Biological warfare and the coevolutionary arms race

An excerpt from the Berkeley Evolution Library http://evolution.berkeley.edu/evolibrary/article/biowarfare_01

Local legend has it that during the 1950s, three hunters were found dead at their campsite in Oregon. Nothing was stolen, and there was no evidence of foul play. Investigators scoured the scene, but found nothing more unusual than a newt boiled in the hunters' coffee pot — probably scooped from the stream along with their water. What caused the death of these hunters? Edmund Brodie Jr. (a.k.a. "Butch"), a biologist at The Oregon College of Education, wanted to find out.

Pretty but deadly

To investigate the mystery of the hunters' deaths, Butch began by studying the newts. Rough-skinned newts, like the one found in the coffee pot, live along the west coast of the United States. The newts' brown backs blend into their surroundings; but when disturbed, the newts do something strange: they curl their heads and tails towards each other to show off their bright orange bellies. Why? Well, other brightly colored animals like monarch butterflies and coral snakes are poisonous or venomous. Their bright colors warn predators, "Back off, I'm dangerous!" Perhaps, Butch reasoned, the orange belly of the rough-skinned newt sends a similar message — perhaps the newts are poisonous.

Rough-skinned newts usually blend into their surroundings, but when disturbed they curl up to reveal a bright orange underside.

Nasty newts!

To test his hypothesis that the newts were poisonous, Butch injected potential predators, like birds and reptiles, with different concentrations of a newt skin solution or offered them newts to eat. When exposed to the toxin, these predators became wobbly or weak in the knees. They sometimes vomited, stopped moving, or had a fall in blood pressure. All in all, the predators became very sick when exposed to newts. The evidence supported the hypothesis that newts had evolved a defensive poison. Shortly thereafter, chemists identified the newts' poison as a neurotoxin called tetrodotoxin, or "TTX."

Neurotoxins are incredibly dangerous. They act directly on nerve cells, which control everything you do. Some neurotoxins, like the tetanus toxin (which you can pick up by stepping on rusty nail carrying the tetanus bacteria) over-stimulate nerve cells, and cause all of the victim's muscles to contract at once. The muscles contract so intensely that they can break bones! Other neurotoxins, like TTX, cause nerves to stop functioning completely. A person poisoned by TTX stops breathing when his or her nerve cells stop sending the signal to breathe — but the victim's heart and brain keep working until they are depleted of oxygen several minutes later. Near death survivors of TTX-poisoning recall being paralyzed and unable to protest when others mistakenly declared them dead!

Nastier than nasty newts

Butch discovered that newts are loaded with TTX. In fact, they are SO poisonous that just one newt could kill over 100 people! Humans don't eat newts, but a bird might. One of these newts could kill 200 herons or 2000 kingfishers! The discovery of extreme toxicity in newts explained what killed the hunters — but it also led to a new question: why would a newt evolve to be so toxic? Isn't all that TTX, quite literally, overkill?

After all, newts use a lot of energy making TTX — energy that they could otherwise spend on finding food, growing faster, or reproducing. Why would a newt waste all that energy making more TTX than it needed? In evolutionary terms, it doesn't make sense. Natural selection should cause newts to evolve TTX levels just high enough to protect them from

predators. Evolutionary theory predicts that natural selection will weed out newts with too little or excess TTX so that newt populations should be equipped with just enough TTX to kill their predators and no more. But if that's true, then how did newts evolve to be so poisonous?

The mystery of too much TTX

Butch knew from his earlier experiments that TTX defends newts against predators, so he came up with another hypothesis for their overproduction of TTX. Maybe there was some predator out there that could withstand large amounts of TTX and that had caused the newts to evolve excessive levels of TTX. That predator, Butch reasoned, must be resistant to the effects of TTX and can eat the newts while avoiding suffering the effects of the toxin. Butch hypothesized that newts produce so much toxin because a predator had evolved resistance to TTX.

As luck would have it, another biologist in Oregon observed something quite surprising: common garter snakes were eating newts despite their toxicity! After hearing about newt-eating garter snakes, Butch hypothesized that the snakes had evolved resistance to the newt's toxin. To test this idea he offered newts to garter snakes in the lab. The snakes gobbled up several of these otherwise deadly animals in a row! While just the tail tip of one newt could kill a full grown human, the 1/2 pound garter snakes slurped down whole

newts easily. Butch concluded that garter snakes have indeed evolved resistance to the deadly defenses of the dangerous newts.

Butch suspected that the snakes had evolved this resistance in response to newt toxicity and newt toxicity, in turn, may have evolved in response to snake resistance — in other words, the two species may have been evolving in response to each other.

Coevolution

When two or more organisms each evolve in response to each other, we call it coevolution. Butch hypothesized that TTX-laden newts were favored because they could avoid getting eaten by garter snakes, and that garter snakes with TTX-resistance were favored because they could survive encounters with deadly newts — in other words, that newt toxicity and snake resistance had coevolved. Specifically, Butch hypothesized that snake/newt coevolution was a bit like an arms race, with each side evolving stronger and stronger weapons and defenses to match those of their opponent. According to this hypothesis, newts would have evolved toxicity in response to selection from snakes, and then the snakes would have evolved resistance in response to selection from newts, which allowed the newts to evolve slightly greater toxicity — in response to which snakes evolved greater resistance...and so on. The story seemed to make sense. If the snakes and newts were engaged in an arms race, it would help explain why the newts were so extremely toxic.

Recall that newts face an evolutionary tradeoff. Newts that make too much TTX have less energy to produce offspring, but newts that make too little TTX will be eaten. TTX production is costly for newts, but it turns out that TTX resistance is similarly costly for snakes. By measuring the speeds and TTX resistance levels of many snakes, the Brodies discovered that the cost of TTX resistance is slower average crawling speed even before eating a newt. A snake that is non-resistant can slither away quickly in normal circumstances, but will be killed if it eats a toxic newt. A resistant snake, however, will survive if it eats a toxic newt, but will slither slower in normal circumstances. So snakes face an evolutionary tradeoff, too. Too much resistance results in a much slower snake that is more likely to be eaten by snake predators, but too little resistance would mean death for any snake that tried to eat a newt. Thus, we'd expect snakes to evolve just enough resistance to eat a newt but no more so.

A garter snake eating a newt

Fire ants invade and evolve

An excerpt from the Berkeley Evolution Library http://evolution.berkeley.edu/evolibrary/article/_0/fireants_01

In the late 1930s, a small but threatening invader arrived in the United States: the Argentine fire ant (Solenopsis invicta). Having hitched a ride from its homeland, S. invicta set up outposts in Alabama — nests inhabited by thousands (and sometimes hundreds of thousands) of tiny red workers (all female — males just hang around for reproductive purposes) and single queens. Over the next 20 years, S. invicta spread throughout the South, inflicting painful stings, building large nests, and generally annoying human populations.

At left, workers tend to the queen in a single-queen colony. At right, a pasture dotted with numerous fire ant nests.

What is it about S. invicta that's made their U.S. invasion so successful? As it turns out, the answer is an evolutionary one.

Fire ants in the U.S.A.

In the 1970s, scientists discovered a new and more threatening form of S. invicta in Mississippi. Each of these colonies supported multiple queens. The multiple queen form — called polygyne ("poly" = many, "gyne" = female) — poses a greater hazard than the single queen form — called monogyne ("mono" = one). The tightly-packed, multiple-queen colonies drive out native ant species and other animals. Today, multiple-queen colonies dominate Texas, and may be ready to spread throughout the country.

Both multiple- and single-queen colonies thrive in Argentina, but the ants there don't seem to take over as they have in Texas — where, in some places, you could hopscotch across a field, landing only on ant nests. Let's take a look at how natural selection might favor colonies with different numbers of queens, how genes can influence queen number, and how that knowledge might help us slow the S. invicta invasion.

An evolutionary game

In order to understand the evolution of a behavior (like whether a colony will accept multiple queens or remain loyal to just one), it often helps to think of evolution as a game. In this game, the "players" are the organisms. As in chess, the organisms can adopt different "strategies" — but, unlike chess, these strategies are behaviors that are encoded in the players' genes. The "winner" of this evolutionary game is the player who gets the most genes into the next generation by having a lot of offspring. Evolutionary theory predicts that the winning strategy for getting genes into the next generation will spread through a population via natural selection.

To understand how ant queens might fare in this evolutionary game, imagine two S. invicta queens ready to leave their home nests. One queen — let's call her Mono — carries a gene that causes her to leave her home nest and start a colony by herself. So Mono always has a "go it alone" strategy — she doesn't get any help from other ants, but she doesn't have to share her resources with them either.

The other queen — let's call her Poly — carries a gene that causes her to try to stay in her own nest and reproduce alongside her mother or to join another colony and reproduce alongside the queen there. So Poly always has a "cooperative" strategy — she welcomes the help of other ants and shares her resources with them.

The "go it alone" strategy is a great one — if you can make it work. If it works and the queen does manage to start a new colony, she will get a

bonanza of genes in the next generation — the entire colony (and any queens it produces) will be the founder's direct descendents. However, if the "go it alone" strategy fails, it will be a complete flop. A queen that tries to start a colony by herself and fails is likely to die without getting a single gene into the next generation. The "cooperative" strategy is a safer bet — joining another nest is pretty likely to yield some genes in the next generation, just not a "bonanza" of genes.

Situation 1 — Sparse population Imagine that Mono and Poly live in an area with a lot of unoccupied nest sites available (see graphic below). In this situation, Mono would "win" — she would start a new nest, and the entire colony would end up carrying her genes. Mono's "go it alone" strategy works well when there are enough resources for loners to build a colony from scratch. Poly would "lose" in this situation because — as one of several queens in a nest — she wouldn't have the resources to reproduce as much as Mono. Poly's "cooperative" strategy backfires here — she shares her resources with others, even though there is plenty to go around.

Situation 2 — Dense Population On the other hand, if Mono and Poly live in an area where the good nest sites are already taken (see graphic below), Poly would "win" and Mono would "lose." Mono's genes would cause her to try to start a colony by herself — but because space is limited, she wouldn't be able to find a nest site and would die without leaving any offspring. "Going it alone" is likely to be a complete failure when things are crowded — very, very few solo queens would be able to successfully start a nest. However, Poly's "cooperative" strategy works great in this situation. She doesn't have to find the space and resources to start a whole new colony — she just needs to find a colony willing to accept her and share some resources — either her own or a closely related nest. Poly would get to reproduce — and would get some of her genes into the next generation. Cooperation pays when times are tough and space and resources are limited.

So for S. invicta queens, the winning strategy for getting genes into the next generation depends on whether or not good nest sites are available: with nest sites available, start a colony by yourself; with very few available nest sites, join other queens in a multiple-queen colony.

The hypothesis that natural selection is responsible for the increasing frequency of multiple-queen colonies is a good one — provided that scientists can establish one crucial fact: that queen number (one or many) is genetically controlled. After all, selection can only work on traits that have a genetic basis — if ants learn to be loyal to a single queen or if some environmental cue causes them to accept new queens, natural selection cannot be responsible for the switch. We often think of genes as controlling physical characteristics, like eye color. But genes can also affect behaviors. For many years, showing a genetic basis for behavior affecting queen number seemed like a daunting requirement, since the genetics of complex behaviors were assumed to be complex as well. But in 2002, evolutionary biologists found their golden gene.

Simple gene, complex behavior

Hidden away among the 15,000 fire ant genes, the Gp-9 gene lurks. This simple gene codes for an odor-binding protein — a molecule that helps determine which scents an animal can detect. That might seem like an insignificant job, but Gp-9 seems to really shake up the way that fire ants behave.

There are two versions of Gp-9, B and b, and which version of the gene an ant carries may spell the difference between how that ant reacts to an invading queen: submission ("God save the Queen!") or mutiny ("Off with her head!"). Worker ants seem to accept or reject an invading queen based on both how the new queen smells and what odor-binding proteins the worker has. Bb workers in multiple queen colonies encourage polygyny by accepting new Bb queens and executing new BB queens. BB workers in single-queen colonies encourage monogyny by executing all invading queens.

Genotype BB Genotype Bb Genotype bb

BB queen produces BB workers; BB workers execute all invading queens

Bb queen welcomed in colonies of Bb workers; Bb workers execute invading BB queens

bb queens are weak; usually die before laying eggs

The evolutionary upshot of all the evidence seems to be that Gp-9 influences queen number and that selection has acted on the gene, favoring the B form of the gene when starting a colony alone was advantageous, and favoring the b form when banding together with other queens was advantageous. Thus, as S. invicta spread throughout the South forming tightly-packed colonies, ants and colonies with the b form of the gene may have been successful and reproduced a lot, causing the b version of the gene to be more common.

It Takes Teamwork: How Endosymbiosis Changed Life on Earth

An excerpt from the Berkeley Evolution Library http://evolution.berkeley.edu/evolibrary/article/endosymbiosis_01 In 1966, microbiologist Kwang Jeon was studying single-celled organisms called amoebae, when his amoebae communities were struck by an unexpected plague: a bacterial infection. Literally thousands of the tiny invaders — named x-bacteria by Jeon — squeezed inside each amoeba cell, causing the cell to become dangerously sick. Only a few amoebae survived the epidemic.

The blob-like form of an amoeba

However, several months later, the few surviving amoebae and their descendents seemed to be unexpectedly healthy. Had the amoebae finally managed to fight off the x-bacterial infection? Jeon and his colleagues were surprised to find that the answer was no — the x-bacteria were still thriving inside their amoebae hosts, but they no longer made the amoebae sick. There were more surprises when Jeon used antibiotics to kill the bacteria inside an amoeba — the host amoeba also died! The amoebae could no longer live without their former attackers. Jeon discovered that this was because the bacteria make a protein that the amoebae need to survive. The nature of the relationship between the two species had changed entirely: from attack and defense to cooperation.

When two become one

Jeon's colonies of amoebae seem perfectly happy living with their permanent guests, the x-bacteria, inside of them. This kind of relationship — two or more different species living in close association — is called symbiosis.

Each amoeba and its x-bacteria work together for mutual benefit — but they are still separate organisms. Each bacterium or amoeba divides on its own, gets its own energy, uses its own genes, and makes its own proteins (mostly!). However, with their close relationship, it seems possible that after many years of evolving together, these cells could become not just a team, but a single integrated organism with a common set of genes and proteins. A future scientist discovering the descendents of Jeon's amoebae might not guess that this one "amoebacterium" was once two distinct organisms.

Evidence like this points to the likelihood that the "merging" of two simple organisms has also happened under natural conditions. Long ago in evolutionary history, two cells formed a symbiotic team that, over millions of years, evolved into a single organism. The result of this union was the first eukaryotic cell — the type of cell that makes up the human body. We humans owe our existence to two bacteria that teamed up in a symbiotic relationship over a billion years ago!

From prokaryotes to eukaryotes

Living things have evolved into three large clusters of closely

related organisms, called "domains": Archaea, Bacteria, and

Eukaryota. Archaea and Bacteria are small, relatively simple

cells surrounded by a membrane and a cell wall, with a

circular strand of DNA containing their genes. They are called

prokaryotes.

Virtually all the life we see each day — including plants and animals — belongs to the third domain, Eukaryota. Eukaryotic cells are more complex than prokaryotes, and the DNA is linear and found within a nucleus. Eukaryotic cells boast their own personal "power plants", called mitochondria. These tiny organelles in the cell not only produce chemical energy, but also hold the key to understanding the evolution of the eukaryotic cell.

The complex eukaryotic cell ushered in a whole new era for life on Earth, because these cells evolved into multicellular organisms. But how did the eukaryotic cell itself evolve? How did a humble bacterium make this evolutionary leap from a simple prokaryotic cell to a more complex eukaryotic cell? The answer seems to be symbiosis — in other words, teamwork.

Evidence supports the idea that eukaryotic cells are actually the descendents of separate prokaryotic cells that joined together in a symbiotic union. In fact, the mitochondrion itself seems to be the "great-great-great-great-great-great-great-great-great granddaughter" of a free-living bacterium that was engulfed by another cell, perhaps as a meal, and ended up staying as a sort of permanent houseguest. The host cell profited from the chemical energy the mitochondrion produced, and the mitochondrion benefited from the protected, nutrient-rich environment surrounding it. This kind of "internal" symbiosis — one organism taking up permanent residence inside another and eventually evolving into a single lineage — is called endosymbiosis.

How important is endosymbiosis?

Endosymbiosis explains the origin of mitochondria and chloroplasts, but could it also explain other features of the eukaryotic cell? Maybe. Endosymbiotic origins have been suggested for many structures, including flagella (structures like the tail of a sperm), cilia (hair-like structures that help in locomotion), and even the nucleus — the cell's command center! However, scientists are still actively debating whether or not these structures evolved through endosymbiosis. The jury is out while more evidence is gathered.

In her theory of endosymbiosis, Lynn Margulis emphasizes that during the history of life, symbiosis has played a role not just once or twice, but over and over again. Instead of the traditional tree of life branching out from a few common ancestors to many descendent species, Margulis proposes that branches have separated, and then come together again many times as individuals of different species set up symbiotic relationships and formed new organisms. This process formed an interconnected tree of life in which organisms have multiple ancestors, even from different domains. As eukaryotes, our ancestors include both the bacteria that became mitochondria, and the archaebacterium that was the host cell.

Why have endosymbiosis and symbiosis been so important to evolution? Why cooperate at all? The answer to these questions points us to one of the basic processes of evolution: natural selection. As Darwin observed, organisms that are fit enough to succeed in the game of survival have a good chance of passing on their genes to the next generation. Any survival or reproductive advantage can help a species out-compete another species or simply avoid becoming extinct itself. It seems likely that the first eukaryotic cells gained a slight edge over their neighbors when the mitochondria, a rich source of energy, moved in with them. Like Kwang Jeon's x-bacteria and amoebae, the mitochondria and their hosts relied more and more on each other in order to survive. Eventually, neither could succeed alone — but as a team they produced millions of descendents, establishing a whole new domain of life.

Mantis shrimp shoulder their evolutionary baggage and bluff

An excerpt from the Berkeley Evolution Library http://evolution.berkeley.edu/evolibrary/article/mantisshrimp_01

Imagine yourself facing some little, unexpected problem — being trapped, along with several hundred deadly vipers, in a rapidly flooding room, with the walls closing in. The ideal solution (being rescued by a SWAT team equipped with anti-venom and scuba gear) might not be available to you. Likewise, it's too late to have avoided the situation in the first place — you weren't anticipating this particular problem when you decided to explore the ancient, rumored-to-be-cursed temple. You are constrained — the only tools you have to solve your problem are what you happen to have in your backpack: some loose change, duct tape, and several drinking straws. Perhaps, you can figure out some solution, but it won't be ideal.

Evolution, too, is constrained, as is the adventurer above

who wound up in an unexpected situation. Evolution

works with whatever tools are available at the moment —

not duct tape and drinking straws, but bodies, behaviors,

and genes. This on-the-fly problem solving often leads to

solutions that are less than ideal, but "good enough."

Here, you will learn how the traits that a lineage inherits

from its ancestors may constrain its future evolution.

Specifically, we will see how the exoskeleton has altered

the course of evolution for arthropods, and how one

group of organisms, the mantis shrimp, have evolved

"good enough" solutions within the constraints introduced

by the exoskeleton.

The mantis shrimp has evolved "good enough" solutions within the constraints of its exoskeleton.

The arthropods

Almost six hundred million years ago, long before vertebrates existed, some sea-dwelling animals evolved a hard, armor-like body covering, and things just haven't been the same since. Those organisms were the ancestors of insects, spiders, centipedes, millipedes, the extinct trilobites, lobsters, and their relatives — an enormously successful group of organisms known as arthropods. And for better or worse, all arthropods have inherited some basic characteristics of those first ancestors, including their armor-like body covering — the exoskeleton (exo = outside).

The exoskeleton: the good, the bad, and the creepy-crawly

The "crunch" of squishing a cockroach is caused by the exoskeleton — a mosaic of hard plates that protect and support the soft, inner body of the arthropod. Humans and other vertebrates have an internal skeleton made of bones, but arthropods do not — only their exoskeletons keep them from collapsing into soft blobs.

The exoskeleton is a great evolutionary innovation. With this jointed "suit of armor" operated by muscles attached to it on the inside, arthropods, like the pillbug above, can move around quite efficiently. Beyond support, the exoskeleton provides protection against predators and strength against prey. And it protects arthropods from the environment, for example, keeping ocean-dwellers from being affected by small changes in saltiness and preventing land-dwellers from drying out.

But as you will see, the exoskeleton also constrains arthropod evolution by introducing a big problem: how to grow.

Growing up in a suit of armor

How do arthropods grow while wearing form-fitting suits of armor? The solution that evolution has hit upon is molting — the shedding of the exoskeleton.

The hard, outer covering on this pillbug is the only skeleton it has.

To illustrate this solution, we'll look at it from the mantis shrimp's point of view. Mantis "shrimp" are predatory, ocean-dwelling crustaceans that range in size from 2-40 cm — but they're not the true shrimp that you might eat at a seafood restaurant.

Top and side views of mantis shrimp Some species wield club-like appendages to smash unsuspecting crabs, snails, and clams, and others have sharp, spiny

appendages adapted for spearing fish. These weapons are formidable; they are deadly, strong enough to break the glass of an aquarium, and move 50 times faster than you can blink. Mantis shrimp rely on a hard exoskeleton for the strength and power of their spearing and clubbing weapons, but that same exoskeleton makes it difficult to grow from juvenile to adult. Like all arthropods, they must molt in order to grow. Every couple months, when its exoskeleton is getting a bit tight, a mantis shrimp splits his or her outer body covering and wriggles out of it, leaving behind an almost perfect, but empty, mantis shrimp mold, complete with eyes and limbs.

The newly emerged mantis shrimp is slightly larger than before, but its new outer covering has not yet hardened. The animal must survive up to several weeks of vulnerable "squishiness" before its new armor is fully protective.

The dangers of molting!

Molting is a dangerous undertaking for mantis shrimp and most other arthropods. First, molting itself is not a slam-dunk. Imagine trying to get out of a head-to-toe scuba suit when you've got a nasty sunburn, only worse. Arthropods shed not just their outer body covering, but their eye surfaces, the inner lining of their foregut and hindgut, and even the lining of the internal passageways leading to the respiratory system! It's all too easy for an arthropod to tear off an eyeball or leg, or to get stuck (leading to death) while shedding its armor. When and if the arthropod manages to free itself, it will be largely incapacitated while its armor hardens. Though a newly molted mantis shrimp may look strong and can move around, without a hard exoskeleton, the animal makes easy prey and cannot defend itself — just imagine trying to punch a bully with a boneless arm!

The exoskeleton is part of the mantis shrimp's evolutionary heritage — its "baggage." It is so central to the way a mantis shrimp's body is built that evolution simply cannot produce a mantis shrimp without an exoskeleton. The exoskeleton is useful in many ways, but having one also means that you have to go through the risky process of molting — it's not an ideal solution to the problem of growth, but mantis shrimp must make do.

This horseshoe crab died while molting.

Making the best of an imperfect situation

So there is no way out of molting for mantis shrimp, but that's not the end of the evolutionary story.

Many species of mantis shrimp live in underwater cavities, which provide protection. However, these cavities are in low supply. So mantis shrimp may compete fiercely with one another for homes, even staging fights in order to evict an opponent.

For these mantis shrimp, survival is tightly coupled to ruling a good roost. And, as you might guess, a molting mantis shrimp really needs a protective nook, or cavity. Unfortunately, a molting mantis shrimp is also poorly equipped for defending its cavity against invaders. Since it can't avoid molting entirely, over the course of evolution, natural selection favored any individual with characteristics that helped it defend its home during molting.

In order to learn about adaptations that mantis shrimp evolved for protecting their cavities, biologists performed a series of experiments. They selected a territorial species of mantis shrimp, Neogonodactylus bredini, and staged fights in laboratory aquaria between animals of different sizes and in different stages of their molting cycle. And they discovered something surprising: mantis shrimp have evolved several strategies for keeping their cavities — and frequently try to "trick" their opponents by bluffing.

Imagine an N. bredini mantis shrimp quietly enjoying the safety of her cavity when a challenger appears outside. If she is not molting, the resident is likely to fight to defend her home — she may raise her forelegs in a threatening pose and then strike at the challenger. In this situation, the cavity defender is likely to win, because her body is protected within her cavity and her opponent's is vulnerable. This is bad news for the opponent, who could be killed by the well-protected defender!

If the cavity defender is molting, however, it's a different story. A molting mantis shrimp, still in the "squishy" stage, cannot fight successfully — she will only injure herself if she tries to strike at her opponent. In this case, the defending mantis shrimp has three options: she may flee, hide, or bluff.

Mantis shrimp would probably make quite wily poker players because they are so sophisticated about their bluffing strategies. For example, before bluffing, they seem to size up their opponent to determine how likely he or she is to be intimidated by the bluff — if the defender sees that her opponent is much smaller than her, she is more likely to try the bluff. If the defender sees that her opponent is larger than her, she is more likely to flee.

Mantis shrimp even try to "teach" their neighbors to take threats seriously, so that their bluffs are more effective. For example, a mantis shrimp whose exoskeleton is still hard but who is about to molt will increase her use of threatening displays, holding out her arms and then backing the display up with vicious strikes. This seems to give her neighbors the idea, "Whoa! Don't mess with her!" After all, a cavity-invader risks death by attempting to oust a fully-protected mantis shrimp from her cavity. Then when the cavity defender actually goes into molt, loses her exoskeletal protection, and can no longer follow up a threat with a strike, her neighbors remember "not to mess with her" and shy away from her empty threats.

Selecting the strategies, naturally

Over the course of mantis shrimp evolution, strategies for protecting ones cavity evolved through natural selection. Sometime in the evolutionary past, N. bredini did not have such sophisticated cavity defense strategies. However, some individuals happened to carry genes that generated behaviors that helped them protect their cavities and survive to reproduce. These individuals had more offspring than those without the handy genetic variation, and the good cavity defenders passed their genes for cavity defense on to their offspring. In this way, genes for good cavity defense spread through the population. After that, any small genetic variants that arose through mutation and happened to help with cavity defense spread in the same way. Hence, natural selection acted on the behavior of mantis shrimp, leading to a variety of behavioral strategies for avoiding injury and defending one's cavity.

Of course, as defensive strategies evolved, so did offensive strategies — behaviors or other adaptations that would help an intruder take over a cavity. Any offensive strategies good enough to overcome the defense strategies would also be favored by natural selection and would likewise spread through the population. But these offensive strategies would have to be sophisticated: simply ignoring threats from cavity defenders is dangerous because if the defender is not bluffing, the intruder may be killed. Over the course of the lineage's evolution, offensive and defensive strategies continued to escalate as mantis shrimp struggled to hold on to and acquire safe homes.

A mantis shrimp in its cavity

The Monterey Pine through geologic time

by Frank Perry, Research Associate, Santa Cruz Museum of Natural History reproduced from the Monterey Bay Paleontological Society Bulletin, July–September, 2004. © 2004 Frank Perry

"The present is the key to the past." This well-known geological axiom was first championed by Scottish naturalist James Hutton in the late eighteenth century. Hutton proposed that modern-day Earth processes — such as erosion, sedimentation, volcanism, etc. — could, over long periods of time, leave behind the rock record we see today. It was not necessary to invoke extraordinary events or "catastrophes" to explain the past. Geologic processes that anyone could see during his or her lifetime had acted much the same way throughout earth history. This concept is more formally known as the Principle of Uniformitarianism. It mostly holds true, although now we know that very rare events, such as asteroid impacts, have also played important roles in the history of the Earth and of life.

Less appreciated but just as important is that the past can be a key to better understanding the present. A splendid example of this is the Monterey Pine. Recent discoveries

about its fossil history not only help us to better understand its present distribution, but also may help shape future conservation efforts.

I have long had a fondness for Monterey Pines. Several large Monterey Pines grew next door when I was a kid, so these were the first pines with which I became acquainted. Each was very tall, perhaps a hundred feet, with a craggy trunk three or four feet wide at the base. A carpet of brown needles blanketed the ground beneath them. In the highest branches, kestrels would perch, sometimes giving off a loud screech. On very hot days — a rarity where I lived — the trees emitted an eerie cracking sound as some of the cones opened.

Closed-cone pines The Monterey Pine, along with the Bishop and Knobcone Pines, belong

to an informal taxonomic category known as the "California closed-cone

pines." In most species of pine, a set of cones matures annually,

opening and dropping their seeds in the fall. With the closed-cone

pines, however, many of the cones remain sealed with resin and

attached to the branches. In the case of the Knobcone Pine, the cones

can remain closed for decades. Monterey Pine cones may open after a

few years, but remain on the tree.

Botanists call this closed-cone trait serotiny (sahr-ROT-in-ee). Most scientists believe that serotinous cones evolved as an adaptation to dry climates with occasional forest fires. The heat from fires sweeping through the crowns of the trees causes the resin seal to melt. In as little as a day after the fire, the cones open, at once releasing a seed supply saved up from multiple years. Although the burned mature trees usually die, many new trees soon sprout from the seeds, recolonizing the area more quickly than other tree species.

Distribution Today, the Monterey Pine has a very restricted distribution. It is native to just five areas: the Año Nuevo-Swanton area (San Mateo and Santa Cruz Counties), the Monterey Peninsula and Carmel (Monterey County), Cambria (San Luis Obispo County), and Guadalupe and Cedros Islands off Baja California in Mexico. The species was more widely distributed at various times in the past. Fossil cones or needles of the Monterey Pine have been recorded from 20 localities in California, including the famous La Brea Tar Pits. Most of the localities are near the coast and range from Tomales Bay in the north to Chula Vista near San Diego and on the Channel Islands. The one inland locality is in Riverside County. The fossils range in age from middle Miocene through Pleistocene.

Range of Monterey Pines today.

Localities where fossil cones of Monterey Pines

have been found.

Fossil cones The cones of this species are distinctive and one of the easiest to recognize from fossils. They are asymmetrical with large,

smooth, bulbous umbos (knoblike protuberances) on the scales. The scales do not have sharp prickles like some pine

species.

Most of the Pleistocene cones are carbonized (also called coalified). Basically, the wood has turned into coal. In this process, the molecules that make up the cellulose begin to break down after prolonged burial. Hydrogen and oxygen atoms are released in the form of water, carbon dioxide, and methane gas. This leaves behind carbon — hence the term carbonized.

Evolutionary history: early hypotheses The evolutionary history of the Monterey Pine (Pinus radiata) and its close relative, the Bishop Pine (Pinus muricata), first piqued the curiosity of paleontologists in the early 1900s. In the 1930s Herbert Mason suggested that the disjunct populations of these pines could be explained by Tertiary-age islands. He hypothesized that the populations had been isolated on offshore islands millions of years ago, and that after these islands became part of the mainland, the pine's insular distribution persisted. It was a reasonable theory at the time, but geologic studies in recent decades have found no evidence for such islands, at least in central and northern California. In addition, later discoveries show that the Monterey Pine was much more widely distributed only a few tens or hundreds of thousands of years ago, well after the end of the Tertiary Period.

In a series of studies during the last half of the 1900s, paleobotanist Daniel Axelrod used geologic, fossil, and associated floristic evidence to better piece together the evolutionary and geographic history of the Monterey Pine. According to Axelrod, the closed-cone pines originated in Central America from an ancestor related to the modern Pinus oocarpa. The closed-cone pines spread northward into California about 15 million years ago. By this time, Pinus radiata had already evolved into a distinct species. The Monterey Pine's widespread fossil distribution along the California coast led Axelrod to believe that it flourished throughout the Pleistocene. He theorized that it was not until a warm, dry period 4,000 to 8,000 years ago that it was driven to near extinction, surviving in the form of five small populations. He called this warm period the Xerotherm. Today it is more commonly referred to as the Climatic Optimum or early Holocene warm period.

A recent hypothesis A few years ago, Connie Millar, a scientist with the United States Forest Service, developed a revised theory of the pine's

history based on new fossil evidence and a refined understanding of climate change during the Pleistocene.

Marine sediments and ice cores from the Atlantic, Pacific, and Arctic oceans have now given us high-resolution climate records going back hundreds of thousands of years. Paleotemperatures have been determined from the shells of fossil organisms and from gases trapped in the ice, sometimes datable to individual years. These scientific breakthroughs have enabled scientists to paint a much more detailed picture of climate change than was ever before possible.

These records show that there have been at least 11 ice ages over the past million years. Each lasted for about 90,000 years. Between the glacial periods were warm interglacial periods of about 10,000 years, like the one we live in now. The Climatic Optimum (Axelrod's "Xerotherm"), when temperatures were even warmer than at present, appears to have had analogs in other interglacial periods. This casts doubt on Axelrod's theory, which relied on the premise that the early Holocene warming was unique. The previous interglacial period (125,000 to 111,000 years ago), for example, apparently had peak temperatures at least two degrees centigrade warmer than the period 8,000 to 4,000 years ago.

Another important line of evidence has come from fossil pollen. Pollen is extremely durable and can last for millions of years. The tiny grains have distinctive morphologies and in many cases are identifiable to species. Fossil pollen samples collected from cores of lake, bog, and marine deposits can show what plants were living nearby and how the flora in a particular area changed over time with changes in climate.

For her study of the Monterey Pine, Millar drew upon pollen evidence from sediment cores in the Santa Barbara Channel. According to Millar, the Monterey Pine "was least abundant during full interglacials (i.e., the Holocene and previous interglacials), when oaks dominated coastal habitats, and was also uncommon during the cold periods of the glacials, when junipers dominated. Monterey Pine, as well as other coastal pines, increased dramatically in abundance and shifted in coastal location during climate periods intermediate between these extremes — that is, at times such as the end of the ice ages (climate warming), during ‘interstadial periods’ (warmish intervals within the ice ages), and at the end of interglacials (climate cooling)."

She also found that, "Times of abundance of Monterey Pine correlated also with increases in charcoal abundances in the sediment cores, corroborating that fire plays an important role in dispersal and spread of Monterey Pines by opening cones and preparing seed beds."

Shifts in distribution and today's populations There is no evidence that the Monterey Pine was ever continuously distributed along the California coast at any one time over the past two million years. Instead, its populations were always fragmented. The species expanded, shifted, or colonized new sites during periods of favorable climate. Its range contracted and some forest stands died out during periods of unfavorable climate.

Millar used this revised interpretation of the tree's history to suggest a revised conservation strategy. Many biologists are concerned about the survival of the five native stands of Monterey Pine. On Guadalupe Island, for example, the population has long been threatened by goats. In 1964 only 320 trees remained. By 1992 the population had dwindled to 150. On the mainland, urbanization, fire suppression, genetic contamination, and pine pitch canker (a deadly fungal disease ravaging many stands) all threaten the trees.

Domesticated Monterey Pines are common in the United States as garden trees, and small ones are used for Christmas trees. Although the species is not likely to become extinct, it is important that the wild populations be preserved. In other parts of the world, domesticated Monterey Pines are a major forest tree. In fact, it is the world's most planted conifer. Over 10 million acres have been planted. There are 2 million acres in Australia, 3 million in New Zealand, and millions more in Chile, Uruguay, Argentina, Spain, South Africa, and Kenya. Selective breeding has produced fast-growing trees with straight trunks — ideal for the forest industry. Most of these countries have active breeding programs to develop still better strains for lumber and paper manufacturing. DNA studies have shown that the material from which these foreign plantings were developed was not very diverse genetically. On the contrary, the native forests are genetically diverse. According to Millar, the diverse germplasm in these native populations have "inestimable value." It could, for example, help with the development of strains that are more resistant to certain insect pests and diseases.

Given the Pleistocene history of the species, she recommends expanding conservation efforts beyond the five native populations. For example, "neo-native" forests could be planted in areas where the pine lived only a few thousand years ago such as near Point Reyes, Point Sur, Santa Barbara, and San Diego. This would help ensure survival of genetically

A fossil Monterey Pine cone from the Pleistocene La Brea Tar

Pits, Los Angeles County.

diverse forests. When Dr. Millar spoke to the Monterey Bay Paleontological Society a few years ago, she told the tragic story of a park along the northern California coast where all the Monterey Pines were cut down. The trees were perfectly healthy, but were removed because they were a "non-native" species. Yet, a few thousand years ago the trees might have grown there naturally.

One of the great values of fossils is that they enable us to see the present from the perspective of geologic time. As the Monterey Pine clearly shows, this perspective can give us a greater appreciation for the present and help us plan better for the future.

So the next time you visit the Monterey Peninsula or drive by the trees along Highway 1 near Año Nuevo State Reserve, take a closer look at the Monterey Pines and their remarkable cones. Consider their clever survival strategy. Think of them not as relicts of the Ice Age on the verge of dying out, but as trees lying in wait for the next shift towards a cooler climate.