong and thin and light. Flexible enough to bend a bit to change shape, but rigid enough to support the weight of the bird and the forces of air-speed, drag, lift and gravity that all flying organisms have to contend with. In modern birds, muscles in the skin at the base of the feathers allow for independent control of each feather. This allows birds to have amazing control over the shape of their wing.

Flight is a fascinating example of how a complex structure or function can arise by natural selection. In these three cases, natural selection favored three very different, and ultimately successful, experiments in how to get an animal airborne. All three were very different solutions, which is worth remembering. Most of the problems out there, even really hard ones, probably have many different solutions, you just have to tinker and figure them out.

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