The Botanical Counterattack, And an Arms Race Unseen

Roses use their thorny exteriors to fend off herbivores.
Roses use their thorny exteriors to fend off herbivores.

By Cody Sullivan
BU News Service

Underneath a swaying canopy of gold, burnt orange, and crimson runs the sun dappled Meadow Road.  The leaves of exotic Chinese maple trees litter the lawns surrounding Arnold Arboretum’s thoroughfare.  The distant groundskeepers, with their roaring lawn mowers and wire rakes, have yet to manicure the maples’ lawns.  Leaving the cover of trees behind, and emerging into the crisp air of a bright autumn afternoon, an elegant display of roses and arching trellises awaits me

Strolling onto the lawns of the Bradley Rosaceous Collection, I am surrounded by botanical bastions.  Separated from the grassy walkways, by barricades of flat stones, are roses in all forms: leafy plants, bristling bushes and woody trees. But the deep green leaves, and flowers ranging from pale pink to blood red, cannot conceal the rose’s thorns.  With their defensive lances at the ready, these rose bushes are prepared for the herbivorous onslaught plants face every day, all around the world.

The rose standing guard is armed with a few large and sturdy thorns, while across the grass pathway is a bush densely covered in small, almost hair-like thorns.  Both variations on the thorny theme will prick, pierce, and even impale herbivores.  A thorn stabbing into an animal’s delicate gums, or shredding the tender tongue, is enough to discourage would-be herbivores.

Roses represent a group of plants that use mechanical defenses against herbivores –– and they’re not the only ones: berry bushes are another well-known example of a species that uses its structure to repel attacks, and even a tree trunk’s bark blocks invasions. But physical defenses can’t keep all predators away. As I examine this rose’s defenses, I notice that some of its arrowhead shaped leaves have holes, despite their piercing protection.

Insects, which nimbly avoid rose thorns, commonly munch rose leaves, and seem to have the upper hand in the plant-herbivore war.  But not so fast: plants have evolved, and consistently update, a cache of chemical toxins and deterrents used to fend off tiny herbivores capable of avoiding thorns.

Deep inside each individual leaf, beneath its cuticle, and behind the barriers of the cell wall is a plant cell’s chemical war factory.  The cytoplasm of plant cells converts essential, everyday molecules into toxins, irritants, and bitter tasting defensive compounds.

All cells, whether in a plant or an animal, produce basic molecular building blocks, chiefly amino acids, nucleic acids, or fatty acids.  These “primary metabolites” enter the military production line and the addition of a hydroxide molecule here, or a carbon group there, modify these compounds into secondary metabolites which function as biologic defensive weapons.  A plant’s artillery comes in three forms: phenolics, terpenoids, or nitrogen containing compounds.  All three types work as either unpleasant tasting deterrents, or toxins that cause a range of symptoms from mild burning, to severe pain or nausea, and sometimes even death – the cyanide in hemlock is a secondary compound that can kill when administered in high enough doses.

When implementing their chemical defenses, plants use two different strategies.  The first strategy is to wait for attack, then react.  Chemicals used in this approach are called phytoalexins, and are only produced after an herbivore, fungus, or even a pathogen has latched onto a leaf and begun its assault.  The second strategy is to always be prepared for attack.  A plant that subscribes to this tactic constantly produces chemical weapons and stores them inside its cells, commonly in vacuoles, and then releases these phytoanticapans once the fight has commenced.

Each time a battle occurs between an herbivore and a plant, an exchange of information occurs.  Both sides come away from an encounter experienced with their enemy’s weaponry, spurring on a never ending arms race.  Plants will develop a new chemical only to have herbivores evolve a resistance to it, rendering that defense ineffective, and prompting the plant to create another new compound.  This arms race has resulted in around 100,000 different metabolite defenses evolving in plants worldwide.

Gazing at the chewed-up rose, I am reminded by the insect bites that even with a plant’s formidable defenses, mechanical and chemical, they still sometimes lose the battle.  Plants have a particularly difficult time surviving attacks when their aggressors are invasive pests or herbivores that a plant has not evolved a specific toxin for.  With nowhere to run, a plant’s only option is to stand and fight, however futile the attempt.

This unseen arms race and silent warfare is waged all across the pristine and orderly Bradley Rosaceous Collection.  Yet the war is not contained to just roses, or to Jamaica Plain’s Arnold Arboretum: globally, the battle rages.

Old Skulls in a Murky New World

River Dolphins surfacing.  Photo by Cody Sullivan
River Dolphins surfacing. Photo by Cody Sullivan

By Cody Sullivan
BU News Service

While this topic is not in the news, nor is it a particularly large area of scientific study, I have been fascinated by the pink river dolphin’s skull for five years and counting.

My obsession started in the summer of 2009 when I worked in the Amazon as a research assistant on a pink river dolphin study. I loved these dolphins from the moment I first saw one. My skull morphology craze started in 2010 when I learned that pink river dolphins have primitive skulls.

First, I will present a brief introduction about why the shape of a dolphin’s skull matters. Echolocation is a well-known aspect of dolphin life. Dolphins send out bursts of high frequency sound that bounce off objects around them; the timing and directionality of these sound rebounds describe the surrounding environment. Most dolphins tell which direction a sound is coming from because the bones in their skulls are asymmetric. The sounds hit the skull on side sooner than the other, allowing a dolphin to determine which direction it came from. But like I said, only most dolphins have this skull asymmetry, and guess which dolphin doesn’t have an asymmetric skull, Pink River Dolphins!

Pink River Dolphins, or botos, are almost completely blind and live in the muddy Amazon River, and the Amazon’s flooded forests during the wet season – which add another level of navigational complexity with trees, roots, and submerged vegetation. It is plausible to assume that botos navigate through the flooded forests and murky waters by using echolocation as most dolphins do, but their skulls are almost entirely symmetric. By having a uniform skull, botos receive echolocation sound waves at the same time. They shouldn’t be able to tell direction with that skull shape.

The symmetric skull is a very primitive trait compared to marine dolphins and their highfalutin asymmetry. I realize that the two dolphins have been geographically isolated for years upon years, and they have taken divergent evolutionary paths from each other, but why has no parallel evolution occurred? Birds and bats evolved flight separately. It would stand to reason that two closer related species could evolve the same skull shape. River dolphins also have a different ecologic role and environment than marine dolphins, but still, an orienting mechanism is needed. I am so confused by this. I am also confused, confounded and annoyed that there is no answer. What else could help a blind dolphin swim through a flooded forest? I need to know!

There have been two recent studies (this one here, and this one here) on skulls, sound generation and reception, and echolocation in dolphins, both marine and riverine, but neither satisfies me. How does a boto tell left from right?
Someone please answer this question for me!

Keeping Pace with Humans: Urban Evolution

Photo by Comfreak
Photo by Comfreak

By Cody Sullivan
BU News Service

Scrambling inside the walls of a New York City apartment, rummaging through trash, or sometimes caught in a spring-loaded trap is a pest well known to many urban dwellers: mice.  In big cities, mice are nothing more than a problem to exterminate, yet these same urban environments make mice a fascinating example of how humans can impact evolution.

Developed areas, such as New York City, have become ecosystems unto themselves filled with animals such as mice, starlings, and opportunistic weeds sprouting from a sidewalk’s fissures.  This urban ecology – far different than the forest or plains ecosystems where the animals first evolved – alters how inhabitants behave, from finding food to picking a place to live.

Now, it’s clear that urban ecosystems modify more than just the habits of its residents; they change the very DNA of the animals and plants living within their concrete jungles.

“Urbanization acts as an incredibly strong impetus for adaptation,” said Max Lambert, a PhD student at Yale University who is studying landscape ecology and evolution.

Urbanization pushes evolution at a rapid rate compared to the rates of evolution in nonurban areas.  Certain animals and plants evolve faster in cities because of the constant light, noise pollution, and modern landscapes.  These stressors require animals to change their behavior and physiology; and if they don’t adapt, they die.  Researchers use common garden experiments to determine whether a trait is a result of evolutionary adaptation or what scientists call plasticity.

“Adaptive traits are not due to the environment affecting a trait within one generation, as plastic responses are,” said Dan Warner, an evolutionary biology professor at the University of Alabama at Birmingham.  Plastic responses are physical or behavioral changes that are not grounded in an individual’s genetics, but are instead a result of how that individual was raised.

Hawksbeak weed, Crepis sancta, evolved in response to living in cities.  Because paved streets and buildings provide only a patchy growing environment, the weed adapted to release fewer seeds. That way, they don’t waste energy by making seeds that won’t grow anyway.  In 2008 Pierre Cheptou, of the French National Center for Scientific Research, discovered that Hawksbeak evolution happens very quickly, within five to twelve generations.

The killifish, a common species along the East Coast salt marshes, also evolved to survive nearby development.  Marshes near urban areas started becoming polluted in the 1940s, giving the killifish the option to either evolve or die, as many species do when urbanization arrives.  The killifish, thanks to a large population size and genetic diversity, survived by evolving in just a few dozen generations an extreme resistance to the chemical stress placed on them by pollution.

“Some luck is involved,” said Andrew Whitefield, a professor of biology at the University of California Davis, who is one of the researchers investigating killifish adaptation.  “Mutations that allow evolution usually are already present in large populations [such as the killifish].”

Many species, especially vertebrates, do not have large population sizes that beneficial mutations can hide in.  These helpful mutations then can reemerge once needed for big city survival.

A vertebrate that does show quick evolution in cities is the New York City white-footed mouse, as discovered by Professor Jason Munshi-South and his colleagues at Fordham University.  Just as the birds Darwin discovered on different islands in the Galapagos evolved different adaptations, so too have isolated populations of mice in city parks evolved uniquely, but not yet enough to become separate species.

While the existence of mice in cities, weeds along sidewalks, or fish offshore may seem trivial to the well-being of a city, these animals and plants all contribute to a healthy ecosystem.  Fish, for example, help maintain healthy salt marshes, and salt marshes act as a barrier against storms, sea level rise, and erosion.

Hurricane Katrina in 2005 and Hurricane Sandy in 2012 were so devastating partly because the offshore marshes along Louisiana and the northeastern coast were unhealthy or destroyed.  While the killifish may or may not be a linchpin in the marsh ecosystem, the ability for marsh animals to rapidly adapt and survive human development and maintain healthier marshes so that urbanites, human or otherwise, may have naturally built in protection when the next major storm hits.

Child Care Is Expensive for Sea Otter Mothers

A sea otter resting in Elkhorn Slough, CA. Credit: Hanae Armitage


By Cody Sullivan
BU News Service

Raising a child is expensive, but few species have it worse than the sea otter. Sea otter mothers nearly double their daily energy costs by the time their pup is weaned, according to a team of scientists at the University of California Santa Cruz. Sea otters already lose significant amounts of heat to their cold, Californian, watery environment. But adding child care to that energy loss can leave a sea otter mother with an energy bill that is sometimes too high to overcome.

A study done by a team of researchers, led by Nicole Thometz, measured the amount of energy that sea otter pups demand. They found that upon birth of a pup, sea otter mothers face a 17% increase in their daily energy budget, then by the time the pup is completely weaned–six months later–it jumps to a 96% increase. The findings were published in the June 2014 edition of the Journal of Experimental Biology.

In addition to lactating a sea otter mother has to spend extra time foraging to support both herself and a pup. Older pups can actually require more milk from their mother and more energy despite being able to forage for themselves, because pups have higher activity levels.

Otters reproduce every year. If a mother is already in poor condition when giving birth, she will sometimes abandon her pup early on. “There is an initial two week window [after birth] that is most important for a mother in determining if she will keep a pup,” says Thometz. A female may abandon a pup to preserve her own health for the next breeding season, when she might be more successful.

Poor body condition can result in a potentially deadly syndrome named “end lactation syndrome” that affects a mother after weaning her pup – a combination of symptoms including nutritional deficiency, generally poor health, and an increase in susceptibility to disease. Scientists have noted an increase in end lactation syndrome, female strandings, and mortalities over the last twenty years. Experts think the problems might result from more competition for resources as the otter populations have increased in their central range of Big Sur and Monterey Bay. Interestingly, otter populations on the fringe of this range do not face the same rise in end lactation syndrome. They still haven’t recovered from when the fur trade decimated their populations throughout the 1800s.

Thometz plans to quantify the cost of lactation to further understand why sea otter mothers experience such hardship when raising pups. While a reproductive strategy that can result in a mother’s death or pup abandonment may seem overly expensive, it is not uncommon for strange behaviors to evolve. As Thometz says, “Evolution does not always produce the most effective process.”