Mutual Respect
By Aaron Hoover
Zoologist Todd Palmer's chance observation reveals new insights into the unlikely bond between Africa's smallest insects and its largest mammals
Like all the best stories in science, this one begins with an unintended observation.
Todd Palmer was a doctoral student at a wildlife research center in central Kenya. Busy with his work and deadlines, he was paying virtually no attention to the fenced-in tracts of native Acacia trees he walked by each day en route to his research site.
Then one day something odd struck him. The Acacias in the enclosures, created by another scientist for a project on cows and elephants, looked spindly and unhealthy. That was weird, he thought, since for a decade the electric fences had been preventing elephants, giraffes and other mammals from munching the trees. In those conditions, the Acacias, iconic trees of the African savannah also known as whistling thorns, should be thriving, not dying.
It proved a rich reflection. Palmer — in 2003 a doctoral student at the University of California at Davis and visiting scientist at the Mpala Research Foundation’s 48,000-acre wildlife conservation area — spent the next two years unlocking a mystery in plain sight.
His conclusion: The Acacias “hire” ants to protect them from large plant-eating mammals, and without these mammals around, the trees seek to “fire” the ants. Fewer, weaker ants wind up living on the trees, which then become vulnerable to a destructive wood-boring beetle and other previously minor foes. The finding is ironic, because it means the trees, and to a lesser extent the ants, depend on the very mammals they team up to ward off. It’s also unsettling, since it shows the vulnerability of what scientists call “mutualism.” The ant-tree relationship evolved over millions of years; yet, after just 10 years of human intervention, it collapsed. The implication is that continued decline of Africa’s large mammals might have the unexpected result of killing off some of its most enigmatic and charismatic trees.
Now an assistant professor of zoology at the University of Florida, Palmer’s provocative paper on the research appeared on the cover of the journal Science in January. Brian Fisher, anentomologist at the California Academy of Science and a pioneer in the study of mutualism in ants, praised the work, calling the paper “wonderful” in a San Francisco Chronicle story.
Palmer, 40, is tall and dark-haired, with strong features that belie an empathetic personality. For him, the response was gratifying not only because it lauded his results, but also because it implicitly endorsed what he happily calls his “boyish” scientific style. Anyone with a ruler, a notebook and an abiding tolerance for ant bites could have done this “incredibly rudimentary” project, he says. The critical part was observing nature closely enough to notice the mystery it had on full display.
“A lot of us think of science as this rarified, mystical discipline, where these old people in lab coats are turning cranks on this big, expensive machinery,” he says. “There is a lot of science that’s highly technical like that, but we know so little about so many places in the world, that there’s still plenty of room for muddy boots ecology.”
Boyish Science
Palmer grew up in Pendle Hill, a Quaker community outside Philadelphia, the son of a sociologist dad and artist mom. He loved catching frogs and exploring the outdoors, but at Oberlin College in Ohio, he signed on as a pre-med major. Then, the summer after his freshman year, he accompanied a professor deep into the wilds of the Rocky Mountains, where he spent hours following the paths of bees as they pollinated plants. “I realized that, wow, you can get paid to ask questions and pursue your curiosity about the world around you,” he says. “And when the work was done at 4:30, I got to go trout fishing in my cutoffs.”
He wound up double-majoring in biology and pre-med, but a stint on a study abroad program in Kenya left a deep impression. The program inspired a post-graduation internship and teaching job at a school for field studies in Kenya. Palmer returned to the U.S. to earn his master’s in zoology from the University of Wisconsin at Madison — then dropped out of his doctoral program to move to San Francisco and paint houses.
“The funny thing was, every night I would come home from painting houses and I would watch the Discovery Channel,” he says. “I would look for the Africa shows and the savannah shows and just think, ‘How am I going to get back to East Africa?’”
He found a way at UC-Davis, where his advisor, ecologist Truman Young, introduced him to Kenya’s Mpala Research Center. Nearly 50,000 acres in size, the research center and neighboring ranch are spread across Kenya’s central highlands at an elevation of about 6,000 feet. Jointly operated by the Smithsonian Institution, Princeton University and the National Museums of Kenya, the reserve is a kind of ecology-based version of classic artists colonies like Yaddo — with a full-time staff, stone huts for visiting scientists, basic laboratories and a satellite Internet connection.
When Palmer arrived, he was convinced he wanted to do his doctoral work on lions or cheetahs. But it wasn’t long before the Acacias and their complex interrelationship with four different species of territorial ants living in the trees’ boughs captured his imagination. That led to his dissertation, which focused on how three of the ant species, far weaker than the fourth, managed to persist. The field work included tying branches of different trees together to spur the ants to go to war.
“I did the kinds of things that little boys do for fun as a way to get my Ph.D.,” Palmer laughs, only half joking.
Vicious Defenders
Palmer began the work that led to the Science article after noticing the unhealthy looking Acacias while halfway through the field work for his dissertation.
Scientists had long recognized that ants and Acacias throughout the tropics live together in intricate, interdependent relationships. Both the ants and the trees benefit from the close cohabitation.
The Acacias have ping pong ball-sized swollen thorns that serve as the kind of nests only biting ants could love. The trees also produce nourishing nectar for the ants at the base of their leaves. In return, the ants swarm in defense of anything that molests the tree — especially hungry elephants, giraffes, antelopes and other herbivores.
Palmer says the as many as 100,000 ants that occupy each tree are nothing if not aggressive, converging in a matter of seconds on any interlopers who jostle a branch. Three of the ant species merely bite, which can be annoying, but a fourth inflicts a painful sting. The result for giraffes and other browsing mammals is a full frontal assault on the head.
“The ants focus on the mucus membranes, so they will run to your eyes, the inside of your nose and your lips to bite you,” he says.
The sickly trees within the fenced enclosures seemed to have fewer thorn nests, so Palmer counted the number of nests on hundreds of fenced and unfenced trees. He also compared the number of “nectaries” where the ants sip the trees’ nectar. The fenced trees had far fewer of both. Not only that, but when Palmer jostled the fenced trees, the seemingly stupefied ants were far less aggressive than their counterparts.
Without mammals around to eat the trees, sheltering fewer, more peaceable ants would not present a cost to the trees, Palmer says. To the contrary, the trees would seem to be better off, because they would not use their precious fluids to support the ants.
But the research revealed that the fewer colonies of weakened ants become less able to defend their territory from a fourth species of ant that, unlike the three others, does not have a mutually beneficial relationship with Acacias.
Instead, this fourth ant species feeds on the ground away from the tree and does not protect it. Even worse for the Acacias, it encourages a destructive, wood-boring beetle whose harmful cavities within the tree then serve as the ants’ home. The result appears to be that the trees untouched by browsing mammals are infested with more of the beetles, making them sickly.
The bottom line: The fenced trees were twice as likely to die as the unfenced ones, and they grew 65 percent more slowly, that despite the fact they were unmolested by their ostensibly worst nemeses, the elephants and other browsers.
Buddha and Darwin
Palmer, who sustains his energy with hourly infusions of exotic green teas, says his results are not only surprising, but also worrying.
Sharing a cup of Lotus Heart Dragon Well with a visitor not long ago in his office in Carr Hall, he noted that the implications of his work go far beyond ants and trees.
From coral reefs to corn fields, many of the world’s most productive ecosystems are underpinned by mutualisms — in the case of coral reefs, an algae and coral share a symbiotic relationship, while many crops rely on insect pollinators. He says his work highlights how such mutualisms may sync within the larger ecosystem in ways that are not immediately obvious. “The broader message,” he says, “is that we really need to better understand how these cooperative interactions are maintained and what factors can precipitate their breakdown, so that we can do something to prevent at least some from falling apart.”
Palmer has no lack of respect for modern science and its immense technological capabilities. In fact, he’s powering up sophisticated computer modeling techniques to project what might happen to the fenced Acacias during the trees’ 200-year lifespan.
But as a committed Buddhist, he says, he tries to see the world as it is, rather than as he might wish it were. That same attitude is useful in ecology and science in general, he says, with many of the best discoveries grounded in a simple observation of something surprisingly plain.
“When you think about it,” Palmer says, “Darwin’s theory of evolution is the most obvious thing on the planet. When Darwin published, there were scientists around the world hitting themselves on the head saying, ‘Why didn’t I see that?’”