O is for Ocean Acidification

By Jonathan Farrow from the Thoughtful Pharaoh

We all know that CO2¬†emissions are warming the planet. ¬†Or at least,¬†most of us¬†do. ¬†What often goes unreported is the effect of carbon dioxide on the worlds’ oceans. ¬†A lot of the CO2¬† that we pump into the air makes its way to the water and is making it more and more difficult for shelled creatures like sea urchins, lobsters, and coral to survive.

Lobster
This is Bob the lobster. This is his “I’m sad because of the increased levels of anthropogenic carbon dioxide that are making my life harder” face. ¬†Image by Pedrosanch

In order to understand why this happens, we need to go back to secondary school chemistry.

Don’t worry, I’ll make sure Jared doesn’t pick on you.

Jared
No Jared! No!     Image public domain

The first lesson we need to recall is about acids.  What is an acid?

Something that bubbles in a flask?  Image by Joe Sullivan
Something that bubbles in a flask? Image by Joe Sullivan

Acids are compounds that have free hydrogen ions floating around. ¬†These hydrogen atoms are quite reactive, so it means the more free hydrogen you have floating around, the more reactive your compound. Acidity is usually measured in pH, which stands for the “power of hydrogen”. ¬†pH is measured on a scale (creatively named the “pH scale”) that ranges from 0 to 14.

Compounds that get a 0 on the scale are exceedingly acidic, meaning they are made up of pretty much just free-floating Hydrogen ions. Compounds that rate 7 are perfectly neutral, like distilled water. Compounds on the other end, near 14, are called “basic” or “alkaline” and instead of having lots of hydrogen ions to give away, they have all sorts of space for hydrogen atoms. ¬†This makes them reactive because they can strip hydrogen from things that don’t usually want to give it away (like Edward Norton’s hand in Fight Club).

The other confusing bit to remember is that the pH scale is logarithmic, meaning that each number you jump actually indicates a multiplication by 10. For example, something with pH 3 (like soda) is 100 times more acidic than something with pH 5 (like coffee).  This means if a large body of water (like the ocean) shifts by even a small pH number, the effect can be very large.

Image by OpenStaxCollege
Image by OpenStaxCollege

The second lesson we need to recall is about equilibrium.

In chemistry, everything tends towards balance. If you combine equally strong acids and bases, they will react together until the result has a pH that is in between.  You might also get a volcano-themed science fair demonstration.

When CO2 combines with water (H2O), they form carbonic acid (H2CO3).  The carbonic acid will break up (dissociate) into bicarbonate (HCO3) and a hydrogen ion (H+).  In a basic environment, the bicarbonate will dissociate further into carbonate (CO32-) and the result will be two hydrogen ions (2H+).

We can visualize this path with a chemical equation:

H2CO3¬† —- ¬† H+ + HCO3– ¬†¬†—- ¬† 2H+ + CO32-

Where this path stops depends on the environment it is in.  In an acidic environment, the balance will tend towards the left, with more hydrogen bound up with the carbonate (because there is no space in the solution for more free hydrogen).  In a basic environment, the balance will tip to the right, releasing more hydrogen and freeing up the carbonate.

Currently, the pH of the ocean sits at about 8.1 (slightly alkaline). ¬†Because of this, there is plenty of carbonate available for creepy-crawly-shellfish to use to build their homes. ¬†Crustaceans and corals combine the free carbonate with calcium to form calcium carbonate (aka limestone, chalk, and Tums). They can’t use bicarbonate (HCO3) or carbonic acid¬†(H2CO3) and find it hard to form anything at all in an acidic environment.

This means that as we add CO2 to the water, we create more carbonic acid and contribute to the acidity of the ocean, dropping its pH.  Not only does this make it hard for the little guys down there trying to make a living, but it also endangers the big chompers that eat the little guys.

The ultimate big chomper.  This is what happens when you jokingly search for
The ultimate big chomper. This is what happens when you jokingly search for “chomper” on wikimedia.

A recent review¬†found that even under the most optimistic emissions scenario, the ocean’s pH is likely to drop to 7.95, affecting 7-12% of marine species that build shells. Under a high emissions scenario, the pH will go down to 7.8, affecting 21-32% of those species.

In order to keep track of the progress of this acidification, researchers from Exeter have proposed using satellites to monitor hard-to-reach bits of the ocean.

Regardless of the pace of the change, scientists agree one thing is certain: the oceans will become less hospitable for shell-builders.  The superficial impact of this for humans will be rising prices on shellfish, but there will be much deeper ramifications throughout marine ecosystems.

And I think we all know who is to blame.

Jared

Thanks Jared.

B is for Bat Echolocation

Microbat Photo by Neal Foster

By Jonathan Farrow from The Thoughtful Pharaoh

Ever wish you could see in the dark?  It would make life a bit easier.  No more tripping over clutter on the ground or feeling walls for a switch.  Humans rely quite heavily on their sight, but some animals can make do by illuminating their surroundings with sound.

Bats are just such an animal. ¬†They belong to a privileged group of organisms including toothed whales (like sperm whales, dolphins, and killer whales) and shrews that use sound to see the world. ¬†By listening for the reflections of their¬†high-frequency clicks, bats¬†are able to build up an accurate picture of the world around them. ¬†The clicks are often too high for humans to hear, sometimes reaching as high as 110 kHz (human hearing generally goes¬†from 20Hz-20kHz). ¬†This amazing superpower is called echolocation but not all bats have it. ¬†Most microbats (usually small, insect-eating, with proportionally large ears) can echolocate using their throat to produce clicks, while megabats (larger, fruit-eating, with large eyes) usually can’t. ¬†Like most rules in biology, though, these distinctions aren’t¬†universal. ¬†Some megabats have evolved¬†echolocation by way of specialized nose structures and others are smaller than big microbats.

An example of a Megabat, waiting for Comissioner Gordon to turn on the signal. Photo by Gerwin Sturm
An example of a megabat, waiting for Comissioner Gordon to turn on the signal.
Photo by Gerwin Sturm
This little microbat can't wait to be free! Photo by Neal Foster
This little microbat can’t wait to be free!
Photo by Neal Foster

So now that you’ve been acquainted with the notion of echolocation and the bat family tree, let’s start talking about some neat things that bats can do with their special ability.

Jamming

Since echolocation is dependent on a bat receiving and interpreting the reflections of sound, it is particularly¬†susceptible to interference. ¬†The biggest source of interference is the bat itself. ¬†Bats produce some of the loudest sounds in nature and have some of the most sensitive ears to register the reflections that come back hundreds of times quieter. ¬†Imagine revving up¬†a Harley Davidson and putting a traffic cone on your ear to hear someone whispering across the room. ¬†It would probably hurt if you did those things at the same time. ¬†You’d be too rattled by the revving to be able to listen to the whisper. ¬†Bats avoid this by temporarily disconnecting their ears as they shriek, then quickly reconnecting them in time to hear the echo.

One particular species of bat, the Mexican free-tailed bat (Tadarida brasiliensis), has been recently observed messing with its competitors’ signals. ¬†By emitting a special signal right when another bat is about to catch an insect, the bats make each other miss. ¬†It’s the bat equivalent of yelling “PSYCH!” when someone is about to shoot a free-throw. ¬†Unlike the obnoxious friend though, the bat version¬†actually works. ¬†The bats’ success rate drops by about 80%. ¬†It’s such an effective strategy that two bats will even hang out near each other, jamming each others’ signals every time one swoops in for a bug, until someone gives up.

Adapting

The same species of bat that jams also lives in close proximity to natural gas fields in New Mexico.  Some of the rigs have compressors that emit a constant, loud noise that can interfere with echolocation calls.  For the Mexican free-tailed bats, whose normal calls fall within the same frequency range as the compressors, the loud wells are avoided when possible.  The bats have also begun to change their calls, making them longer and in a more restricted range of frequencies.  This strategy would make the calls more easily distinguishable from the background din and marks the first time human-made noise has been shown to interfere with bat life.

Sneaking

We know that humans can’t hear a lot of what the bats are “saying” when they are building up a sonar picture because our ears aren’t sensitive to the right frequencies. ¬†This makes sense because, for the vast majority of humans, it really doesn’t matter what the bats are saying. ¬†It’s a whole¬†other issue if you’re a moth about to be eaten. ¬†There’s a lot of (evolutionary) pressure to hear the bats coming in order to avoid getting eaten. ¬†Some noctuids, a rather large family of moths, have evolved bat-sensing ears that warn the insect of impending disaster. ¬†If the bat is far enough away, the moth will make a break for it, otherwise it will just start flying erratically in random directions to try and make the bat miss. ¬†The Pallas long-tongue bat (Glossophaga soricina) still manages to get a meal by using only ultra-high-frequency, low intensity calls to find moths and by going silent on approach. ¬†This stealth mode doesn’t trip the moth’s defences.

Stealth bats.  Also happen to have the fastest metabolism of any known mammal. Photo by Ryan Somma
Stealth bats. Also happen to have the fastest metabolism of any known mammal.
Photo by Ryan Somma

For more information on echolocation and bats, check out:

The Bat Conservation Trust, a UK charity devoted to all things bat

This Scientific American article about how echolocation works

This study about Mexican free-tailed bat jamming

This study about Mexican free-tailed bat adapting

This study about Pallas long-tongue bat sneaking

Want to adopt a bat with Avon Bat Group?

Microbat Photo by Neal Foster

A BIG thank you to Kiri and Stuart from the Avon Bat Group for coming to Enter the Bat Cave and showing off the beautiful little animals they have in their care. Everyone really enjoyed getting up close and personal with an animal that is so rarely seen by the general public.

Stuart and Kiri with their bats and Bish the bear from Bristol Improv Theatre
Stuart and Kiri with their bats and Bish the Bear from Bristol Improv Theatre

If you are interested in supporting Avon Bat Group, adopting a bat, or just want to know more about your new favourite mammal; you can find more information on their website at www.avonbatgroup.org.uk, or like their facebook page for regular updates. We hope that after all you learnt during the evening, you can really appreciate how important their work is.

Thank you to everyone who came last night, we hope you had a great evening! Keep an eye on the website for more information about our next event and for another of Antony’s excellent infographics about Enter the Bat Cave.