Thursday, December 07, 2023

A new 66 million-year history of carbon dioxide offers little comfort for today

A massive study sharpens the outlook on greenhouse gases and climate

COLUMBIA CLIMATE SCHOOL

WANING ICE — IMAGE:
THE EDGE OF THE GREENLAND ICE SHEET, WHERE RECENT MELTING HAS LEFT BARE GROUND.
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CREDIT: KEVIN KRAJICK/EARTH INSTITUTE

A massive new review of ancient atmospheric carbon-dioxide levels and corresponding temperatures lays out a daunting picture of where the Earth’s climate may be headed. The study covers geologic records spanning the past 66 million years, putting present-day concentrations into context with deep time. Among other things, it indicates that the last time atmospheric carbon dioxide consistently reached today’s human-driven levels was 14 million years ago—much longer ago than some existing assessments indicate. It asserts that long-term climate is highly sensitive to greenhouse gas, with cascading effects that may evolve over many millennia.

The study was assembled over seven years by a consortium of more than 80 researchers from 16 nations. It appears today in the journal Science.

“We have long known that adding CO2 to our atmosphere raises the temperature,” said Bärbel Hönisch, a geochemist at Columbia University’s Lamont-Doherty Earth Observatory, who coordinated the consortium. “This study gives us a much more robust idea of how sensitive the climate is over long time scales.”

Mainstream estimates indicate that on scales of decades to centuries, every doubling of atmospheric CO2 will drive average global temperatures 1.5 to 4.5 degrees Celsius (2.7 to 8.1 Fahrenheit) higher. However, at least one recent widely read study argues that the current consensus underestimates planetary sensitivity, putting it at 3.6 to 6 C degrees of warming per doubling. In any case, given current trends, all estimates put the planet perilously close to or beyond the 2 degrees warming that could be reached this century, and which many scientists agree we must avoid if at all possible.

In the late 1700s, the air contained about 280 parts per million (ppm) of CO2. We are now up to 420 ppm, an increase of about 50%; by the end of the century, we could reach 600 ppm or more. As a result, we are already somewhere along the uncertain warming curve, with a rise of about 1.2 degrees C (2.2 degrees F) since the late 19th century.

Whatever temperatures eventually manifest, most estimates of future warming draw information from studies of how temperatures tracked with CO2 levels in the past. For this, scientists analyze materials including air bubbles trapped in ice cores, the chemistry of ancient soils and ocean sediments, and the anatomy of fossil plant leaves.

The consortium’s members did not collect new data; rather, they came together to sort through published studies to assess their reliability, based on evolving knowledge. They excluded some that that they found outdated or incomplete in the light of new findings, and recalibrated others to account for the latest analytical techniques. Then they calculated a new 66-million-year curve of CO2 versus temperatures based on all the evidence so far, coming to a consensus on what they call “earth system sensitivity.” By this measure, they say, a doubling of CO2 is predicted to warm the planet a whopping 5 to 8 degrees C.

The giant caveat: Earth system sensitivity describes climate changes over hundreds of thousands of years, not the decades and centuries that are immediately relevant to humans. The authors say that over long periods, increases in temperature may emerge from intertwined Earth processes that go beyond the immediate greenhouse effect created by CO2 in the air. These include melting of polar ice sheets, which would reduce the Earth’s ability to reflect solar energy; changes in terrestrial plant cover; and changes in clouds and atmospheric aerosols that could either heighten or lower temperatures.

“If you want us to tell you what the temperature will be in the year 2100, this does not tell you that. But it does have a bearing on present climate policy,” said coauthor Dana Royer, a paleoclimatologist at Wesleyan University. “It strengthens what we already thought we knew. It also tells us that there are sluggish, cascading effects that will last for thousands of years.”

Hönisch said the study will be useful for climate modelers trying to predict what will happen in coming decades, because they will be able to feed the newly robust observations into their studies, and disentangle processes that operate on short versus long time scales. She noted that all the project’s data are available in an open database, and will be updated on a rolling basis.

The new study, covering the so-called Cenozoic era, does not radically revise the generally accepted relationship between CO2 and temperature, but it does strengthen the understanding of certain time periods, and refines measurements of others.

The most distant period, from about 66 million to 56 million years ago, has been something of an enigma, because the Earth was largely ice free, yet some studies had suggested CO2 concentrations were relatively low. This cast some doubt on the relationship between CO2 and temperature. However once the consortium excluded estimates they deemed the least dependable, they determined that CO2 was actually quite high—around 600 to 700 parts per million, comparable to what could be reached by the end of this century.

The researchers confirmed the long-held belief that the hottest period was about 50 million years ago, when CO2 spiked to as much as 1,600 ppm, and temperatures were as much as 12 degrees C higher than today. But by around 34 million years ago, CO2 had dropped enough that the present-day Antarctic ice sheet began developing. With some ups and downs, this was followed by a further long-term CO2 decline, during which the ancestors of many modern-day plants and animals evolved. This suggests, the paper’s authors say, that variations in CO2 affect not only climate, but ecosystems.

The new assessment says that about 16 million years ago was the last time CO2 was consistently higher than now, at about 480 ppm; and by 14 million years ago it had sunk to today’s human-induced level of 420 ppm. The decline continued, and by about 2.5 million years ago, CO2 reached about 270 or 280 ppm, kicking off a series of ice ages. It was at or below that when modern humans came into being about 400,000 years ago, and persisted there until we started messing with the atmosphere on a grand scale about 250 years ago.

“Regardless of exactly how many degrees the temperature changes, it’s clear we have already brought the planet into a range of conditions never seen by our species,” said study coauthor Gabriel Bowen, a professor at the University of Utah. “It should make us stop and question what is the right path forward.”

The consortium has now evolved into a larger project that aims to chart how CO2 and climate have evolved over the entire Phanerozoic eon, from 540 million years ago to present.

Temperatures and atmospheric concentrations of carbon dioxide over the past 66 million years. Bottom numbers indicate millions of years in the past; right-hand numbers, carbon dioxide in parts per million. Hotter colors indicate distinct periods of higher temperatures; deeper blues, lower ones. The solid zigzagging line charts contemporaneous carbon dioxide levels; shaded area around it reflects uncertainty in the curve.

CREDIT

Adapted from CenCO2PIP, Science 2023

JOURNAL

Science

DOI

10.1126/science.adi5177

METHOD OF RESEARCH

Meta-analysis

SUBJECT OF RESEARCH

Not applicable

ARTICLE TITLE

Towards a Cenozoic History of Atmospheric CO2

ARTICLE PUBLICATION DATE

8-Dec-2023

Geoscientists map changes in atmospheric CO2 over past 66 million years

Carbon dioxide has not been as high as today's concentrations in 14 million years thanks to fossil fuel emissions now warming the planet.

Peer-Reviewed Publication

UNIVERSITY OF UTAH

Atmospheric CO2 — IMAGE:
THIS GRAPHIC INDICATES EARTH’S ATMOSPHERIC CONCENTRATIONS OF CO2, EXPRESSED IN PARTS PER MILLION (PPM), THROUGHOUT THE CENOZOIC ERA FROM PRE-INDUSTRIAL TIMES BACK 65 MILLION YEARS. THESE ARE ESTIMATES BASED ON PROXIES ENCODED IN THE GEOLOGICAL RECORD. THE COLOR-CODED BARS REPRESENT GLOBAL TEMPERATURE RECONSTRUCTED FROM INDEPENDENT PROXY DATA. THE DASHED LINE REPRESENTS WHERE CO2 CONCENTRATIONS STAND TODAY AT 420 PPM. CREDIT: GABE BOWEN, UNIVERSITY OF UTAH.

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CREDIT: GABE BOWEN, UNIVERSITY OF UTAH

Today atmospheric carbon dioxide is at its highest level in at least several million years thanks to widespread combustion of fossil fuels by humans over the past couple centuries.

But where does 419 parts per million (ppm)—the current concentration of the greenhouse gas in the atmosphere—fit in Earth’s history?

That’s a question an international community of scientists, featuring key contributions by University of Utah geologists, is sorting out by examining a plethora of markers in the geologic record that offer clues about the contents of ancient atmospheres. Their initial study was published this week in the journal Science, reconstructing CO2 concentrations going back through the Cenozoic, the era that began with the demise dinosaurs and rise of mammals 66 million years ago.

Glaciers contain air bubbles, providing scientists direct evidence of CO2 levels going back 800,000 years, according to U geology professor Gabe Bo wen, one of the study’s corresponding authors. But this record does not extend very deep into the geological past.

“Once you lose the ice cores, you lose direct evidence. You no longer have samples of atmospheric gas that you can analyze,” Bowen said. “So you have to rely on indirect evidence, what we call proxies. And those proxies are tough to work with because they are indirect.”

"Proxies" in the geologic record

These proxies include isotopes in minerals, the morphology of fossilized leaves and other lines of geological evidence that reflect atmospheric chemistry. One of the proxies stems from the foundational discoveries of U geologist Thure Cerling, himself a co-author on the new study, whose past research determined carbon isotopes in ancient soils are indicative of past CO2 levels.

But the strength of these proxies vary and most cover narrow slices of the past. The research team, called the Cenozoic CO2 Proxy Integration Project, or CenCO2PIP, and organized by Columbia University climate scientist Bärbel Hönisch, set out to evaluate, categorize and integrate available proxies to create a high-fidelity record of atmospheric CO2.

“This represents some of the most inclusive and statistically refined approaches to interpreting CO2 over the last 66 million years,” said co-author Dustin Harper, a U postdoctoral researcher in Bowen’s lab. “Some of the new takeaways are we're able to combine multiple proxies from different archives of sediment, whether that's in the ocean or on land, and that really hasn't been done at this scale.”

The new research is a community effort involving some 90 scientists from 16 countries. Funded by dozens of grants from multiple agencies, the group hopes to eventually reconstruct the CO2 record back 540 million years to the dawn of complex life.

At the start of the Industrial Revolution--when humans began burning to coal, then oil and gas to fuel their economies--atmospheric CO2 was around 280 ppm. The heat-trapping gas is released into the air when these fossil fuels burn.

Looking forward, concentrations are expected to climb up to 600 to 1,000 ppm by the year 2100, depending on the rate of future emissions. It is not clear exactly how these future levels will influence the climate.

But having a reliable map of past CO2 levels could help scientists more accurately predict what future climates may look like, according to U biology professor William Anderegg, director the U’s Wilkes Center for Climate & Policy.

"This is an incredibly important synthesis and has implications for future climate change as well, particularly the key processes and components of the Earth system that we need to understand to project the speed and magnitude of climate change,” Anderegg said.

Today's 419 ppm is the highest CO2 in 14 million years

At times in the past when Earth was a far warmer place, levels of CO2 were much higher than now. Still, the 419 ppm recorded today represents a steep and perhaps dangerous spike and is unprecedented in recent geologic history.

“By 8 million years before present, there's maybe a 5% chance that CO2 levels were higher than today,” Bowen said, “but really we have to go back 14 million years before we see levels we think were like today.”

In other words, human activity has significantly altered the atmosphere within the span of a few generations. As a result, climate systems around the globe are showing alarming signs of disruption, such as powerful storms, prolonged drought, deadly heat waves and ocean acidification.

A solid understanding of atmospheric CO2 variation through geological time is also essential to deciphering and learning from various features of Earth’s history. Changes in atmospheric CO2 and climate likely contributed to mass extinctions, as well as evolutionary innovations.

During the Cenozoic, for example, long-term declines in CO2 and associated climate cooling may have driven changes to plant physiology, species competition and dominance, which in turn impacted mammalian evolution.

“A more refined understanding of past trends in CO2 is therefore central to understanding how modern species and ecosystems arose and may fare in the future,” the study states.

JOURNAL

Science

DOI

10.1126/science.adi5177

METHOD OF RESEARCH

Systematic review

SUBJECT OF RESEARCH

Not applicable

ARTICLE TITLE

Toward a Cenozoic history of atmospheric CO2

ARTICLE PUBLICATION DATE

8-Dec-2023

Ancient stars made extraordinarily heavy elements

Peer-Reviewed Publication

NORTH CAROLINA STATE UNIVERSITY

How heavy can an element be? An international team of researchers has found that ancient stars were capable of producing elements with atomic masses greater than 260, heavier than any element on the periodic table found naturally on Earth. The finding deepens our understanding of element formation in stars.

We are, literally, made of star stuff. Stars are element factories, where elements constantly fuse or break apart to create other lighter or heavier elements. When we refer to light or heavy elements, we’re talking about their atomic mass. Broadly speaking, atomic mass is based on the number of protons and neutrons in the nucleus of one atom of that element.

The heaviest elements are only known to be created in neutron stars via the rapid neutron capture process, or r-process. Picture a single atomic nucleus floating in a soup of neutrons. Suddenly, a bunch of those neutrons get stuck to the nucleus in a very short time period – usually in less than one second – then undergo some internal neutron-to-proton changes, and voila! A heavy element, such as gold, platinum or uranium, forms.

The heaviest elements are unstable or radioactive, meaning they decay over time. One way that they do this is by splitting, a process called fission.

“The r-process is necessary if you want to make elements that are heavier than, say, lead and bismuth,” says Ian Roederer, associate professor of physics at North Carolina State University and lead author of the research. Roederer was previously at the University of Michigan.

“You have to add many neutrons very quickly, but the catch is that you need a lot of energy and a lot of neutrons to do so,” Roederer says. “And the best place to find both are at the birth or death of a neutron star, or when neutron stars collide and produce the raw ingredients for the process.

“We have a general idea of how the r-process works, but the conditions of the process are quite extreme,” Roederer says. “We don’t have a good sense of how many different kinds of sites in the universe can generate the r-process, we don’t know how the r-process ends, and we can’t answer questions like, how many neutrons can you add? Or, how heavy can an element be? So we decided to look at elements that could be made by fission in some well-studied old stars to see if we could start to answer some of these questions.”

The team took a fresh look at the amounts of heavy elements in 42 well-studied stars in the Milky Way. The stars were known to have heavy elements formed by the r-process in earlier generations of stars. By taking a broader view of the amounts of each heavy element found in these stars collectively, rather than individually as is more common, they identified previously unrecognized patterns.

Those patterns signaled that some elements listed near the middle of the periodic table – such as silver and rhodium – were likely the remnants of heavy element fission. The team was able to determine that the r-process can produce atoms with an atomic mass of at least 260 before they fission.

“That 260 is interesting because we haven’t previously detected anything that heavy in space or naturally on Earth, even in nuclear weapon tests,” Roederer says. “But seeing them in space gives us guidance for how to think about models and fission – and could give us insight into how the rich diversity of elements came to be.”

The work appears in Science and was supported in part by the National Science Foundation and the National Aeronautics and Space Administration.

-peake-

Note to editors: An abstract follows.

“Element abundance patterns in stars indicate fission of nuclei heavier than uranium”

DOI: 10.1126/science.adf1341

Authors: Ian U. Roederer, North Carolina State University; Nicole Vassh, TRIUMF (Tri-University Meson Facility) Vancouver, Canada; Erika M. Holmbeck, Carnegie Observatories, California; Matthew R. Mumpower, Los Alamos National Laboratory; Rebecca Surman, University of Notre Dame; John J. Cowan, University of Oklahoma; Timothy C. Beers, University of Notre Dame; Rana Ezzeddine, University of Florida; Anna Frebel, Massachusetts Institute of Technology; Terese T. Hansen, Stockholm University, Sweden; Vinicius M. Placco, NSF’s National Optical-Infrared Astronomy Research Laboratory; Charli M. Sakari, San Francisco State University
Published: Dec. 7, 2023 in Science

Abstract:
The heaviest chemical elements are naturally produced by the rapid neutron-capture process (r-process) during neutron star mergers or supernovae. The r-process production of elements heavier than uranium (transuranic nuclei) is poorly understood and inaccessible to experiments so must be extrapolated by using nucleosynthesis models. We examined element abundances in a sample of stars that are enhanced in r-process elements. The abundances of elements ruthenium, rhodium, palladium, and silver (atomic numbers Z = 44 to 47; mass numbers A = 99 to 110) correlate with those of heavier elements (63 ≤ Z ≤ 78, A > 150). There is no correlation for neighboring elements (34 ≤ Z ≤ 42 and 48 ≤ Z ≤ 62). We interpret this as evidence that fission fragments of transuranic nuclei contribute to the abundances. Our results indicate that neutron-rich nuclei with mass numbers >260 are produced in r-process events.

JOURNAL

Science

DOI

10.1126/science.adf1341

METHOD OF RESEARCH

Data/statistical analysis

SUBJECT OF RESEARCH

Not applicable

ARTICLE TITLE

Element abundance patterns in stars indicate fission of nuclei heavier than uranium

ARTICLE PUBLICATION DATE

8-Dec-2023

The evolutionary paradox behind the unusual mating strategy of the ruff

Peer-Reviewed Publication

SMBE JOURNALS (MOLECULAR BIOLOGY AND EVOLUTION AND GENOME BIOLOGY AND EVOLUTION)

Images of male ruffs — IMAGE:
MALE PHENOTYPES IN THE RUFF. (LEFT) INDEPENDENT RUFF MALES INTERACTING AT A LEK. (RIGHT) A SATELLITE RUFF MALE WITH PALE ORNAMENTAL FEATHERS AT A LEK. PHOTOS COURTESY OF TOM SCHANDY.
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CREDIT: TOM SCHANDY

In the colorful world of avian courtship, the ruff (Calidris pugnax) is in a league of its own. Breeding in marshes and wet meadows across Eurasia, the males of this medium-sized sandpiper species are well-known for their distinctive mating strategies, which range from flamboyant territorial displays to cunning mimicry. These behaviors, along with striking differences in plumage, are determined by a single genetic region known as a supergene. Supergenes are clusters of genes that control complex traits. They are often associated with a chromosomal inversion, in which the gene order is reversed along the chromosome compared with the wild-type allele; this serves to suppress recombination, allowing a set of traits to be co-inherited. While there are potential benefits to preserving favorable combinations of genetic variants, this lack of recombination can also lead to the accumulation of deleterious mutations within the supergene over time. However, a new study published in Molecular Biology and Evolution, titled "Low mutation load in a supergene underpinning alternative male mating strategies in ruff (Calidris pugnax)," has revealed a remarkable evolutionary paradox, as the supergene that underlies male mating strategy in the ruff exhibits a surprisingly low mutation load. The study's findings therefore challenge our understanding of the evolution and persistence of supergenes in nature.

Ruff males have long captured the attention of scientists and birdwatchers alike due to their showy mating displays and outlandish plumage, resembling the extravagant collars worn in the sixteenth century that inspired the species’ name. There are actually three distinct types of male ruffs, known as Independents, Satellites, and Faeders, which differ in behavior, plumage, and body size. “Independents have spectacular ornamental feathers, and these males defend territory on the lek [mating grounds],” says Leif Andersson, the lead author of the new study. “Satellites have light-colored ornamental feathers and do not defend territory on the lek but allow Independent males to dominate them. This behavior helps Independent males attract females that are ready to mate; the advantage for the Satellites is that they get access to the mating ground without the need to spend energy defending territory on the lek. Faeders are non-territorial, female-mimics with no ornamental feathers. They sneak around on the lek and try to mate with females.”

Interestingly, the Satellite and Faeder phenotypes are determined by the presence of an inversion that harbors about 100 genes. “The Faeder haplotype is an intact inversion while the Satellite haplotype originated after genetic recombination between the Independent and Faeder haplotypes,” continues Andersson. In addition to carrying one of the inverted haplotypes, all Satellite and Faeder males carry one Independent haplotype, as the presence of two copies of the inversion (in the recessive or homozygous state) is lethal.

The ruff supergene has long puzzled Andersson and his research team. “When we first discovered the ruff supergene,” says Andersson, “we were amazed that the sequence divergence between the inversion alleles and the wild-type allele was as high as 1.4%. This is higher than the sequence divergence between humans and chimpanzees and suggested a split about 4 million years ago based on the estimated substitution rate for birds. The inversion alleles are recessive lethal, most likely because the inversion breaks an essential gene. Thus, the question that emerged is how can a recessive lethal be maintained for 4 million years?”

To investigate this mystery, the researchers employed cutting-edge genomic sequencing techniques to create highly contiguous genome assemblies for both the Independent and Satellite haplotypes. They used these assemblies alongside previously published whole-genome data to assess the mutational load of the inverted supergene. As noted by Andersson, “Population genetic theory predicts that supergenes should accumulate genetic load [e.g., deleterious mutations] due to relaxed purifying selection, in particular if the supergene is a recessive lethal like the ruff supergene is.”

Surprisingly, however, the researchers found no substantial accumulation of repetitive elements and only a modest mutation load on the Satellite and Faeder haplotypes. This unexpected finding forced the study’s authors to reassess their assumptions about the ruff supergene. “I really had to reevaluate the way that I thought about supergenes as we continued to find evidence of recent purifying selection where there should not have been any,” notes Andersson.

The authors propose two potential scenarios to resolve this paradox. First, the inversion may have only recently acquired its recessive lethality. If an older version of the supergene was more common and not a recessive lethal, recombination could occur in ruffs carrying two copies of the inversion, allowing deleterious mutations to be removed through purifying selection. An alternative hypothesis, which is favored by the authors, is that the supergene was introduced by introgression from another species or subspecies. In this scenario, hybridization between a ruff and another species led to introduction of the supergene into the ruff genome, and its persistence was then favored by selection because it kept together alleles contributing to a successful male mating strategy. While the study authors were unable to identify the lineage that may have contributed the inversion, they note that given the estimated timeline, the donor species may now be extinct.

This study highlights the complex forces governing male mating strategies in the ruff and supergenes in general. “Inversions are easy to find with modern genomic tools but are difficult to understand,” notes Andersson. “However, it should be very interesting to analyze gene expression in multiple tissues from the different morphs and try to understand which of the genes in the inversion contribute to the spectacular differences between morphs.” While their genomic data have so far unearthed two potential candidate genes—one involved in testosterone metabolism and one that may influence ornamental feather coloration—additional transcriptomic data are needed to answer this question. Unfortunately, such data may be difficult to obtain: “The major challenge with this suggested gene expression study,” says Andersson, “is that this is a wild species, and it is not easy to put together the large collection of samples that will be needed for a comprehensive analysis.” Despite this hurdle, further research into this remarkable model system promises to provide a deeper understanding of the origin, persistence, and evolutionary trajectories of supergenes.

JOURNAL

Molecular Biology and Evolution

DOI

10.1093/molbev/msad224

METHOD OF RESEARCH

Observational study

SUBJECT OF RESEARCH

Animals

ARTICLE TITLE

Low Mutation Load in a Supergene Underpinning Alternative Male Mating Strategies in Ruff (Calidris pugnax)

ARTICLE PUBLICATION DATE

7-Dec-2023

MUTUAL AID

Wild birds lead people to honey — and learn from them

The greater honeyguide can recognize distinct vocal signals to help people in Africa locate bee colonies

Peer-Reviewed Publication

UNIVERSITY OF CALIFORNIA - LOS ANGELES

Male greater honeyguide — IMAGE:
A MALE GREATER HONEYGUIDE ON A BRANCH SEARCHES FOR WAX.
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CREDIT: BRIAN WOOD

Key takeaways

People in parts of Africa communicate with a wild bird, the greater honeyguide, to locate bee colonies and harvest their honey and beeswax.
A study by UCLA anthropologist Brian Wood and other authors show how this partnership is maintained and varies across cultures.
They demonstrate the bird’s ability to learn distinct vocal signals traditionally used by different honey-hunting communities.

In parts of Africa, people communicate with a wild bird — the greater honeyguide — in order to locate bee colonies and harvest their stores of honey and beeswax.

It’s a rare example of cooperation between humans and wild animals, and a potential instance of cultural coevolution.

UCLA anthropologist Brian Wood and University of Cape Town ornithologist Claire Spottiswoode were lead authors on a study showing how this valuable partnership is maintained and varies across cultures. Their article, “Culturally determined interspecies communication between humans and honeyguides,” was published in Science.

“Our study demonstrates the bird’s ability to learn distinct vocal signals that are traditionally used by different honey-hunting communities, expanding possibilities for mutually beneficial cooperation with people,” Wood said.

“Honeyguides seem to know the landscape intimately, gathering knowledge about the location of bee nests, which they then share with people, Spottiswoode said. “People are eager for the bird’s help.”

The honeyguides also benefit from locating the colonies: They eat the leftover honeycomb.

The study’s findings build on research published in 2014 that showed the immense benefits of this relationship for the Hadza people. Honeyguides increased Hadza hunter-gatherers’ rate of finding bee nests by 560% and led them to significantly higher-yielding nests than those found without honeyguides. This prior research also found that 8%–10% of the Hadza's yearly diet was acquired with the help of honeyguides.

Spottiswoode and Wood’s study was done in collaboration with the Hadza in Tanzania, with whom Wood has been conducting research since 2004, and the Yao community of northern Mozambique.

Their prior work in both communities documented differences in how each culture attracts honeyguides. Among the Hadza, a honey-hunter announces a desire to partner with the bird by whistling. (Listen to the Hadza vocal signal.)

In Mozambique, Yao honey-hunters do so with a trilled “Brr! ...” followed by a guttural “ ... hmm!” (Listen to the Yao vocal signal.)

Using mathematical models and audio playback experiments, the team studied these signals, their utility to people and their impacts on birds.

They experimentally exposed honeyguides in Tanzania and Mozambique to the same set of prerecorded sounds. This enabled the researchers to test whether honeyguides had learned to recognize and prefer the specialized signals that local honey-hunters used — or were innately attracted to all such signals.

The honeyguides in Tanzania were over three times more likely to cooperate when hearing the calls of local Hadza people than the calls of ‘foreign’ Yao. The honeyguides in Mozambique were almost twice as likely to cooperate when hearing the local Yao call, compared to the ‘foreign’ Hadza whistles.

The study proposes that differences in honeyguide-attracting signals are not arbitrary, but make practical sense. While honey-hunting, both the Hadza and Yao encounter mammals, but only the Hadza hunt them, using bows and arrows. The Hadza’s hunting might explain the less conspicuous whistles they use. Filmed interviews show Hadza hunters explaining that they can evade being detected by their prey because their whistles “sound like birds.”

“Not just among the Hadza, but in hunting cultures around the world, people use whistles as a form of encrypted communication — to share information while avoiding detection by prey,” Wood said.

Conversely, the guttural trill-grunt signal the Yao use to communicate with the honeyguide can help scare off animals they find dangerous.

Although both humans and birds can learn new signals, the authors propose that the mutually beneficial relationship between birds and people spawns local traditions of human-bird communication that remain stable over time.

“The benefits of the honey-hunter-honeyguide relationship should produce long-lasting, ‘sticky’ traditions,” Wood said.

JOURNAL

Science

ARTICLE TITLE

“Culturally determined interspecies communication between humans and honeyguides”

Honeyguide birds learn distinct signals made by honey hunters from different cultures

Peer-Reviewed Publication

AMERICAN ASSOCIATION FOR THE ADVANCEMENT OF SCIENCE (AAAS)

African honeyguide birds understand and respond to the culturally distinct signals made by local human honey hunters, suggesting cultural coevolution between species, according to a new study. Although the animal kingdom is full of interspecific mutualism, systems in which humans successfully cooperate with wild animals are rare. One such relationship involves the greater honeyguide (Indicator indicator), a small African bird known to lead humans to wild bees’ nests. Humans open the nests to collect honey, and the honeyguides eat the exposed beeswax. Human honey hunters in different parts of Africa often use specialized and culturally distinct calls to signal they are looking for a honeyguide partner and to maintain cooperation while following a guiding bird. For example, honey hunters from the Yao cultural group in northern Mozambique use a loud trill followed by a grunt (“brrr-hm”). In contrast, honey hunters from the Hadza cultural group of northern Tanzania use a melodic whistle. These successful calls have been maintained in these groups for generations. In a series of field experiments across these areas, Claire Spottiswoode and Brian Wood investigated whether honeyguides are more likely to respond to signals of their local human culture than to those of another culture or to arbitrary human sounds. Spottiswoode and Wood discovered that honeyguides in the Yao area were more than three times more likely to initiate a guiding response to the Yao’s distinct call than the Hadza’s whistle. Conversely, honeyguides in the Hadza area were more than three times as likely to respond to the Hadza’s whistle than the Yao’s brrr-hm. According to the authors, the geographic variation and coordination between signal and response observed in this behavioral system suggests cultural coevolution between honeyguides and humans has occurred. In a related Perspective, William Searcy and Stephen Nowicki discuss the study and its findings in greater detail.

For reporters interested in trends, this study builds on previous work published in a July 2016 Report in Science, which demonstrated the reciprocal signaling in honeyguides and honey hunters in Mozambique.

JOURNAL

Science

DOI

10.1126/science.adh4129

ARTICLE TITLE

Culturally determined interspecies communication between humans and honeyguides

ARTICLE PUBLICATION DATE

8-Dec-2023

Grunt or whistle: successful honey-hunters know how to communicate with wild honey-seeking birds

Peer-Reviewed Publication

UNIVERSITY OF CAMBRIDGE

Hadza honey-hunters communicating with wild birds — AUDIO:
HADZA HONEY-HUNTERS IN TANZANIA COMMUNICATE WITH HONEYGUIDES USING A MELODIC WHISTLE
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CREDIT: CLAIRE SPOTTISWOODE

In many parts of Africa, humans cooperate with a species of wax-eating bird called the greater honeyguide, Indicator indicator, which leads them to wild bees’ nests with a chattering call. By using specialised sounds to communicate with each other, both species can significantly increase their chances of accessing calorie-dense honey and beeswax.

Human honey-hunters in different parts of Africa use different calls to communicate with honeyguides. In a new study, researchers have discovered that honeyguide birds in Tanzania and Mozambique discriminate among honey-hunters’ calls, responding much more readily to local than to foreign calls.

“We found that honeyguides prefer the calls given by their local human partners, compared to foreign calls and arbitrary human sounds. This benefits both species, since it helps honey-hunters attract a honeyguide to show them hard-to-find bees’ nests, and helps honeyguides to choose a good partner to help them to get at the wax,” said Dr Claire Spottiswoode, an evolutionary biologist at the University of Cambridge’s Department of Zoology and the University of Cape Town, and joint lead author of the paper.

Hadza honey-hunters in Tanzania communicate with honeyguides using a melodic whistle, whereas Yao honey-hunters in Mozambique use a trill followed by a grunt.

The experiments showed that honeyguides in the Kidero Hills, Tanzania are over three times more likely to cooperate with people giving the local Hadza whistle, than people giving the ‘foreign’ Yao trill and grunt. And the honeyguides in the Niassa Special Reserve, Mozambique are almost twice as likely to cooperate in response to the local Yao trill and grunt, than the ‘foreign’ Hadza whistle.

The phenomenon seems to be self-reinforcing: honeyguides learn to recognise that a specific call indicates a good honey-hunting partner, and humans are more successful in attracting the birds if they use this call.

People who use a different call are less likely to attract a bird to guide them to the honey – so it’s in their interests to stick to the sounds used locally.

“Once these local cultural traditions are established, it pays for everyone – birds and humans – to conform to them, even if the sounds themselves are arbitrary,” said joint lead author Dr Brian Wood, an anthropologist at the University of California, Los Angeles.

The researchers compare this to different human languages, in which the sounds of words are arbitrary, but everyone has agreed on their meaning.

Spottiswoode added: “Just as humans across the world communicate using a range of different local languages, people across Africa communicate with honeyguide birds using a range of different local sounds.”

Like language, these culturally determined calls convey an underlying meaning – signalling a desire to partner with the bird to find honey.

The study is published today in the journal Science.

It is likely that cultural factors relating to the wider hunting practices of different groups have helped to shape the precise design of their honey-hunting calls.

For example, the melodic whistle made by Hadza honey-hunters in Tanzania to attract honeyguide birds sounds like a bird call. This reduces the risk of frightening away the prey animals they’re trying to hunt at the same time.

In contrast, the loud trill followed by a grunt made by Yao honey-hunters in Mozambique, sounds distinctively human. This may be a good way for them to frighten away dangerous large animals like elephants and buffalo.

The findings build on work published in 2016, which found that honeyguide birds in Mozambique respond to the calls of human honey hunters.

The researchers work closely with the Yao and Hadza honey-hunting communities in Africa, whose guidance they have relied on for over a decade.

“It’s such a privilege to witness cooperation between people and honeyguides – these are birds who specifically come to seek us out. The calls really sound like a conversation between the bird and the honey-hunters, as they move together towards a bees’ nest,” said Spottiswoode.

Humans are useful collaborators to honeyguides because of our ability to subdue stinging bees with smoke and chop open the nest, providing wax for the honeyguide and honey for ourselves.

This relationship is a rare example of cooperation between humans and wild animals. Wild honey is a high-energy food that can provide up to 20% of the calorie intake for honey-hunters – and the wax they share or discard is a valuable food for the honeyguide.

“What’s remarkable about the honeyguide-human relationship is that it involves free-living wild animals whose interactions with humans have evolved through natural selection, possibly over the course of hundreds of thousands of years,” said Spottiswoode.

She added: “This ancient, evolved behaviour has then been refined to local cultural traditions – the different human call sounds – through learning.”

The research is a collaboration between researchers at the University of Cambridge, the University of Cape Town, and the University of California Los Angeles, and many honey hunters who inspired and supported the experiments.

Yao honey-hunters in Tanzania use a trill followed by a grunt to communicate with honeyguides

Male honeyguide in Mozambique

In many parts of Africa, humans cooperate with a species of wax-eating bird called the greater honeyguide, *Indicator indicator,* which leads them to wild bees’ nests with a chattering call.

Yao honey-hunters using fire and tools to harvest a bees' nest in Mozambique

Humans are useful collaborators to honeyguides because of our ability to subdue stinging bees with smoke and chop open the nest, providing wax for the honeyguide and honey for ourselves.