I’m having a lot of fun with my ultrasonic microphone (Ultramic 200k). It’s opened up a parallel universe of flying mammals and calling insects and the evolutionary arms race between them.
In early August of this year, I found myself camping in a gorgeous spot in the canyon country of Utah – Calf Creek. I had not planned on camping there, but I ran into a flash flood on the way to my original destination of the Devil’s Backbone, and was forced to change direction. It was pouring when I arrived at Calf Creek, and I had to wait for a pause in the rain to set up my tent. I had no sooner got it set up, when it started raining again. The rain finally stopped just before sunset, and I cooked dinner surrounded by soggy vegetation and a roaring creek. As the light started to fade, I could see several bats flying around overhead, so I set up my ultrasonic mic and started to record. I recorded at least two species, possibly a little brown myotis (Myotis lucifugus) and a fringed myotis (Myotis thysanodes). In the recording, slowed down 7x, you can hear the two different bats, and even hear some of the calls echoing off of the sandstone cliff across from my campsite. You can also hear to faint feeding buzz, as a bat makes a grab at an insect.
Calf Creek the morning after
Bats eat a tremendous number of insects, including mosquitoes, beetles and moths. But the insects don’t take this lying down, so to speak. Many insects can hear the ultrasounds that bats produce when they echolocate, and use a variety of evasive maneuvers if they hear a bat approach. Some drop to the ground, and some produce a variety of loops, spirals and changes of speed and direction in an effort to shake off their pursuers. Moths are even known to produce loud ultrasounds of their own to “jam” the bats sonar.
On my recent trip to southeastern Arizona and southwestern New Mexico, we spent one night camped at the confluence of the east and middle forks of the Gila River. After my friend retired to the back of her truck to read, I set up the ultrasonic mic on the table we had brought along with us. As I watched the level meter to see if the computer was registering sounds that I could not hear, I heard a quiet shuffling sound nearby. I turned to the source of the sound, and my headlamp illuminated a striped skunk about 10 feet away. I used to study skunks, and knew that if I sat quietly, it would continue on its way, which it did. I dreamed that night that a bear was trying to get in my tent. I was trying to scream to alert my friend in her truck, but couldn’t enunciate – my mouth didn’t want to work. When I described the dream to my friend the next day, she said, “Hmm, so that explains that noise…” The dirt surrounding my tent was littered with skunk tracks in the morning.
East fork of the Gila River
The recorder picked up a couple of bats, plus some loud clicks which I think are defensive sounds against bats. Some insects are known to produce broadband sounds that inform potential predators that they are nasty to eat – in other words, aposematic sounds. Other insects cheat the system by imitating these sounds, even though they are quite edible. These are called Batesian mimics, and it’s an acoustic version of the mimicry you see between viceroy and monarch butterflies. In the following recording, you can hear the bat issuing its hunting calls, and you can hear some loud clicks. The recording was slowed down by 9x.
The ultrasonic crickets in my backyard also respond to ultrasonic predators – by going quiet. In the following recording, you can hear a chorus of bush crickets (Tettigoniidae) that one by one get quiet as a bat approaches. As the bat leaves, they start calling again. The recording was slowed down by 7x. I think the bat might be a southwestern myotis (Myotis auriculus).
Bats are not only predators, they are also prey. Hibernating bats succumb to skunks and badgers if they roost within reach. Snakes and ringtails are known to wait near the entrance of bat roosts to snatch bat as they leave for their evening foraging. I was fortunate enough several years ago to witness the nightly exodus of millions of Mexican free-tailed bats (Tadarida brasilensis) from their maternity roost on the Armendaris Ranch in central New Mexico (note: the Armendaris is private property and permission is required to enter. Commercial tours are available). I didn’t have an ultrasonic mic with me, but I did have a video camera, and recorded the amazing spectacle. The bats start their evening flight just before sunset, when it’s still light out. The hawks have caught on, and Swainson’s Hawks, Peregrine Falcons, and even the nearby-reintroduced Aplomado Falcons greet the bats – with talons extended. I’ve included a short clip of video in which you can see the Swainson’s hawks grab the bats in mid-air and eat them while in flight. You can also hear the wing beats of millions of bats flying by, as they make their nightly journey to feast on mosquitoes along the Rio Grande River.
References:
Sales, G. and D. Pye. 1974. Ultrasonic communication by animals. Chapman and Hall, London.
Hayes, M.A. 2008. Bats in the Paradox Valley area and Gunnison Gorge National Conservation Area: Results of mist-netting and acoustic surveys during 2008. Report to BLM. https://www.blm.gov/pgdata/etc/medialib/blm/co/field_offices/uncompahgre_field/documents/biological.Par.61217.File.dat/Hayes%202008%20Siders%20BLM%20Report.pdf. Accessed 11/2012.
Recording notes:
Bat recordings made with a Ultramic 200k (Dodotronics, Inc.) and an Acer Iconia W500 tablet. Recordings were amplified, and subject to high and low pass filtering to reduce hiss. Video was recorded with a Sony Handycam.