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1. The structural and motivational role of the unique lip‐flip movement in the gelada ( Theropithecus gelada ) facial display repertoire

   https://doi.org/10.1002/ajpa.24031  

   Lazow, Stefanie P.; Bergman, Thore J. (February 2020, American Journal of Physical Anthropology)

   Abstract ObjectivesHuman language represents an extreme form of communicative complexity. Primate facial display complexity, which depends upon facial mobility, can be used as a model for the study of the evolution of communicative complexity. The gelada (Theropithecus gelada) is the only primate that can produce a lip‐flip eversion. This study investigates the role of the lip‐flip relative to the bared‐teeth display to understand its role in generating communicative complexity. Materials and methodsWe reviewed videos of gelada social interactions. We utilized the facial action coding system (FACS) to define structural component action units (AUs) of each display. We inferred display motivation from the behaviors of the display sender. ResultsThe lip‐flip was used only in combination with the essential AUs of the bared‐teeth display, serving as an optional structural element added to produce a structural variant. Both the bared‐teeth display with and without a lip‐flip occurred most frequently with nonaggressive, submissive behaviors. The lip‐flip was more frequently preceded by approach than the bared‐teeth display, especially in males. The lip‐flip was also present in the majority of structurally blended facial displays though the motivation of the non‐lip‐flip parent display often dominated. DiscussionThe lip‐flip may potentially function as an indicator of benign intent after an approach or as an intensifying component of nonaggressive intent. Adaptations to increase facial mobility in geladas via facilitating the lip‐flip may promote increased communicative complexity through increased conspicuousness and motivational signaling specification or intensification. 

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2. Assessing male gelada chest patches: color measurement and physiological mechanisms

   https://doi.org/10.1007/s42991-021-00211-5  

   DeLacey, Patricia M.; Perlman, Rachel F.; Sen, Sharmi; Schneider-Crease, India; Chiou, Kenneth L.; Lemma, Alemayehu; Ayele, Ferehiwot; Higham, James P.; Lu, Amy; Snyder-Mackler, Noah; et al (February 2022, Mammalian Biology)
3. Bridging Captive and Wild Studies: Behavioral Plasticity and Social Complexity in Theropithecus gelada

   https://doi.org/10.3390/ani11103003  

   Palagi, Elisabetta; Bergman, Thore J. (October 2021, Animals)

   Cognitive ethology explores the ability of animals to flexibly adapt their behavior to rapid physical and social environment fluctuations. Although there is a historical dichotomy between field and captive studies, recently, a growing interest in questions that sit at the intersection of cognitive and adaptive perspectives has helped bridge this divide. By focusing on Theropithecus gelada, we discuss the three main reasons why this hybrid approach is extremely successful. First, captive and wild studies provide data at different social, spatial, and temporal scales that can be synthesized to give a fuller picture of the behavior. Secondly, apparently conflicting results from captive and wild settings are powerful tools to explore behavioral flexibility and latent behavioral tendencies. Third, the different settings provide ways of validating and exploring behaviors that are noticed in the other setting. Although we were able to bring together our captive and wild research to demonstrate these ideas, we could have obtained a more integrated vision on the proximate and ultimate gelada behavioral and cognitive strategies if we had considered this hybrid approach from the beginning. We hope that this manuscript stimulates scholars in designing their studies by taking into account the incredible potential of a complementary captive-wild research approach. 

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4. Elemental ratios of carbon and nitrogen track weaning in a graminivorous primate (Theropithecus gelada)

   Reitsema, LJ; Snyder-Mackler, N; Beehner, JC; Bergman, TJ; Lu, A (April 2017, Am J Biol Anthropol)