Tarsier Goggles: a virtual reality tool for experiencing the optics of a dark-adapted primate visual system

Abstract

Abstract Charles Darwin viewed eyes as the epitome of evolution by natural selection, describing them as organs of extreme perfection and complication. The visual system is therefore fertile ground for teaching fundamental concepts in optics and biology, subjects with scant representation during the rise and spread of immersive technologies in K-12 education. The visual system is an ideal topic for three-dimensional (3D) virtual reality learning environments (VRLEs), and here we describe a 3D VRLE that simulates the vision of a tarsier, a nocturnal primate that lives in southeast Asia. Tarsiers are an enduring source of fascination for having enormous eyes, both in absolute size and in proportion to the size of the animal. Our motivation for developing a tarsier-inspired VRLE, or Tarsier Goggles, is to demonstrate the optical and selective advantages of hyperenlarged eyes for nocturnal visual predation. In addition to greater visual sensitivity, users also experience reductions in visual acuity and color vision. On a philosophical level, we can never know the visual world of another organism, but advances in 3D VRLEs allow us to try in the service of experiential learning and educational outreach.

Full text

Gochmanetal. Evo Edu Outreach (2019) 12:9
https://doi.org/10.1186/s12052-019-0101-6
COMMENTARY
Tarsier Goggles: avirtual reality tool
forexperiencing theoptics ofadark-adapted
primate visual system
Samuel R. Gochman1,2* , Marilyn Morano Lord3, Naman Goyal2, Kristie Chow2, Benjamin K. Cooper2,
Lauren K. Gray2, Stephanie X. Guo2, Kylie A. Hill2, Stephen K. Liao2, Shiyao Peng2, Hyun J. Seong2, Alma Wang2,
Eun K. Yoon2, Shirley Zhang2, Erica Lobel2, Tim Tregubov2 and Nathaniel J. Dominy1*
Abstract
Charles Darwin viewed eyes as the epitome of evolution by natural selection, describing them as organs of extreme
perfection and complication. The visual system is therefore fertile ground for teaching fundamental concepts in optics
and biology, subjects with scant representation during the rise and spread of immersive technologies in K-12 educa-
tion. The visual system is an ideal topic for three-dimensional (3D) virtual reality learning environments (VRLEs), and
here we describe a 3D VRLE that simulates the vision of a tarsier, a nocturnal primate that lives in southeast Asia. Tarsi-
ers are an enduring source of fascination for having enormous eyes, both in absolute size and in proportion to the size
of the animal. Our motivation for developing a tarsier-inspired VRLE, or Tarsier Goggles, is to demonstrate the optical
and selective advantages of hyperenlarged eyes for nocturnal visual predation. In addition to greater visual sensitiv-
ity, users also experience reductions in visual acuity and color vision. On a philosophical level, we can never know the
visual world of another organism, but advances in 3D VRLEs allow us to try in the service of experiential learning and
educational outreach.
Keywords: Constructivist learning theory, Tarsius bancanus, Tarsier, Virtual reality, Optics, Natural selection, Vision
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Background
e eye is an exquisite anatomical structure and fer-
tile ground for demonstrating core concepts in phys-
ics (optics) and biology (evolution by natural selection),
a pattern that began with Darwin himself. He described
eyes as “organ[s] of extreme perfection and complication”
(Darwin 1859, p. 186) and he used them as foil for oppo-
sition in one of his most-quoted sentences:
To suppose that the eye with all its inimitable con-
trivances for adjusting the focus to different dis-
tances, for admitting different amounts of light, and
for the correction of spherical and chromatic aber-
ration, could have been formed by natural selection,
seems, I freely confess, absurd in the highest degree.
Despite his ‘confession,’ Darwin never doubted the evo-
lution of complex eyes, a view that has since received
overwhelming support (Lamb etal. 2007; Gregory 2008).
At the same time, the eyes and visual systems of animals
are wonderfully diverse, a fact that fuels the pages of biol-
ogy textbooks and fires our natural curiosity. Cronin etal.
(2014) put it this way: “We humans are visual creatures.
We are also introspective and curious, a combination that
makes us all by nature amateur visual ecologists (even
if we don’t know it). Because our world is dominated by
visual sensations, we naturally wonder how other animals
see their particular worlds.” On a philosophical level, we
can never know the visual world of another organism
(Nagel 1974), but the emergence and spread of immersive
technologies enables us to try in the service of construc-
tivist pedagogies (Colburn 2000), as a “way of seeing”
fundamental concepts in optics and evolution (Scott etal.
1991).
Open Access
Evolution: Education and Outreach
*Correspondence: samuel.r.gochman.18@dartmouth.edu;
nathaniel.j.dominy@dartmouth.edu
1 Departments of Anthropology and Biological Sciences, Dartmouth
College, Hanover, NH 03755, USA
Full list of author information is available at the end of the article

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Gochmanetal. Evo Edu Outreach (2019) 12:9
3D virtual reality learning environments (VRLEs)
ree-dimensional (3D) virtual reality learning environ-
ments (VRLEs) are well suited to constructivism, espe-
cially when students must form 3D representations of
course material or interact with a learning environment
to construct knowledge (reviews: Huang et al. 2010;
Merchant et al. 2014). Accordingly, the development
and deployment of 3D VRLEs has expanded rapidly in
K-12 and higher education, especially medical education
(Wu etal. 2013; Jang etal. 2017); indeed, the anatomi-
cal education of medical students is a major catalyst for
3D VRLE technology. e practical value of 3D VRLEs
for learning human anatomy hints at wider applications
within K-12 biological education. For example, the prin-
ciples of natural selection and evolution is another topic
that invites constructivist pedagogies (Kalinowski et al.
2013; Lee etal. 2017; Prins etal. 2017). Here we describe
a 3D VRLE with this goal in mind. It is intended to dem-
onstrate the principles of visual optics and natural selec-
tion in a way that constructs knowledge and stimulates
user reflection on diverse worldviews. e inspiration
for our 3D VRLE is the tarsier, a primate with an extreme
visual system.
Tarsiers andtheir visual world
Tarsiers are small (113–142 g) nocturnal primates
(Fig.1a). ey are an enduring source of fascination for
having enormous eyes, both in absolute size and in pro-
portion to the size of the animal (Fig.1b). Polyak (1957)
concluded that the eye size relative to body size of tar-
siers is unmatched by any living vertebrate. e extreme
eye size of tarsiers is most likely related to the absence of
a tapetum lucidum, the mirror-like structure that results
in ‘eye shine’ (Cartmill 1980).
A tapetum lucidum is prevalent among nocturnal
mammals, including nocturnal primates, because it
increases photon capture and visual sensitivity under low
light levels. e absence of a tapetum lucidum in tarsiers
is therefore puzzling, and it is interpreted as evidence of
an ancestral shift from nocturnality to diurnality followed
by a reversion to nocturnality with a diurnally-adapted,
tapetum-free eye (Cartmill 1980; Martin and Ross 2005).
us, the hyper-enlarged eyes of tarsiers are widely
viewed as a compensatory adaptation to improve visual
sensitivity at night in the absence of a tapetum lucidum.
To appreciate why enlarged eyes are advantageous at
night, we can use the dimensions of tarsier eyes to cal-
culate the corresponding parameters for humans. For
example, the eye-to-brain volume ratio of tarsiers (see
Fig.1b) can be scaled to human dimensions (see Appen-
dix for calculations), to produce an eye with a diameter of
13.6cm, the approximate volume of a grapefruit (Fig.2a).
e biological plausibility of this thought experiment
is attested by the eyes of colossal squid (Mesonychoteu-
this hamiltoni), which are nearly twice as large (Nilsson
etal. 2012). Yet, the optic axes of these hypothetical eyes
would never align with the visual axes of human bin-
ocular vision, so we merged the eyes to bring the optic
and visual axes into alignment (Fig.2b). In theory, such
tarsier-inspired eyewear would enhance the visual sen-
sitivity of human users (Fig.2c), as the enlarged corneas
would capture more photons under low light levels.
Physical eyewear could demonstrate these principles,
but virtual “lenses” enable the use of filters and interac-
tive elements, essentially transcending physical limita-
tions to create specialized environments for intentional
exploration. Such a VRLE is exciting because it can bet-
ter convey visual sensitivity at night by simulating the
benefits of having high densities of rod photorecep-
tors—tarsiers have > 300,000/mm2, whereas humans
have ~ 176,000/mm2 (Collins etal. 2005). It can also sim-
ulate other aspects of tarsier vision. For example, the vis-
ual acuity of Philippine tarsiers is estimated at 8.89 c/deg
(Veilleux and Christopher 2009), a minimum resolvable
angle that can be simulated for human users (Caves and
Johnsen 2017). Another distinguishing trait of tarsiers is
red-green colorblindness. is trait varies among spe-
cies, but each phenotype can be simulated (Melin etal.
2013a, b; Moritz et al. 2017). Lastly, a VRLE can simu-
late the visual field of tarsiers (186°), which we calculated
by summing the visual angle of each eye (156.5°; Fig.1c)
Fig. 1 a Bornean tarsier (Tarsius bancanus) under nocturnal
conditions; note the extreme dilation of the pupil (photograph by
David Haring, reproduced with permission). b Anatomical preparation
of the eye and brain of T. bancanus (modified from Sprankel 1965),
illustrating the comparable volume of the two structures (Castenholz
1984). The eyes of T. bancanus are therefore enormous, both in
absolute size and in proportion to the size of the animal. c Geometry
of the tarsier eye (modified from Castenholz 1984) illustrating our
calculation of the visual angle

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Gochmanetal. Evo Edu Outreach (2019) 12:9
and subtracting the area of binocular overlap (127°; Ross
2000).
Collectively, these traits of the tarsier visual system are
predicted to result in superior vision (relative to humans)
at night, and they are widely interpreted as adaptations
for visual predation—tarsiers are exceptional among pri-
mates for being 100% faunivorous (Ross 2004; Moritz
etal. 2014, 2017). For humans to appreciate the optical
and selective advantages of tarsier eyes for accomplish-
ing this visual challenge (navigation and predation in the
dark), we conceived and developed a VRLE in the service
of education, science communication, and existential
reflection. e result—which we call Tarsier Goggles—
can simulate human and tarsier vision under varying
ambient lighting conditions.
Design oftheVRLE
We developed several virtual environments for users
to explore within a classroom setting, each of which
allows users to alternate between human and tarsier
vision, highlighting corresponding differences in bright-
ness, acuity, and color vision. In VR, users begin in an
open space (Fig.3a, b) where they can choose to receive
guidance, including tutorials for interface controls and
prompts for user behaviors. e first learning environ-
ment, “Matrix,” is a 3D lattice of beams that emphasizes
human-tarsier differences in visual acuity and color vision
(Fig. 3c, d). e second learning environment, “Laby-
rinth,” is a dark maze-like space that is practically opaque
under human visual conditions but navigable as a tarsier,
demonstrating the advantages of tarsier visual sensitiv-
ity (Fig.3e, f). e third learning environment, “Bornean
Rainforest,” is modeled on the dipterocarp rainforests of
Borneo at night (Fig.3g, h). In this final setting, users can
navigate between trees, applying knowledge from previ-
ous environments to discover a new worldview—to both
experience the worldview of a tarsier and to appreciate
why natural selection favored such large eyes. For orien-
tation purposes, two-dimensional (2D) video capture of
the preceding progression is available as Additional file1;
Fig. 2 a Hypothetical size of tarsier eyes when scaled to human dimensions. We used the mean interpupillary distance reported for humans
(6.3 cm) to merge the eyes and align the optic and visual axes. b Scaling of the hypothetical eyes in relation to a human head. c Rendering to
simulate the scaled eyes on a human user

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Gochmanetal. Evo Edu Outreach (2019) 12:9
however, we recommend that instructors are present to
guide first-time student users.
Tarsier Goggles is available for free online (see Avail-
ability of data and materials). It is intended to enrich the
classroom when the curriculum turns to optics or evo-
lution, topics that have natural and enduring synergies.
With even basic awareness of the relevant scientific prin-
ciples, students or members of the public can wear a VR
headset and reflect on how they currently experience the
world and how they might through the eyes of another.
Development
At the time of writing, Tarsier Goggles was built in Unity
2018 with SteamVR for the HTC Vive and Vive Pro head-
sets. We used the Virtual Reality Toolkit (VRTK), an
open source library of scripts for Virtual Reality develop-
ment, to create some menu options and user “teleporta-
tion”. is action enables user navigation through each
VR environment; it also simulates the vertical clinging-
and-leaping behavior of tarsiers when users explore the
Bornean Rainforest. We built all other functionalities
such as the splash screens and tutorial. For visual effects,
we used and modified Unity’s built-in post-processing
stack as well as the Colorblind Effect asset created by
Project Wilberforce. Our GameObjects, which include
trees, grass, bushes, and other virtual structures, were
designed and built in Maya as well as downloaded from
the Unity Asset Store.
Assessments anddiscussion
Our initial assessments of Tarsier Goggles were ad hoc
and opportunistic, stemming from five demonstrations
across a wide range of settings and ages (Fig.4). Demos
were conducted during two on-campus events at Dart-
mouth that were open to students, faculty, and their
families. In addition, we conducted a demo at a profes-
sional meeting of biological anthropologists, a group
familiar with tarsier visual adaptations. In one case, we
worked with middle school (6th grade) students visit-
ing a nonprofit environmental education, research, and
avian rehabilitation center in Vermont. Collectively, this
broad mix of users (n ≈ 35) provided important feedback
for improvements, many of which were implemented in
subsequent iterations. Overall, pilot users experienced
the effects that we intended—they integrated optical and
biological concepts to enrich their understanding of eye
evolution and tarsiers.
ey also reflected on their experiences. As one Dart-
mouth professor of engineering put it, “We all think we
are seeing what everyone else sees, but in fact we are all
seeing something different. I feel connected to animals
in a way I haven’t been before.” Another adult user in the
profession of science education and outreach added, “It’s
not just speculating. It’s actually having it in front of my
eyes.” Notably, some children described brief sensations
of disorientation, which is not uncommon in VR. Nau-
sea from prolonged use of VR has been reported (Madary
and Metzinger 2016), and it is something that educators
should consider when using the technology. Best prac-
tices for using VRLEs are still in development.
Formal assessment of Tarsier Goggles occurred at an
independent private secondary school in New Hamp-
shire serving ∼ 300 students. We focused on two courses,
Fig. 3 Screen captures from each VRLE in Tarsier Goggles. Paired
images simulate the vision of humans (left) and tarsiers (right) under
identical twilight conditions, revealing differences in visual sensitivity
(brightness), acuity, and color discrimination. a, b VR environment
where users can elect to receive guidance. c, d The “Matrix”
VRLE contains a lattice of beams that is intended to emphasize
human-tarsier differences in visual acuity and color vision. e, f The
“Labyrinth” VRLE is intended to emphasize human-tarsier differences
in visual sensitivity by challenging users to navigate a dark (scotopic)
environment. g, h The “Bornean Rainforest” VRLE enables naturalistic
exploration within the understory of a lowland dipterocarp forest

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Gochmanetal. Evo Edu Outreach (2019) 12:9
Anthropology (12th grade; 18 student users) and Inquiry
to Science (9th grade; 8 student users). We used a cen-
tral meeting room equipped with a large monitor, which
allowed us to project images and orient students to the
physical appearance of tarsiers (Fig.1a) and the relative
size of their eyes (Fig.1b). We also played a brief, muted
video of tarsier foraging behavior, in which it is evident
that tarsiers are nocturnal visual predators. In the spirit
of constructivism (Colburn 2000), there was no prepara-
tory content related to visual anatomy, optics, or natural
selection. Instead, we immersed students in the VRLE
immediately, allowing them to experience and construct
for themselves the adaptive advantages of having enor-
mous eyes at night. Each user trial was 5min; however,
classmates were able to view the user’s learning environ-
ment via the monitor (Fig.5). is configuration stirred
considerable commentary and discussion among other
students, enriching the learning experience beyond the
individual user.
Fig. 4 Pilot testing of Tarsier Goggles was ad hoc and opportunistic, but it generated uniform marvel and constructive feedback from a wide range
of users. a Adults without formal training in evolutionary biology tended to view the experience as reflective. b Professional biologists tended to
focus on the anatomical and physiological parameters informing the VR simulation of tarsier vision. c Many middle school students valued the
gaming aspects of Tarsier Goggles; i.e., overcoming visual ‘impairments’ to explore some learning environments. d Younger children had difficulty
mastering the hand controls, and they sometimes attempted to reach for objects in the virtual environment
Fig. 5 A student-user experiences Tarsier Goggles during formal
assessment. The student stands in front of a projection of the internal
experience for classmates to view (photograph by Dustin Meltzer,
reproduced with permission)

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Gochmanetal. Evo Edu Outreach (2019) 12:9
Our post-survey instrument contained 9 open-ended
questions (Table1), and it was administered immediately
after use of the VRLE. e present discussion of student
reactions (see Additional file2) will focus on the suita-
bility of our VRLE for fulfilling constructivist principles.
For example, when students were asked how Tarsier Gog-
gles differs from traditional classroom content, 22 of 26
(85%) respondents expressed a preference for the VRLE
(cf. question 4). As one student put it, “Instead of hearing
what life is like, you [can] actually experience it.” Other
questions assessed whether students grasped the learning
objective; i.e., that larger eyes capture more light, which
increases visual sensitivity and is advantageous for see-
ing prey at night. We found that user responses varied
according to the nuance of the question. For example, 22
of 25 (88%) respondents understood that large eyes are
advantageous (cf. question 8), and 23 of 25 (92%) recog-
nized that tarsier eyes are more sensitive than our own
(cf. question 9), but only 15 of 25 (60%) could articulate
why on the basis of optical principles (cf. question 7).
One student put it this way: “Large eyes means more light
can hit the retina? I’m not positive, I assume it allows
more light in.” is inquisitive response—expressed
as conjecture—is a testament to the seven principles
of constructivism, and we agree with Colburn (2000)
that post-demonstration discussion or lecture content
should verify or elaborate on the knowledge constructed.
Accordingly, we developed a potential lesson plan with
an eye to Next Generation Science Standards (see Addi-
tional file3).
Colburn (2000) argued that classroom demonstra-
tions are at their best when they challenge student pre-
conceptions, forcing them to account for discrepancies
between their expectations and observations. Accord-
ingly, we asked students if they were surprised by the
differences in tarsier and human visual systems, and 12
of 18 (67%) respondents answered affirmatively (cf. ques-
tion 6). We attribute this marginally equivocal result to
our use of Fig. 1b as an orientation tool. One student
said, “I wasn’t that surprised that their vision was that
good. eir eyes are slightly larger than their brain so I
would have thought their vision would be better.” Such a
response reveals twin outcomes: first, it demonstrates the
fulfillment of our learning objective; and second, it raises
questions about the sequence of learning materials. For
this student, prior exposure to Fig.1b put Tarsier Goggles
into the position of confirming rather than challenging
expectations, which diminished its effect. An alterna-
tive approach in the spirit of constructivism would be to
expose students to the VRLE and then prompt them to
predict the proportions depicted in Fig.1b (and perhaps
Table 1 Post-survey instrument together withour scoring criteria andresults. Individual responses toeach question are
available inAdditional le2
Question Criterion Responses
satisfying
criterion
Total
responses Proportion
satisfying
criterion
1. What was the object of this virtual reality (VR)
experience? User identified the goal of showing the experi-
ence of another species’ visual system 19 26 73%
2. How did this recognition influence your behav-
ior in VR? What strategies did you employ and
why?
User identified their intent to experience differ-
ences in the visual systems 15 26 58%
3. What conclusions can you draw about vision
among other species (for example, tarsiers) from
this VR experience?
User explained that visual systems may differ
between species 24 26 92%
4. How did this VR experience differ from a tradi-
tional classroom experience? User identified aspect(s) preferable to traditional
classroom experience 22 26 85%
5. Does this change the way you think about adap-
tation and vision among different species? User expressed advanced understanding of adap-
tation or visual differences among species 15 19 79%
6. Did the differences between human and tarsier
visions surprise you or match your expectations?
Explain.
User expressed surprise 12 18 67%
7. How might the tarsier’s vision be adaptive or
helpful in its environment? What are its limita-
tions?
User accurately identified ways tarsier vision might
be helpful in low-light, visually busy environ-
ments
15 25 60%
8. What benefit(s) might relatively large eyes
provide for the tarsier? User accurately identified potential advantages of
large eyes 22 25 88%
9. Which would be more effective in low-light
environments: tarsier vision or human vision? User expressed that tarsier vision would be more
effective than human vision in low-light environ-
ments
23 25 92%

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Gochmanetal. Evo Edu Outreach (2019) 12:9
Fig. 2c, which would require them to converge on the
same calculations in the Appendix).
Taken together, we believe that Tarsier Goggles has the
potential for widespread application. It is poised to com-
plement middle and secondary school curricula in optics
and biology; and it is a form of experiential learning that
promotes user reflection. In some cases, reflection is the
express goal of a VR simulation; for example, In the Eyes
of the Animal (http://iteot a.com) is a multisensory artistic
exploration and technical achievement. An advantage of
Tarsier Goggles is that it is designed to be integrated with
educational curricula and is targeted to address specific
scientific concepts. It may even extend into a museum
settings, where people of all backgrounds could enhance
their understanding of optics and natural selection via
technology that might be new or generally unavailable
to them. Further, this VR experience can be expanded
to other senses—some tarsiers enjoy exceptional hear-
ing (Ramsier etal. 2012)—or to other visual systems. For
example, we have experimented with incorporating the
vision of strigiform owls as an example of convergent
evolution with tarsiers (Moritz etal. 2014, 2017). Other
applications could include human visual impairments,
which could further promote greater empathy.
Conclusions
Applications of VR to science education and outreach are
certain to increase greatly over the next years. Here we
developed a VR tool Tarsier Goggles to simulate the visual
sensitivity, acuity, and red-green colorblindness of tarsi-
ers, and the advantages of these traits under dim condi-
tions. We found that user experiences of these traits were
overwhelmingly positive, indicating an improved concep-
tual understanding of natural selection and visual optics.
It also had a strongly reflective effect, with users describ-
ing an evolved outlook on their own perceptual systems
especially in comparison to those of species that they
have not considered before. ese experiences are prom-
ising for future applications in education and personal
use with the potential to cast new light on the world of a
fascinating animal.
Additional les
Additional le1. 2D video progression of the learning environments in
Tarsier Goggles.
Additional le2. Data containing user responses to questions listed in
Table 1. Responses that neglected to answer the question at hand were
omitted from the data.
Additional le3. Lesson plan to accompany Tarsier Goggles.
Abbreviations
3D: three-dimensional; VR: virtual reality; VRLE: vir tual reality learning environ-
ment; 2D: two-dimensional.
Authors’ contributions
SRG and NJD conceived the project and implemented the parameters for
simulating tarsier vision. SRG, NG, SZ, SXG, SP, LG, EKY, KAH, HJS, KC, and SL
designed the environments and user interface. NG, KC, SL, BKC, SP, EKY, and
AW were developers and EL and TT directed design and development opera-
tions. SRG and NJD wrote the paper with contributions from MML, NG, KC, and
SL. All authors read and approved the final manuscript.
Author details
1 Departments of Anthropology and Biological Sciences, Dartmouth College,
Hanover, NH 03755, USA. 2 Digital Arts, Leadership, & Innovation Lab, The Wil-
liam H. Neukom Institute for Computational Science, Department of Com-
puter Sciences, Dartmouth College, Hanover, NH 03755, USA. 3 Kimball Union
Academy, Meriden, NH 03770, USA.
Acknowledgements
We thank the following individuals for technical and practical advice through-
out the course of this project: John Allman, Anna Autilio, Chris Collier, Andy
Cooperman, Lorie Loeb, Theo Obbard, Callum Ross, and Michele Tine.
Competing interests
The authors declare that they have no competing interests.
Availability of data and materials
Tarsier Goggles is available for free at https ://dali-lab.githu b.io/tarsi er. Users
should follow the link to the GitHub page where they can read written instruc-
tions for using the VR and download the zip folder that contains the.exe file
and the data folder, both of which will need to be in the same directory to
function. If the user experiences problems within VR, restart the application.
Funding
Funding was received from the Digital Arts, Innovation, & Leadership Lab
(DALI) at Dartmouth College. Additional funding was received from the Claire
Garber Goodman Fund, Department of Anthropology, Dartmouth College
(Grant to SRG and NJD), the Kaminsky Research Fund, Division of Undergradu-
ate Advising & Research, Dartmouth College (Junior Research Scholarship
to SRG), and The William H. Neukom Institute for Computational Science,
Dartmouth College (Travel Grant to SRG).
Appendix
Scaling eye dimensions
We assumed a spherical eye geometry per Schultz (1940),
and we used the following formula to scale the eye pro-
portions of the tarsier for human dimensions:
where
TDH, S = scaled transverse diameter for human
VH, S = scaled volume for human
VT = volume for tarsier
VH = volume for human
CDH, S = scaled corneal diameter for human
CDT = corneal diameter for tarsier
TDT = transverse diameter for tarsier.
TD
H, S =233
4πVH, S (eye)=23

3
4π
VT(eye)VH(brain)
VT(brain)
=233
4π
(2.03 cc)(
∼1400 cc)
2.14 c
=13.63758 cm
CD
H, S =
CDT
TDT
TDH, S

=
1.59 cm
1.85 cm
(13.63758 cm)=
11.75271 cm

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Received: 9 December 2018 Accepted: 19 February 2019
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