The Snails that Farm

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Littoraria grazing on Spartina marsh grass. (source)

Us humans really like to talk up our skills at farming. And while it’s true that we have domesticated animals and plants to a degree not seen in other life forms, the act of nurturing and harvesting food is actually not really that special, and is broadly observed throughout the animal kingdom. Perhaps the most iconic invertebrate farmers are insects. Leaf-cutter ants, termites, and some beetles have been observed to actively cultivate fungus by gathering plant material to feed it, growing the fungus, protecting the fungus from competition, and then harvesting the fungus to feed themselves and their young. Ants are also known to keep livestock in the form of aphids, which they lovingly protect and cultivate for the sweet nectar they excrete. Such practices are called “high-level food production” because, like human farmers with their seeds and fertilizer, insects have evolved a highly complex symbiosis with their fungus. The fungus has shaped their behavior as much as the ants cultivate the fungus. 

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The marsh periwinkle Littoraria irrorata (source)

Less well understood is the “low-level food production” that may occur more broadly throughout the tree of life. There is less direct evidence of such behavior because it is more indirect and less specialized than high-level food production, but it may be equally advantageous for the cultivator and the cultivated. One study published in 2003 uncovered a simple but powerful relationship between marsh periwinkles of the genus Littoraria and fungus which they cultivate and harvest.

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Close-up of a snail’s radula (source)

Marsh periwinkles are small and not particularly charismatic creatures. Like many snails, they are grazers with a shell, a fleshy foot and a rough, abrasive organ called a radula which they use like sandpaper to graze on pretty much whatever they can get into. Snails are not known as picky eaters. But researcher Brian Silliman of Brown University and Steven Newell of University of Georgia noticed that these innocuous snails regularly undertake the risky, low reward activity of grazing above the water on the blades of swamp grass, stripping off the surfaces of the blades of grass. The researchers were confused why the snails would expose themselves to predation and the harmful open air for such a low-nutrition food.

 

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A typical snail farm, complete with liberally applied fertilizer. Yum.

They discovered that the snails were investing in the future. By stripping away the protective surface of the swamp grass blades and liberally fertilizing the surface of the grass with their droppings, the periwinkles are ensuring that the swamp grass will be infected with an active and very prolific fungal infection. The fungus, unlike the plant it lives on, is of high nutritional value. The researchers demonstrated the active partnership between the snails and fungus by conducting caged experiments where they showed that snails which grazed on grass but not the resulting fungus did not grow as large as snails which were allowed to return and chow down on the fungus. The fungus loves this deal as well. They grow much more vigorously on grass that is “radulated” (rubbed with the snail’s sandpapery radula) than uninjured grass. The fungus grows even faster if the snails are allowed to deposit their poop next to the wounds. The researchers found that this same relationship applies at 16 salt marshes along 2,000 km of the Eastern Seaboard.

The periwinkles don’t really know what they’re doing. They aren’t actively planting fungus and watching proudly like a human farmer as their crop matures. But over millions of years, the snails have been hard-wired to practice this behavior because it works. Snails that abrade a leaf of swamp grass, poop on the wound and come back later to eat the yummy fungus do a lot better than snails which just stick to the safe way of life below the surface of the water. The fungus loves this relationship too. The only loser is the swamp grass, which the researchers unsurprisingly found grows much more slowly when infected with fungus. But marsh grass is the largest source of biomass in swamp environments, and the snails that partner with fungus are able to more efficiently use this plentiful but low-nutrient food source, to the extent that it is now the dominant way of eating for swamp periwinkles on the East Coast of the US, and probably in a lot more places too. The researchers noted that there are likely far more examples of low-level food production that we simply haven’t noticed.

Since this work was published, other teams have discovered that some damselfish like to farm algae, fiddler crabs encourage the growth of mangrove trees, and even fungus get in on the action of farming bacteria. We love to talk up our “sophisticated” high-level food production techniques, but such relationships probably got started at a similarly low level. Our activities as hunter-gatherers encouraged the growth of certain organisms, we stumbled upon them, ate them, kept doing what we were doing and eventually our behavior developed into something more complex. Next time you see a snail munching its way up a blade of grass, consider to yourself whether it knows exactly what it’s doing. Come back later to see the fruits of its labor.

Why funding science makes America stronger

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As a PhD candidate in the last year of my doctorate, I’m currently applying for postdoctoral positions, and running into a lot of difficulty because the fellowships that were previously available are on ice (NOAA, EPA), dead (USGS Mendenhall), or are so selective and prohibitive in their application requirements that they are effectively a lottery (NSF). These fellowships are would support my living and research costs for 1-2 years and allow me to continue my work after graduate school while I look for faculty positions. Right now, I have a front-row seat watching the death of a generation of AMERICAN researchers before they even get started. We can’t get support for our work, and there are not enough privately-funded grants to cover the gap, and there never will be. So many of my peers are giving up and entering the private sector or going abroad.

You might think this is the point in my essay where I assert to you that funding science is the *right* thing to do and produces knowledge which improves the human condition and is the greatest thing that our species can aspire to, yada yada yada. While all of that is true, if you’re reading this, you likely already agree. What you might not be aware of, however, is that science is a smart investment of public funds in the short term. Science is a powerful engine of economic stimulus. Every researcher takes part in an enormous amount of economic activity which produces multiple dollars of payoff for every dollar of AMERICAN federal grant money invested.

Let me give you an example with my research work, which is small potatoes compared to the big labs, which apply for grants that measure in the millions of dollars. I am an isotope biogeochemist. To conduct my work, I require the use of immensely complex machines called mass spectrometers. These devices cost somewhere between the cost of a low end BMW ($45,000) and an Italian supercar (hundreds of thousands). The companies that make these (Thermo-Fisher, PerkinElmer, etc) are multi-billion dollar corporations which employ thousands of skilled workers in AMERICA and depend on the sale, support and upkeep of machines like these to make a profit and provide AMERICAN jobs. The sale of scientific equipment is their bread and butter, so the grant money that I spend running samples on the mass spec is going right towards employing other highly-educated AMERICAN workers. The same goes for the scanning electron microscope that I use, the micromill (the little robot I use to drill my shells), and the vials and everyday supplies I use. All are manufactured by AMERICAN companies.

At my local level, I am not doing the day-to-day work of running the mass spectrometers. My department employs highly educated technicians, some with PhDs of their own. They do the day-to-day work of keeping these engineering marvels running and helping us do our research. I will be writing another blog post soon about these unsung wizards of science, but know that behind every talented researcher you know, there are at least a few talented technicians that enabled them to do the science. And all of those people depend on research money to be able to do those jobs, and are paying taxes, so many of those tax dollars go right back into the AMERICAN Treasury.

The university where I study is the top employer in the county. Universities are the top employers in many counties and even states nationwide. My school makes jobs and generates economic activity which transformed Santa Cruz from a sleepy vacation town into an engine of research and development. Behind the technicians and researchers that do science, there is a vast ocean of administrators needed to do paperwork, administer grant money, interface with the community and assist student needs. All of those people are generating economic activity in their communities, buying food, gas, and living their lives, contributing to a strong, fluid AMERICAN economy. Research is the lifeblood of an economy that cannot easily be automated or outsourced away from AMERICA, but only if we provide the federal funding to keep the machine running.

What I’m trying to articulate to you here is that funding science is not just a feel-good use of your tax dollars. It is a necessary use of your tax dollars. Right now, science is withering away in AMERICA. I can name ten researchers off the top of my head who are throwing in the towel and leaving the field or going abroad because they can’t get funding here. And the discoveries they would have made and the economic activity that they would have generated is happening in CHINA, and EUROPE, and increasingly in ARAB countries, INDIA and the developing world. We need to immediately bring AMERICAN scientific funding back to its historical proportion of the federal budget at least, and hopefully ramp it up if we intend to continue to be the world’s biggest research economy.

I am not begging and wheedling you to fund science for my sake. I will be fine. There are lots of jobs that I can do, and I’m looking into doing these jobs abroad if I can’t find the support I need in AMERICA. But if you believe that AMERICA is great, you should be aware that a lot of that greatness is thanks to the economic stimulus brought about by basic research. Please keep that in mind when you vote, call your representatives, and even when you’re just talking about politics.

What is Conservation Paleobiology?

In undergrad, I felt like my school and internship were training me to be two different types of researcher. At USC, I was majoring in Environmental Studies with an emphasis in Biology. It was essentially two majors in one, with a year of biology, a year of chemistry, a year of organic chem, a year of physics, molecular biology, biochemistry, etc. On top of that, I took courses on international environmental policy and went to Belize to study Mayan environmental history. Meanwhile, I was working at Jet Propulsion Laboratory in Pasadena researching trends in historical rainfall data. I loved both sides of my studies, but felt like neither was exactly hitting the spot of what I would want to spend my career researching. I love marine biology but am not particularly interested in working constantly in the lab, looking for expression of heat shock protein related genes or pouring stuff from one tube into another. On the other hand, I was fascinated by the process of untangling the complex history of rainfall in California, but I yearned to relate this environmental history to the reaction of ecological communities, which was outside the scope of the project.

During my gap year post-USC, I thought long and hard about how I could reconcile these disparate interests. I read a lot, and researched a bunch of competing specialized sub-fields. I realized that paleobiology fit the bill for my interests extremely well. Paleobiologists are considered earth scientists because they take a macro view of the earth as a system through both time and space. They have to understand environmental history to be able to explain the occurrences of organisms over geologic time. I really liked the idea of being able to place modern-day changes in their geologic context. What changes are humans making that are truly unprecedented in the history of life on earth?

But it doesn’t have to be all zoomed out to million-year processes. A growing sub-field known as Conservation Paleobiology (CPB) is focused on quantifying and providing context of how communities operated before humans were around and before the agricultural and industrial revolutions, in order to understand the feasibility of restoration for these communities in this Anthropocene world. Sometimes, this means creating a baseline of environmental health: how did oysters grow and build their reefs before they were harvested and human pollution altered the chemistry of their habitats? I’m personally researching whether giant clams grow faster in the past , or are they reacting in unexpected ways to human pollution? It appears that at least in the Gulf of Aqaba, they may be growing faster in the present day. Such difficult and counterintuitive answers are common in this field.

Sometimes, CPB requires thinking beyond the idea of baselines entirely. We are realizing that ecosystems sometimes have no “delicate balance” as described by some in the environmental community. While ecosystems can be fragile and vulnerable to human influence, their “natural” state is one of change. The question is whether human influence paves over that prior ecological variability and leads to a state change in the normal succession of ecosystems, particularly if those natural ecosystems provide services that are important to human well-being. In a way, the application of paleobiology to conservation requires a system of values. It always sounds great to call for restoring an ecosystem to its prior state before humans. But if that restoration would require even more human intervention than the environmental harms which caused the original damage, is it worth it? These are the kinds of tricky questions I think are necessary to ask, and which conservation paleobiology is uniquely suited to answer.

At the Annual Geological Society of America meeting in Seattle this year, the Paleo Society held the first-ever Conservation Paleobiology session. The room was standing room only the whole time, investigating fossil and modern ecosystems from many possible angles. This field is brand new, and the principles behind it are still being set down, which is very exciting. It’s great to be involved with a field that is fresh, interdisciplinary, and growing rapidly. I look forward to sharing what my research and others find in the future.

Hard shells aren’t actually that hard to make (yet)

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One of the Antarctic bivalve species featured in this study. Source

Like all organisms, bivalves have a limited budget governing all aspects of their metabolism. If they put more energy into feeding (filtering the water), they can bring in a bit more food and therefore fuel more growth, but sucking in water takes energy as well, particularly if there isn’t enough food to be filtered out. Bivalves also periodically have to grow gonadal material and eggs for reproduction, expand their body tissue (somatic growth) and of course, grow their shells (made of of a mineral called carbonate). All of these expenditures are items in a budget determined by the amount of energy the bivalve can bring in, as well as how efficiently they can digest and metabolize that energy.

If a bivalve is placed under stress, their scope for growth (the max amount of size increase per unit time) will be decreased. Because they’re cold-blooded, bivalves are limited by the temperature of their environment. If temperatures are low, they simply can’t sustain the chemical reactions required for life at the same rate that endotherms like us can. They also may have to shut their shells and stop feeding if they’re exposed by the tide, or are tossed around by a violent storm, or attacked by predators or toxins from the algae that they feed on.

When their budget is lower, they have to make painful cuts, much like a company lays off employees if their revenues are lower. The question is which biological processes get cut, and when? My first chapter (submitted and in review) has settled temperature being the primary control on seasonal shell growth. Bivalves at high latitudes undergo annual winter shutdowns in growth, which create the growth bands I use to figure out their age, growth rate, etc. We’d be a lot closer to accurately predicting when bivalves suffer from “growth shutdowns” if we had hard numbers on how much energy they actually invest in their shells. A new study from a team led by Sue-Ann Watson of James Cook University attempts to do just that.

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Diagram relating the growth bands of Antarctic soft-shelled clam with a chart showing the widths of those bands. Source

Collecting a database of widths for the annual growth rings of bivalve and gastropod (snail) species from many latitudes, Watson and her team were able to get a global view of how fast different molluscs grow from the equator to the poles. Because the unit cost of creating carbonate is determined by well-understood chemistry, they were able to create an equation which would determine the exact number of Joules of energy used for every bivalve to grow their shells.

They still needed a total energy budget for each species, in order to the percent of the energy budget that each bivalve was investing in their shells. They drew on a previous paper which had calculated the standard metabolic rates for each species by carefully measuring their oxygen consumption. We could do the same for you if you sat in a sealed box for an extended period of time while we measured the exact amount of oxygen going in and CO2 going out. Dividing the amount of energy needed to grow the shell by the total amount of energy used in the organism’s metabolism would give us a percent of total energy that the bivalve dedicates to adding growth layers to its shell.

That number is…not very large. None of the bivalves or gastropods they looked at put more than 10% of their energy into shell growth, and bivalves were the lowest, with less than 4% of their energy going into their shell. Low-latitude (more equatorward) bivalves have the easiest time, putting less than 1% of their energy into growth but getting way more payoff for that small expenditure. High-latitude polar bivalves have to work harder, because the lower temperatures they experience mean the reactions needed to create their shells are more expensive. In addition, most of that energy is going into the protein-based “scaffolding” that is used to make the shell, rather than the crystals of carbonate themselves. Organisms right now don’t have to put a whole lot of effort into making their protective shells, which could explain why so many organisms use shells for protection. That is good, because if shells were  already breaking the bank when it came to the bivalves’ growth budget, they wouldn’t have a lot of room to invest more energy in the face of climate change. Unfortunately, as the authors note, these budgets may need to change in the face of climate change, particularly for bivalves at the poles. As the oceans grow more acidic due to human CO2 emissions, growing their shells will start to take up more of their energy, which is currently not a major part of their budget.

A cold-water ecosystem dominated by Antarctic scallops. Source

Right now, the cold waters of the poles are refuges for organisms that don’t deal well with shell crushing predators. As polar regions warm, such predators will begin to colonize these unfamiliar waters. Polar bivalves may encounter the double whammy of needing to spend more energy to make the same amount of shell, but also find that it is no longer enough to protect them from predators that easily crack open their protective coverings.

I found this study to be an elegant and thoughtful attempt to fill in a gap in our current understanding of how organisms grow and how energy budgets are influenced by environmental variables like temperature. I instantly downloaded the paper because it answered a question that has long been on my mind. Maybe can sneak its way into my manuscript during the review process!

Back on social media!

I had taken a break from logging into Facebook/Twitter/Reddit for the last couple months because it was stressing me out. I was feeling inundated by political news that were making me feel overwhelmed and not in control of the information I was processing. So I disconnected for a while and my mind began to feel a lot clearer. The battery life on my phone also improved by at least 3x.

I realized that while I appreciate and respect the views of the people I follow, as a collective the news they were sharing was crowding out my own personal views on the issues. I seriously felt like I was being radicalized after being subjected to a firehose of competing political opinions. I was worried I was contributing to that problem and doing the same thing to other people.

But I like using Facebook and Twitter to keep in touch with my family, friends and colleagues. It is valuable to me to see pictures of your pets, news about whether you’re safe from the latest natural disaster, and links to your latest paper that’s headed straight to my references folder. So I’m back on FB/Twitter for those things, but I’ve set some strict rules for myself.

  • Only can log in on my computer. The phone makes it too easy and compulsive to log in frequently and scroll through. And I have to log out immediately after viewing, with only one login per day permitted.
  • No more than one post per day on any of the networks.
  • No more than one political post per month.
  • No replies to anyone that I don’t personally know.
  • No Reddit. I actually haven’t missed that one at all.

I hope I can find a way to engage and stay in touch with you all while not crowding out real life! Talk to you soon.

Things I wish I knew/did earlier in grad school

  • Keep a journal of every research-related idea you have and every research-related action you take. Seriously, find the most frictionless way you can keep notes and stick to it. Your brain will thank you later.
  • Ask for help whenever possible, but with the knowledge that many of the issues you have will have no troubleshooting manual.
  • Crude, hacked together and done is better than perfect and never finished.
  • Work when you feel productive. Sleep when you feel less productive. Use the benefits of being a self-scheduled researcher to your advantage.
  • Don’t feel guilty to be involved in grad student life and service. These activities give a mission and direction to your research.
  • Take on a mentee. It is such a massive boost to your own productivity to take charge of managing and encouraging another less experienced person’s work. It will push you to practice what you preach.
  • Make sure your family and loved ones know what you do and what is expected of you, so they aren’t upset when you aren’t free to talk or have to work a late night.
  • Don’t hold on to the paper you’re working on too long. Chances are that there is someone out there doing a similar project based on an idea that they had at the same time as you, and you don’t want to get scooped.
  • If someone more experienced than you who you respect disagrees with your findings, that doesn’t mean you’re wrong.
  • Don’t be afraid to overhaul a project you have based on new information. This is the stage of your career when you are not invested in a theory or particular method. You can quickly change tack to use new analyses and pursue new research questions with little or no cost. Your committee will understand.
  • NEVER show off how much you work. We all work a ton (yes, you do too, don’t let that impostor syndrome get you) and there is no need to hero-worship based on how many hours we work a week.

No Man’s Sky: An Environmentalist’s Review

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Approaching a beautiful lake-filled green planet, which unfortunately turned out to be covered with hostile robots.

I’m entering the atmosphere of this planet in full knowledge that I don’t have the fuel to lift off again. Normally, this wouldn’t be an issue, but the weather strongly tends towards acid rain and the average temperature is well above 150°F. So when I’m landed, I’m going to have to be quick about harvesting some plutonium (an odd choice for fuel considering its rarity on Earth) to use for my ship’s launch thrusters. When I get out to do so, my life support systems immediately begin using power and I realize my harvesting tool is low on juice as well. I could easily die here, alone and hundreds of light years from the nearest real-life human.

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My home base, on a frozen lake on the tundra planet I call home. One of the strange hog-faced, antlered bipedal herbivores is in the foreground.

The main enemy in No Man’s Sky is scarcity. Everything from your space-suit’s life support systems to the hyperdrive you use to travel between planets requires some amount of resources to power and repair. Most planets are not friendly in conditions. They can be frozen to -200 degrees, or +200 or both in a day. Windstorms and acid rain can sap away at your suit’s life support systems. You are truly subservient to the environment in No Man’s Sky.

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Weird hopping pineapple creature in the foreground in this strange fungus-dominated toxic world, where acid rain quickly sapped my suit’s life support systems.

There is a stunning variety of animal and plant life present in the game. All environments and the inhabitants thereof are generated by an algorithm, meaning that the game’s creators can’t be fully aware of all the billions of worlds existing in their fictional galaxy. I have seen flying worms, giant predatory dinosaur-like creatures that chase me on sight, and what can only be described as a hopping pineapple. Each planet is its own ecosystem. Most creatures are uninterested or fearful of you, though a few do seem to chase me or attack in defense. Some attack each other.

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A creature reminding me of a terror bird on a burning hot desert planet.

To be fair, the ecosystems are somewhat limited. I have yet to see truly giant trees rivaling the redwoods of my own real-life planet. I haven’t seen icebergs or glaciers, because each planet only has one real biome. There are no ice-caps or climate differences on each world, which is disappointing, but typical in science fiction (think Hoth or Tatooine from Star Wars). I haven’t seen a running river, which would likely be too computationally expensive to generate. All oceans and lakes seem to be static at a certain sea level. There are no differences in gravity between worlds, most likely to simplify gameplay. Star systems have planets and moons are not to realistic scale, with planets and moons far closer than they should be, probably for visual effect.

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Triceratops-like creature and a weird tubby feathered animal, with a crashed freighter in the background.

Some of the design limitations are interesting from an environmental perspective. There are orbiting space stations, but no cities to speak of. The sparse planetary settlements have at most a few inhabitants in a few buildings. Did the civilization of this fictional galaxy suffer some calamity which decimated its population? Or did they make a conscious decision to spread out and dismantle their cities in subservience to environmental preservation? Perhaps No Man’s Sky is the most extreme manifestation of the Kuznets curve. As human societies mature and living situations improve, their policies begin to value conservation and public health instead of economic growth.

In this way, the civilization of No Man’s Sky has achieved a near-environmental utopia. The player’s actions, however are interesting in their persistence. When you destroy a plant or harvest resources, they do not return. The changes that you make are truly persistent, and the game does not fill back in the gaps. As I rode my buggy over the surface of my own planet, I felt a twinge of regret running over the strange coral-like creatures in the warm canyons between stretches of tundra, because they will not regenerate.

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Remnants of a disbanded city? This world was a frozen archipelago with a yellow sky.

No Man’s Sky has been heavily criticized by many players, who felt it didn’t live up to the hype it received before release. Several updates have been added to improve the core gameplay and story in the last year. Regardless of the improvements of the game itself, I find the concept and environmental ideas in the game to be engrossing. As a real-life naturalist, the experience of exploration and nature-watching in a game is also fun, with the added novelty of knowing that every creature I see has likely never been observed before.

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A modern settlement, in harmony with nature.

You are Isotopes (Part III)

This is the third part of a series about isotopes and why they’re useful and interesting to scientists.

Isotopes are the flavors of elements. And because our universe is made up of atoms of elements, every object can be thought of as a delicious smoothie of flavors. Scientists like me are trying to reverse engineer those mixtures and pick out individual tastes, in order to answer questions about our world.

For example, I work with giant clams. These guys build enormous shells made of a mineral called calcium carbonate: CaCO3. That means that every molecule in a clam’s shell contains a calcium atom, a carbon and three oxygens. But as you might know from reading the previous entries in this series, not all of those atoms are the same. They are a mixture of different flavors. We have some carbon-12 and 13 in there (so named for their atomic weights), and some oxygen-16, 17 and 18. Here I’m focusing on the stable isotopes, which are not radioactive and are called “stable” because they’re not going to self-destruct. There are radioactive isotopes in there too, but I don’t use those nearly as often in my work.

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Officer, this is a pile of giant clam powder, I swear!

I am measuring stable isotopes of carbon and oxygen in my shell samples. To do this, I take a sample of powder, grind it up, weigh it, and put it into tiny little cups. We only need a very small sample: about 50 micrograms of shell material. A typical pill of tylenol contains over 300 mg of active ingredient, so about 6,000 of my samples will fit in a single tylenol regular strength pill, if you suddenly decided you needed a giant clam prescription.

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Simplified representation of what’s happening in a mass spec. Source

This tiny sample is one of thirty that I can measure at a time. Those samples are reacted with acid and the CO2 gas that is released as a result of the reaction can be processed by a machine called a mass spectrometer. The mass spec, which is in the Stable Isotope Laboratory in my building, ionizes the molecules in that gas (gives them a bit of electric charge) and then those ions are flung through an electromagnetic field. That beam of charged gas is flung around a curve. That curve is where the magic of making a mass spectrum happens.

Think of the atoms in the CO2 gas from my sample as a bunch of racecars exiting the straightaway and starting around the curve on the racetrack. Only these racecars vary in weights. And the race organizers have greased the track at the curve so that they fling into the sides of the track when they try to turn. As the racecars fling into the sides of the track, they will separate according to their mass. The lighter cars will be able to make it further around the curve before they meet their demise because they have less inertia forcing them forward, whereas the SUVs in the race will barrel forward straight into the sides of the track. At the end, you have a spectrum of racecars poking out of the walls of the track, with SUVs first, then the coupes, then the compact cars and then the motorcycles, which almost made it around the bend, but not quite. Atoms in the mass spectrometer act the same way, and we measure how many collisions happen along each point of the bend in order to not only “weigh” the sample of gas, but also figure out how many molecules of each weight there are!

It turns out that it is quite difficult to measure the exact number of atoms of a particular isotope in gas, however. It is much more economical and feasible for the purposes of most researchers to simply compare our mass spectrum to the results from a standard. Much like there is a literal standard kilogram and standard meter in a lab somewhere in France which is used to keep track of how much mass is actually in a kilogram, there is a standard used by all researchers like me to describe our samples of carbonate.

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A collection of belemnite fossils from the Pee Dee formation, similar to the one used for the PDB standard. Source

The most common standard used is from a belemnite fossil from the Pee Dee formation in North Carolina. Belemnites are extinct squid-like creatures that formed an internal shell, and one of those internal shells was fossilized, unearthed by a researcher and ground up to become the reference for all other researchers following. Samples of the carbonate in its fossil had more carbon-13’s per unit mass than most other fossil specimens known.  Almost everything you measure will be “lighter” in terms of carbon, because carbon-12 is naturally so common on our planet.

Scientists needed a convenient way to put a number on this, so a simple formula was developed which would allow us to quickly communicate to each other how isotopically “heavy” or “light” a particular sample is in comparison to the Pee Dee Belemnite. The formula isn’t that important for our purposes but the units of its output are in parts per thousand, or “per mil” for short (same idea of how we shorten parts per hundred to “percent”).

The symbol for per mil is a percent sign with an extra little loop at the end: ‰. To make the shorthand complete, we also need to note that this is how much the carbon-13 to carbon-12 isotope ratio of a sample differs from the Pee Dee Belemnite. We do so, we use the Greek delta symbol (δ), commonly used in science and math to represent “difference or change from.” So a sample that has a carbon-13 to carbon-12 ratio which is 20 parts per thousand less than that of the Pee Dee Belemnite is written -20 ‰ δ13CPDB. There are other samples that can be used as well, including Standard Mean Ocean water (SMOW), and the Vienna Pee Dee Belemnite (VPDB). It’s important to note which you are using so that people know the scale of your measurement!

Phew, hopefully that didn’t confuse the hell out of you! Next time, I’ll talk about how different δ13C (and for oxygen isotopes, δ18O) can tell us different details about the life of an organism. Here’s a cute gif of a scallop as a chaser after all that science you read.

Local thief spotted in my backyard

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The brown-headed cowbird (Molothrus ater) is a notorious thief. This is merely a young thief which I observed in my yard; it’s still in training, but when it is an adult it will look more like this:

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Source: Wikipedia

It will search out a nest of an unsupecting bird, perhaps a sparrow or other songbird, and it will sneakily lay an egg when the parents are away. The egg will hatch and the other birds will raise it as their own. This practice is known as brood parasitism. The unsuspecting sap that raises the young cowbird will unfortunately feed its own young proportionately less and its fitness will suffer as a result.

Sometimes, the victim figures out that one of the eggs isn’t its own and disposes of it. If the cowbird sometimes returns to the scene of the crime and if it discovers its egg is missing, it may destroy the nest in a darkly Darwinian form of payback called “mafia behavior.” Sometimes, a cowbird egg is an offer you can’t refuse.