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BIODIVERSITY
Published: 06 February 2023
doi: 10.3389/frym.2023.914617
HABITAT DESTRUCTION IS AFFECTING THE FACIAL
FEATURES OF ORANGUTANS
Alexandra E. Kralick 1* and Kate McGrath 2,3
1Department of Anthropology, University of Pennsylvania, Philadelphia, PA, United States
2Department of Anthropology, State University of New York Oneonta, Oneonta, NY, United States
3Department of Anthropology, The George Washington University, Washington, DC, United States
YOUNG REVIEWER:
PARITHI
AGE: 10
Orangutans are endangered primates that live in Asian forests. Each
year, these forests are becoming more stressful to live in because of
habitat destruction and deforestation. The stress of habitat change
may aect some orangutans more than others. As male orangutans
age, some of them grow big cheek pads on their faces, called flanges
(flanged males), while others keep their kid faces (unflanged males).
Scientists still do not understand why orangutans grow up to become
one type or the other, but stress might help explain it. To test this,
we examined orangutan teeth for signs of stress that formed when
they were kids. We found that unflanged males have less severe stress
lines, meaning lower levels of childhood stress, compared to flanged
males. Nowadays, most orangutans have high levels of stress due to
habitat destruction and climate change. As a result, adult unflanged
males may be disappearing from the wild altogether.
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Kralick and McGrath Habitat Destruction and Orangutan Biology
ORANGUTAN HABITATS ARE STRESSFUL
Orangutans are critically endangered and expected to go extinct in the
next 50 years. As much as 97% of the orangutan population has been
wiped out in the last century. The lives of orangutans are particularly
hard because they eat mostly fruit, which naturally goes in and out
of season and can be hard to find at times. Periods with less fruit are
becoming more common due to environmental changes like habitat
destruction, climate change, and massive wildfires. When fruit is not
CLIMATE CHANGE
A global pattern of
change in the
environment, caused
by humans, that puts
Earth’s ecosystems
under threat.
available, orangutans resort to eating bark. As a result, they go into
starvation mode and lose muscle mass [1]. Starving, losing their homes
to logging, and running from wildfires are all incredibly stressful.
In this study, we assessed how severe stress is recorded in orangutans’
STRESS
An organism’s physical
response to
environmental or social
pressure that forces
that organism to adapt.
bodies and what it can tell us about how they are responding to habitat
destruction. Specifically, we tested whether some types of orangutans
respond to stress dierently than others, based on analyses of their
skin and teeth.
TYPES OF ORANGUTANS
All young male orangutans start o looking the same, but when
puberty hits, male orangutans dierentiate into two distinct types.
PUBERTY
A time between
childhood and young
adulthood when
adolescents go through
sex-specific physical
changes to reach
sexual maturity.
When humans go through puberty, boys grow beards and armpit
hair. While orangutans also get hairier during puberty, they also
develop something unique— some males develop face pads called
flanges that female orangutans find attractive. You may have seen
FLANGE
Cheek pads on the
faces of some adult
male orangutans.
these orangutans at the zoo—they almost look like they have half
frisbees glued to their cheeks (Figure 1B). These males are called
flanged males.
Figure 1
Figure 1
(A) An unflanged male.
(B) A flanged male
(Photograph credit:
Meredith Bastian,
Sungai Lading
Orangutan Field
Project).
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Kralick and McGrath Habitat Destruction and Orangutan Biology
Adult male orangutans that do not develop face flanges still look like
young orangutans as they age, and they are called adult unflanged
males (Figure 1A). Unflanged males can delay the development of
flanges for up to 20 years, or possibly may never flange. In the animal
kingdom, it is unusual for a species to contain two kinds of adult
males. Scientists still do not understand why adult unflanged male
orangutans exist, and they are mysterious because they cannot be
found in zoos. Some researchers have long suspected that stress may
play a role.
HOW MIGHT HABITAT STRESS AFFECT ORANGUTANS?
Stress experienced during early life may help explain why some
young male orangutans turn into flanged adults and others turn into
unflanged adults. Adult unflanged males are lower ranking which
means that they often lose out in fights and have fewer babies
compared to higher ranking flanged males. In some animals, like
baboons, lower-ranking males tend to experience more stress than
higher-ranking males. However, once researchers started measuring
hormone levels to assess orangutan stress, they found that, unlike
baboons, the flanged males are more stressed out, even though
they are higher ranking. So, we developed our hypothesis that adult
unflanged males experienced less severe stress in their childhoods
compared to flanged males.
Hormone studies support our hypothesis. An earlier study showed that
adult unflanged male orangutans have the same or lower levels of
stress hormones as flanged males. However, that study only looked
at adults, not orangutans during their whole lives from childhood
through puberty since that is much trickier to do. Adult unflanged
males can only be found in the wild and tracking orangutans in the
wild across their whole lives to study their stress hormones is much
more challenging than in zoos. Orangutans live high up in the trees
and often move away from the research study area when they grow
up. Therefore, we decided to use a new method to analyze childhood
stress: studying the eects of stress on the teeth of adult orangutan
skeletons from museum collections.
TEETH GROW LIKE RINGS ON A TREE
Both teeth and trees hold incredibly detailed records of their growth.
Trees grow bigger by adding new layers on the outer perimeter of
their trunks every year. Because trees grow more with higher rainfall,
rings from rainy years are particularly thick, and rings from years with
droughts are particularly thin.
Teeth reflect a similar pattern—each week one new layer is created
(Figure 2). You might not have noticed these layers yourself because
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Kralick and McGrath Habitat Destruction and Orangutan Biology
they are about the thickness of a single strand of hair! Orangutan teeth
grow in two sets, just like ours. The first set of teeth grow when a baby
is still inside its mother’s womb. Around the time when the baby teeth
start appearing in the mouth, a second set of teeth are still growing
inside the jaw. These adult teeth keep growing throughout childhood
and, when they were done, they push the baby teeth out and grow
into the open spots.
STRESS EVENTS LEAVE MARKS ON TEETH
If kids experience stress while their teeth are growing inside their jaws,
that stress leaves a record behind. When a child has a fever or an injury,
for example, their body focuses on healing itself and takes a break from
growing the teeth at that time. This leaves grooves, called stress lines,
STRESS LINES
Grooves on the surface
of teeth caused by
periods of stress
during childhood.
on the teeth that were in the process of forming (Figure 2). Stress lines,
kind of like a year of drought in tree rings, show that the tooth did not
grow as much as usual during those weeks of sickness or injury.
Orangutan teeth are full of both severe and more typical-looking stress
lines (Figure 2). People and animals normally have some stress lines in
their teeth, including from the time when they were born, because
that is a really stressful event for both moms and babies. But, unless
a person went through a period of starvation or a severe illness with
a sustained fever, it is unlikely that they would have really deep stress
lines on their teeth. Some of our previous research found that when
apes or people experience severe stress events, like losing their families
or experiencing very serious injuries, they get stress lines that are
super deep compared to those caused by everyday stressors, like the
common cold.
MEASURING STRESS IN ORANGUTAN TEETH
A century ago, scientists collected the skins, skulls, and skeletons of
orangutans that passed away, and brought them to museums for
researchers to study. We went to the collections of two museums—the
Smithsonian’s National Museum of Natural History in Washington,
D.C., and the Academy of Natural Science in Philadelphia—to study
these orangutan artifacts. We first identified which adult orangutans
were which: flanged males had larger face measurements than did
unflanged males (Figure 3).
We took dental molds from orangutan skulls to measure the stress
lines in the teeth. This process is similar to the one that dentists
use to mold your teeth before you get braces. The mold looks like
an exact copy of the tooth, except it is black because it is made
of a plastic-like material called epoxy (Figure 2). We then used a
high-powered microscope to take 3D pictures of the stress lines and
measure their depth, so that we could figure out the severity of stress
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Kralick and McGrath Habitat Destruction and Orangutan Biology
Figure 2
Figure 2
(A) Example of stress
lines in a real orangutan
canine. (B, C) A replica
of a canine, showing a
major stress line
marked with an asterisk
in both images. The
colored part of panel B
shows changes in
height across the tooth.
(D) Trees grow bigger
by adding new layers
on the outer perimeter
of their trunks every
year. (E, F) Like trees,
teeth reflect a similar
pattern—each week
one new layer is
created (Image credit:
Sarah Crawley).
events experienced by each individual orangutan while they were
growing up [2].
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Kralick and McGrath Habitat Destruction and Orangutan Biology
Figure 3
Figure 3
We measured the face
sizes of male (flanged
and unflanged) and
female orangutans,
from the eye to the ear
(illustrated in a gray
bracket). The box plots
of these measurements
show flanged males
have larger faces than
unflanged males and
females, which tells us
that the faces of adult
unflanged males can be
reliably distinguished
from those of flanged
males (Photograph
credit: istock
photograph).
WHAT DID WE FIND?
We found that flanged males have much deeper stress lines than
unflanged males. This supports our hypothesis that flanged males
experienced more severe stress events during childhood, when their
teeth were forming, compared to adult unflanged males. These results
help us understand why two types of male orangutans may exist.
Our research supports the idea that orangutan males who flange
earlier do so because of greater childhood stress. In the future, it
would be interesting to look at other factors that may help explain
flanging, such as genetic dierences between flanged and unflanged
adult males.
WHY THIS MATTERS
Our results show that a stressful childhood usually leads to a male
orangutan becoming a flanged adult. With orangutan childhoods
becoming more stressful due to environmental changes like habitat
destruction and climate change, it is likely that more male orangutans
are becoming flanged at puberty. Our results also suggest that
fewer orangutans will become adult unflanged males each year,
which means that adult unflanged males may disappear from the
wild altogether.
Among animals, adult unflanged male orangutans exhibit a rare and
unique phenomenon. Since adult unflanged males are only found in
the wild, and since the wild is becoming more stressful and possibly
causing them to disappear, we have precious few years left to study
them. If orangutans as a species have only 50 years left in the
wild, adult unflanged males will likely become extinct long before
that. Unfortunately, we still know so little about how and why adult
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Kralick and McGrath Habitat Destruction and Orangutan Biology
unflanged males exist. Studying them can also provide clues about
how and why our fossil ancestors experienced stress when they were
young. But we must save orangutans from extinction to understand
any of that!
Saving the orangutans is possible—mountain gorillas were once saved
from the brink of extinction. If we work hard enough through sustained
and well-funded international conservation eorts, orangutans too
could be saved from the brink of extinction.
ACKNOWLEDGMENTS
This project received funding from the European Union’s Horizon 2020
research and innovation program under the Marie Sklodowska-Curie
grant agreement No 798117, The Ohio State University President’s
Postdoctoral Scholars Program, the Smithsonian Institution Graduate
Student Fellowship, Penn Museum, the NSF Graduate Research
Fellowship Program (GRFP), and the NSF Graduate Research Internship
Program (GRIP). Thanks to Janet Monge for her contribution to
dental impression collection, thoughtful conversations, sharing of
equipment, and supervision. Thanks to Sarah Crawley for her
animations used for Figure 2. Thanks to Alain Queelec, Éric Pubert,
Yann Heuzé, William Rendu, Emmy Bocaege, Umang Gurung, Debbie
Guatelli-Steinberg, Shannon McFarlin, Donald J. Reid, Sireen El
Zaatari, Kristofer Helgen, Darren Lunde, Ned Gilmore, the Smithsonian
National Museum of Natural History (NMNH), the Academy of Natural
Sciences of Philadelphia (ANSP), and the American Museum of Natural
History (AMNH) for making their collections and equipment available
for this and previous projects.
ORIGINAL SOURCE ARTICLE
Kralick, A. E., and McGrath, K. 2021. More severe stress markers in
the teeth of flanged versus unflanged orangutans (Pongo spp.). Am.
J. Phys. Anthropol. 176:625–37. doi: 10.1002/ajpa.24387
REFERENCES
1. O’Connell, C. A., DiGiorgio, A. L., Ugarte, A. D., Brittain, R. S., Naumenko, D. J.,
Utami Atmoko, S. S., et al. 2021. Wild Bornean orangutans experience muscle
catabolism during episodes of fruit scarcity. Sci. Rep. 11:10185.
doi: 10.1038/s41598-021-89186-4
2. McGrath, K., El-Zaatari, S., Guatelli-Steinberg, D., Stanton, M. A., Reid, D. J.,
Stoinski, T. S., et al. 2018. Quantifying linear enamel hypoplasia in Virunga
Mountain gorillas and other great apes. Am. J. Phys. Anthropol. 166:337–52.
doi: 10.1002/ajpa.23436
SUBMITTED: 07 April 2022; ACCEPTED: 17 January 2023;
PUBLISHED ONLINE: 06 February 2023.
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Kralick and McGrath Habitat Destruction and Orangutan Biology
EDITOR: Didone Frigerio, University of Vienna, Austria
SCIENCE MENTOR: R. Arthee
CITATION: Kralick AE and McGrath K (2023) Habitat Destruction Is Aecting The
Facial Features Of Orangutans. Front. Young Minds 11:914617. doi: 10.3389/frym.
2023.914617
CONFLICT OF INTEREST: The authors declare that the research was conducted in
the absence of any commercial or financial relationships that could be construed
as a potential conflict of interest.
COPYRIGHT © 2023 Kralick and McGrath. This is an open-access article distributed
under the terms of the Creative Commons Attribution License (CC BY). The use,
distribution or reproduction in other forums is permitted, provided the original
author(s) and the copyright owner(s) are credited and that the original publication
in this journal is cited, in accordance with accepted academic practice. No use,
distribution or reproduction is permitted which does not comply with these terms.
YOUNG REVIEWER
PARITHI, AGE: 10
I really like insects and lizards. I also want to keep on learning about physics
and ecosystems.
AUTHORS
ALEXANDRA E. KRALICK
Alexandra Kralick is a Ph.D. Candidate in anthropology at the University of
Pennsylvania in Philadelphia. She studies the growth and development of sex
dierences in the great ape skeleton, in the field of biological anthropology. Her
previous work is on gorilla dental development and wrist bone shape. Kralick is
a former National Science Foundation Graduate Research Fellow and a Leakey
Foundation grantee. She earned her B.S. in biological anthropology from The
George Washington University. You can follow her on Twitter @BioAnthFunFacts.
*akralick@sas.upenn.edu
KATE MCGRATH
Kate McGrath is an Assistant Professor of Biological Anthropology at SUNY Oneonta
in New York. She studies how early life stress aects the growth and development
of bones and teeth. Her previous studies have focused on great apes, humans, and
human ancestors. She is a former Marie Skłodowska-Curie Fellow and her work has
been supported by the European Union, National Science Foundation, and Leakey
Foundation. She earned her B.S. in anthropology from the College of Charleston,
South Carolina, and Ph.D. in human paleobiology from The George Washington
University. You can follow her on Twitter @kateapemcgrath.
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