Female chimps with powerful moms are less likely to leave home

Gaia the chimpanzee grooms with her mom Gremlin at Gombe National Park. A new study finds that female chimps with high-ranking moms are less likely to leave home, instead, they reproduce in the group where they grew up. Photo by Emily Wroblewski

DURHAM, N.C. — In chimpanzee society, males spend their entire lives in the group where they were born, cooperating to defend their territory, while females tend to move away. But some chimp females seem less willing to cut the apron strings.

Funded in part by The Leakey Foundation, new findings from researchers at Duke University and North Carolina State University show that female chimpanzees with high-ranking mothers are more likely to be homebodies.

The study suggests that the perks of having a powerful mom can make it worthwhile for some females to stay and reproduce in the same group where they grew up, despite the risks of inbreeding with male relatives.

For the research, appearing Jan. 20 in the journal Current Biology, primatologists Kara Walker and Anne Pusey analyzed 45 years of dawn-to-dusk observations for 31 female chimpanzees born in Gombe National Park in Tanzania, where Pusey began working with Jane Goodall in 1970.

Chimps are unusual among mammals in that daughters, not sons, typically pick up their roots at puberty and move away from their families. But in Gombe National Park, some chimpanzee females stay put instead of moving out.

Leaving home is hard, and no less so for a chimp. Immigrants risk a great deal — leaving behind the familiar faces and comforts of home to strike out alone on a perilous journey, only to face multiple challenges upon arrival. Compared to stay-at-home females, those who leave are often attacked by resident females when they arrive in a new group. They also get a later start on motherhood.

So why do movers move away and stayers stay put? “I’ve always been intrigued by that question,” said Pusey, professor emeritus of evolutionary anthropology at Duke. “It has driven my research for decades.”

Differences between the sexes in migratory patterns are widespread in mammals and birds, Pusey said. She and colleagues first noticed the pattern at Gombe in the 1970s, but because female chimpanzees don’t leave home until between the ages of 11 and 13, it would take years of observation to unravel the reasons behind it. Now, thanks to Pusey’s efforts to compile and digitize the data and put it all in one database, researchers are starting to find answers.

In the study, the team found very little difference between females who left and those who stayed in terms of things like diet quality, crowding, or the number of unrelated males around when females reached maturity, which suggests they don’t leave due to competition for food or lack of suitable mates.

With four brothers at home, a young female chimpanzee named Flirt left her birth family at puberty to settle elsewhere. Photo by Kara Walker, Duke University

It was only when the researchers looked more closely at the females’ family members that the split began to reveal itself: specifically, they found that females with more brothers were more likely to leave — presumably because they risk inbreeding if they stay.

Brothers and sisters from the same mom usually show little interest in each other, avoiding sex with close kin. But there are exceptions, said Walker, a research assistant professor of behavioral ecology at North Carolina State University. High-ranking males have been known to coerce their sisters into mating with them. For the chimpanzees at Gombe, only one of four known offspring of such close matings made it to adulthood.

“Breeding with a brother is a pretty costly mistake,” Walker said.

The results confirm a decades-old hypothesis, that in many animals that live in groups, the hazards of inbreeding push one sex or the other to start their families elsewhere. But the study also points to a countervailing force that compels some female chimpanzees to stay put: mom. The researchers found that females with high-ranking moms on hand were more likely to stay rooted.

Yes, being the unwitting target of a brother’s misdirected advances might be a pain, but for some, the risk might be offset by the benefits of having a built-in support system.

Living close to mom means she’s able to provide help, such as by sharing prime foraging spots, the researchers say. And whereas females who move away enter their new groups at the bottom of the pecking order, females with powerful moms who choose to stay put benefit from their mother’s social clout and “cut in line.”

Pusey thinks a lot can be learned about the evolution of migratory patterns in humans by studying chimpanzees. Our species also typically sees young women more likely than young men to leave home for marriage.

Even in chimps, she says, “it’s sometimes better for females to stay,” which “suggests that the benefit of having mom around is pretty universal.”

This research was supported by the Jane Goodall Institute, the National Science Foundation (DBS-9021946, SBR-9319909, BCS-0648481, IOS-LTREB-1052693), the National Institutes of Health (R01 AI058715, R01 AI50529, R01 AI58715, P30 AI27767), the University of Minnesota, Duke University, The Leakey Foundation, and the Margot Marsh Biodiversity Fund.

CITATION: “Inbreeding Risk and Maternal Support Have Opposite Effects on Female Chimpanzee Dispersal,” Kara K. Walker and Anne E. Pusey. Current Biology, Jan. 20, 2020.

DOI: 10.1016/j.cub.2019.11.081.

This article was created with material provided by Robin Ann Smith of the Duke University Press Office.

Grantee Spotlight: Mareike Janiak

Mareike Janiak is a postdoctoral scholar at the University of Calgary. She received a Leakey Foundation grant in 2018 for her project entitled “Understanding adaptive radiation through evolution of digestive enzymes.”

Most of my work is computational, but I find ways to still get to the field, like teaching field courses, here at La Suerte Biological Station in Costa Rica. Photo by Amy Schreier

Were you always interested in science?

I wouldn’t say that I liked science as a child, but I definitely liked animals. I don’t remember being encouraged to be interested in science as a kid, it was more the opposite. I remember someone telling me “You’re a girl, you don’t have to be good at math!” The first person to actually spark a strong interest in science, especially biology and evolution, was my biology teacher in my last two years of high school. This intensified during my undergrad years at the University of Texas in Austin, which had excellent opportunities for undergraduate research.

How did you become interested in your current field?

I have to give credit to Dr. Chris Kirk at UT-Austin for that. I took his Intro to Physical Anthropology class because I needed another science credit (I was a psychology major), and his enthusiasm for the field was just infectious! I took several more primatology classes and decided to apply to grad schools for evolutionary anthropology.

When I shared my research at the Pint of Science Festival at Citizen Brewery in Calgary, audience participation was included in the form of edible insects. Photo by Shasta Webb

What is the big question you’re trying to answer through your Leakey Foundation-funded research project?

Our overarching goal is to get a better idea of the adaptations that make humans such “adaptable” and flexible creatures, especially when it comes to what we eat. Primates, in general, can survive on a wide variety of foods, but there are also a lot of species with a range of really specialized diets, like those focused on insects, leaves, or fruit, and all of these foods have different challenges when it comes to digesting them. But we know very little about how the species in these different dietary niches have adapted to digesting their foods.

So we’re looking for links between digestive enzyme adaptations and different diets in primates, but also other mammals to provide a broader comparative context. To do this in a non-invasive way, we turned to genomics – gene sequences contain an incredible amount of information that is available without the need for invasive animal research. We’re using published genomes and are also sequencing genes of interest in additional species.

For this, we designed something called a custom bait kit, which allows us to find and sequence only those regions of interest from a genetic sample. We just received our custom bait kit a couple of weeks ago and were excited to get sequencing, but unfortunately, the coronavirus outbreak has put that on hold. At the moment, all of us in the Melin Lab at the University of Calgary (including me) are doing our part to flatten the curve and are working only from home. Only essential (meaning mostly COVID-19) research is allowed to continue on campus, but I look forward to getting back to the lab at some point soon. In the meantime, we are planning on donating our spare gloves, masks, etc. to a hospital or lab that is testing for coronavirus.

In the Melin lab at the University of Calgary, excising a DNA fragment of interest from an electrophoresis gel. Photo by Gwen Duytschaever.

How did you feel when you found out you were awarded a Leakey Foundation grant?

I was thrilled, to say the least! I was waiting for a train in Penn Station in NYC when I got the email and immediately texted my co-PI (and supervisor extraordinaire) Amanda Melin a string of excited messages (lots of exclamation points!). This project is the focus of my postdoc with Amanda, so getting funding from The Leakey Foundation was crucial to being able to do the project in the way we had envisioned. The academic job market is tough (to say the least), so getting this grant and having the financial means to do my work is hugely important for early career researchers like myself.

What excites you about your work? 

I love the feeling of discovering something about how evolution has shaped a species, even if it’s just a tiny glimpse of the overall picture. The incredible diversity of (animal) life is just fascinating to me and love being able to create and share new knowledge about all the forms that that diversity takes.

Can you tell us about something interesting you’ve discovered as you’ve been doing your research project?

I can’t quite reveal anything about the Leakey Foundation-funded project yet, but I am about to submit a paper that is on a related topic and was very fun to work on. Humans have a long history of intentionally producing (and consuming) alcohol and previous research found that we (and African great apes) have an adaptation in the alcohol dehydrogenase class IV gene that makes us much more efficient at metabolizing those products. We conducted a broad comparative study into this gene across mammals and found that some fruit- and nectar-eating mammals share that adaptation, which is interesting, but maybe more surprisingly, a number of mammals have actually lost that gene! This might provide some insight into those stories of “drunk” animals (like moose and elephants) we often hear about.

Much of my work is heavily computational, so I’ve had to learn (and continue to learn) how to code in R, Python, and bash. I definitely didn’t expect that when I started grad school, but I’ve actually come to really enjoy it.

At the moment, of course, I’m working only from home and the lab work needed for our Leakey Foundation-funded project is on hold. So I’m working on manuscripts for which we already have the data in hand, including one on age-related microbiome changes, which I was supposed to present at AAPA in April. With the meeting cancelled and lab work paused, the priority is now to get this manuscript out!

Why do you think research like yours is important?

I think understanding the evolution of the human diet is incredibly important, given that what we eat has such a large impact on our health.

Last but not least, what’s your favorite hominin?

My answer is obviously any of the Paranthropus species!

A tiny bone from Little Foot’s skeleton adds fresh insights into what our ancestors could do

Little Foot’s skull, with the arrow on the right-hand image indicating the specimen’s atlas. R.J. Clarke/Author supplied

By Amélie Beaudet, University of the Witwatersrand

In his book Wonderful Life, Professor Stephen Jay Gould – an evolutionary biologist, palaeontologist and widely-read popular science author – described the evolution of life in the following way:

Life is a copiously branching bush, continually pruned by the grim reaper of extinction, not a ladder of predictable progress.

Studying Australopithecus, an extinct hominin genus that represents a branch of our family tree, is an excellent way to make more sense of our bushy family tree, and understand better how species emerge, evolve and disappear.

We don’t yet know the identity of Homo‘s direct ancestor, but the most likely candidate is probably one of the Australopithecus species that lived in Africa between 4 and 2 million years ago. But it’s difficult to study the biology and history of Australopithecus; the fossil record for the genus is just too fragmentary.

There have been some major and exciting finds along the way. For example, the discovery of a partial Australopithecus – later nicknamed Lucy – in Ethiopia in 1974 provided valuable information. But Lucy’s skeleton is only 40% complete and lacks important elements – like a complete skull.

A more complete skeleton, named Little Foot by researchers, offers scientists a chance to fill in the gaps in their knowledge. A number of studies have been done on the skeleton over the past few years. My colleagues and I have added to this body of knowledge in a paper that explores Little Foot’s first cervical vertebra, also called the atlas.

Our paper sheds light on an important part of Australopithecus’s anatomy. It helps us understand better how these ancient hominins lived. The findings suggest that this specimen could climb and move in trees. But it may also have been able to walk on the ground. That echoes the results of a previous study we conducted, which focused on Little Foot’s inner ear. The same study also supports the hypothesis of a late emergence of human brain metabolism.

This sort of research brings us closer to our origins and contributes to a thorough portrait of the main characters in human evolutionary history. It also illustrates Gould’s description of our evolution as a “copiously branching bush”.

The first cervical vertebra of Little Foot

The first skeletal elements of Little Foot were unearthed from the Sterkfontein Caves near Johannesburg, South Africa, in 1994 and 1997. The caves are among the richest sites of fossil remains in the world and they form part of what’s known as the Cradle of Humankind.

After 20 years of meticulous excavations by Ron Clarke and his team, Little Foot turned out to be the most complete Australopithecus skeleton ever discovered: it is more than 90% intact. The specimen has been dated to 3.67 million years old.

Various anatomical studies have been recently conducted on Little Foot. For instance, we’ve virtually replicated the inner surface of the braincase to deliver information about brain size, shape and organization. We’ve also studied the shape of the inner ear, which is part of the balance system. The findings told us more about Little Foot’s brain and behavior.

Little Foot’s first cervical vertebra, or atlas, is nearly intact and represents a key component of Australopithecus’ biology because it connects the skull with the rest of the skeleton. It also plays a role in how blood is supplied to the brain via the vertebral arteries.

By studying it we’ve been able to understand more about how Australopithecus moved, specifically their heads and necks, and the blood flow that irrigated their brains. We turned our attention to it in a bid to confirm or contradict previous findings and to find out more about Australopithecus.

The cranial base was filled with sediments. These were physically removed and the skull was scanned using a technique called microtomography at the University of the Witwatersrand, in South Africa. This imaging technique is far more accurate than the classical medical imaging tools and provided us with high-resolution images of the vertebra that could be virtually extracted from the sediments.

Key findings

Our main findings centered on Little Foot’s locomotion – the way it moved; the way this evolved over time; and, its brain metabolism.

First, what we discovered about Little Foot’s head and neck movements indicates that this specimen could climb and move in trees, but this does not exclude the possibility that it may also have walked on the ground. This finding is in accordance with results from our previous study about Little Foot’s inner ear.

Second, we compared the anatomy of Little Foot’s vertebra to two other Australopithecus specimens. One came from the same site as Little Foot, but from a different geological unit that is younger. The second specimen was found in the 1970s in Hadar, Ethiopia – the same site where Lucy was discovered.

A comparative table of the three atlases found in the localities where three iconic Australopithecus specimens were found. Author supplied

The atlas of Little Foot is similar to the one of the Australopithecus specimen from Ethiopia. The additional specimen from South Africa, which comes from the geologically younger deposits of Sterkfontein, is more human-like. These observations could indicate that at least some earlier species of Australopithecus may have spent much more time in trees than the later representatives of the genus.

Finally, our estimation of blood flow supplying Little Foot’s brain shows that the energetic costs of Australopithecus’ brain were lower than those estimated in modern humans. This could be due to Australopithecus’ relatively small brain, a diet that incorporated less meat (and so provided less energy), or because other organs required more energy.

This confirms the late emergence of the human-like brain metabolism that previous studies suggested.

More to come

There is still more work to be done on Little Foot’s skeleton – we are planning more studies using the various tools offered by “virtual paleoanthropology”. These studies and others will help us to shed more light on a crucial part of human ancestry’s family tree.

Amélie Beaudet, Postdoctoral fellow, University of the Witwatersrand

This article is republished from The Conversation under a Creative Commons license. Read the original article.

From the Field: Margaret Buehler

Margaret Buehler is a PhD candidate in the Department of Anthropology at Tulane University. She was awarded a Leakey grant for her research titled “Subordinate male roles in multimale primate groups with high reproductive skew.”

Margaret Buehler with an adult female capuchin.

My research strives to answer a seemingly simple, yet important, evolutionary question about group-living primates: why do specific primates choose to live together? About 70% of group-living (as opposed to solitary) primates reside in groups with multiple adult males and multiple adult females. An initial hunch is that males and females live together for reproduction, but for many multimale-multifemale primate species, the alpha male gets most, or all, reproductive opportunities (high reproductive skew). So why then are subordinate males living in groups? This question is even more perplexing in species characterized by male dispersal because typically males are not related to any other group members, which makes inclusive fitness benefits unlikely. Even though the males themselves may still need to live in social groups to access key resources and avoid predators, it is unclear why females and the alpha male allow additional males to join their groups. As the number of individuals in a group increases, so does energy-expenditure, within-group resource competition, and disease risk. Allowing males to join social groups clearly comes with added costs, so my current research focus is to understand the benefits of living in multimale groups.

An adult female capuchin with dorsal infant grooming a subordinate male.

For most primates that live in multimale groups, female reproductive success increases as the number or proportion of adult males in the group increases (ex: white-faced capuchins, chacma baboons, geladas, black howler monkeys, etc.). This is puzzling because all of these species are characterized by high reproductive skew, so if subordinates are unlikely to reproduce, how are they improving female fitness? Many studies, of white-faced capuchins and other primates, have shown that subordinate males improve fitness, but few, if any, have directly tested how subordinate males impact female reproductive success. Our theory is that subordinate males are protecting (a) key resources necessary for reproductive function, (b) ovulating females, and/or (c) their infants from conspecifics and predators.

In January, my team and I set out to see if, and to what degree, subordinate males are participating in resource, mate, and infant defense to shed some light on the benefits individuals can gain from living in multimale groups even with high reproductive skew. We are collecting behavioral and GPS data on 13 males in 3 study groups to determine how male defensive behavior differs according to group demographics (presence of infants and/or fertile females), proximity to resources, and/or location within the home range (i.e. central areas vs periphery). I will use these data to determine if there is any evidence of subordinate male participation in resource, mate, and/or infant defense in this species.

Photos 3 and 4: Two adult males in the Rosa Maria study group. Cicatriz (left) is identified by two scars on his forehead and one on the left side of his upper lip. Kovu (right) can be recognized by black dots centered below his lower lip and just right of center on his forehead and the placement of his black “cap” is far back on his head.

So far, we have experienced success in the data collection process. For the first three weeks, I trained my three field assistants on data collection methods and how to identify individual monkeys. Luckily, capuchins are easily identified by differences in facial/fur coloration, marks, scars, broken digits, etc. (see photos above), so our research team does not have to capture or collar any of our study individuals.

The downside of this method is that we must learn how to identify all individuals in all of our study groups while they move through the trees, which is no easy task! The other struggle of no-contact tracking is that we must first locate our study groups by searching the forest and stay with them from dawn until dusk to ensure that we will be able to find the group on subsequent days. Despite these struggles, my assistants have risen to the occasion. They quickly developed the skills and stamina necessary to keep up with these groups and collect accurate data. Over the past 5 weeks, we have successfully collected over 50 hours of behavioral data on subordinate males (via continuous focal samples in 10-minute increments) and recorded subordinate male behavior during 8 intergroup encounters.

Photo of the research team during training. Pictured, from left to right Giulia Severino, Zoe Alberts, Margaret Buehler, and Maël Dang Van Sung.

In addition to behavioral data, we are collecting GPS data to analyze how subordinate male behavior varies according to location within their home range. We are taking GPS points at 30-minute increments to establish group home ranges, and we are recording the starting and ending location of all behavioral samples and intergroup encounters. The latter we will use to test how behavior varies by location. For example, we can test if subordinate males more defensive or vigilant when they are near their home range periphery and likely to encounter other groups, or if they are more likely to participate in an IGE if they are near a terrestrial water source. In order to test how ovulating females impact male behavior, we are collecting fecal samples to non-invasively assess female fertility. This process is the most difficult as we must sample each female in each group every 2-3 days, excluding those who are still nursing or visibly pregnant. This requires tremendous patience, as the researcher must follow a single individual closely until she defecates, which can take several hours for each female.  

Margaret follows alpha female Simba while waiting to collect a fecal sample.

At this point, I cannot offer much insight into the answer to our research question. We cannot know the extent of each group’s home range until we finish data collection, and thus, we cannot determine how behavior varies within it. In addition, I cannot determine female fertility until I transport the fecal samples to our lab in the United States. I will need to aggregate the data from many behavioral samples before we can draw conclusions about general behavior patterns. Over the next two months, we hope to collect an additional 100+ hours of focal data, nearly 120 fecal samples, and a few dozen descriptions of intergroup encounters. I look forward to aggregating these data and discovering what is happening here in Santa Rosa. My hope is to use these data to predict long-term patterns and use the rich dataset available on this primate population to test inferences on how exactly subordinate males impact female reproductive success.

Several monkeys in the Los Valles study group just waking up from a morning siesta.

Important information for current Leakey Foundation grant recipients

University travel restrictions and personal health considerations due to COVID-19 may impact Leakey Foundation grant project timelines.

All project end date extension requests due to COVID-19 will be approved. If you are a grantee and your project end date is far in the future, you do not need to request an extension at this time. Immediate extension requests are only necessary if grant spending will be affected by an approaching end date. If you do need an extension, please coordinate with your institution’s sponsored programs office to make a request.