Olsen, AM, Kristensen HG, Iversen KW, Pedersen NH, Pertoldi C, Alstrup AK, Jensen TH, Pagh S. Assessment of Abnormal Behaviour and the Effect of Enrichment on Captive Chimpanzees in Aalborg Zoo. Genet Biodivers J. 2020;4(2):73-91. https://doi.org/10.46325/gabj.v4i2.99

Ottosen MP, Kjaer JW, Andersen M, Bach LA, Pagh S. Chimpanzees Behavioral Response to a Hift in Number of Visitors during Autumn Holiday in Aalborg Zoo. Genet Biodivers J. 2020;4(2):1-60. https://doi.org/10.46325/gabj.v4i2.97

Mellen J, MacPhee MS. Philosophy of Environmental Enrichment: Past, Present, and Future. Zoo Biol. 2001;20(3):211-26. https://doi.org/10.1002/zoo.1021

Puteri S, Ruppert N. Effect of Environmental Enrichment on Activities of Captive Orangutans at Taiping Zoo, Malaysia. Malayan Nat J. 2017;69(4):327-35.

Spillmann B, Van Noordwijk MA, Willems E, Mitra Setia T, Wipfli U, Van Schaik CP. Validation of an Acoustic Location System to Monitor Bornean Orangutan (Pongo pygmaeus wurmbii) Long Calls. Am J Primatol. 2015;77(7):767-76. https://doi.org/10.1002/ajp.22398

Russon AE. The Nature and Evolution of Intelligence in Orangutans (Pongo pygmaeus). Primates. 1998;39(4):485-503. https://doi.org/10.1007/BF02557571

Russon AE, van Schaik VO, Kuncoro P, Ferisa, A, Handayani DP, van Noordwijk MA. Inoovation and Intelligence in Orangutans. In: Wich AS, Atmoko SU, Setia TM, van Schaik CP, editors. Orangutans: Geographic Variation in Behavioral Ecology and Conservation. New York: Oxford UP; 2010. p. 279-98.

Bucken SM, Gürtler W-D. Vergesellschaftung als Verhaltensbereicherung im Zoo - Soziale Interaktion und Raumnutzung bei Orang-Utans, Hulmans und Kurzkrallenottern in der Zoom Erlebniswelt Gelsenkirchen. Zool Garten. 2013;82(1-2):40-59. https://doi.org/10.1016/j.zoolgart.2013.04.004

Delgado RA, Van Schaik CP. The Behavioral Ecology and Conservation of the Orangutan (Pongo pygmaeus): A Tale of two Islands. Evol Anthropol. 2000;9(5):201-18. https://doi.org/10.1002/1520-6505(2000)9:5<201::AID-EVAN2>3.0.CO;2-Y

Dingemanse NJ, Wolf M. Between-Individual Differences in Behavioural Plasticity within Populations: Causes and Consequences. Anim Behav. 2013;85(5):1031-9. https://doi.org/10.1016/j.anbehav.2012.12.032

Fröhlich M, van Schaik CP, van Noordwijk MA, Knief U. Individual Variation and Plasticity in the Infant-Directed Communication of Orangutan Mothers. Roc R Soc B: Biol Sci. 2022;289(1975):1-10. https://doi.org/10.1098/rspb.2022.0200

Dingemanse NJ, Kazem AJ, Réale D, Wright J. Behavioural Reaction Norms: Animal Personality meets Individual Plasticity. Trends Ecol Evol. 2010;25(2):81-9. https://doi.org/10.1016/j.tree.2009.07.013

Condon E, Wehnelt S. The Effect of an Enriched Environment on Behavioural and Hormonal Indicators of Welfare in Oran-utans at Chester Zoo. In: Gilbert TC, editor. Proceedings of The Fifth Annual Symposium on Zoo Research. UK: Federation of Zoological Gardens of Great Britain and Ireland; 2003. p. 53-8.

Elder CM, Menzel CR. Dissociation of Cortisol and Behavioral Indicators of Stress in an Orangutan (Pongo pygmaeus) During a Computerized Task. Primates. 2001;42(4):345-57. https://doi.org/10.1007/BF02629625

Amrein M, Heistermann M, Weingrill T. The Effect of Fission-Fusion Zoo Housing on Hormonal and Behavioral Indicators of Stress in Bornean Orangutans (Pongo pygmaeus). Int J Primatol. 2014;35(2):509-28. https://doi.org/10.1007/s10764-014-9765-5

Claxton AM. The Potential of the Human-animal Relationship as an Environmental Enrichment for the Welfare of Zoo-housed Animals. Appl Anim Behav Sci. 2011;133(1-2):1-10. https://doi.org/10.1016/j.applanim.2011.03.002

Finch K, Waterman JO, Cowl VB, Marshall A, Underwood L, Williams LJ, Davis N, Holmes L. Island Life: Use of Activity Budgets and Visibility to Evaluate a Multi-Species Within-Zoo Exhibit Move. Animals. 2022;12(16):2123. https://doi.org/10.3390/ani12162123

Ross SR, Wagner KE, Schapiro SJ, Hau J, Lukas KE. Transfer and Acclimatization Effects on the Behavior of Two Species of African Great Ape (Pan troglodytes and Gorilla gorilla gorilla) Moved to a Novel and Naturalistic Zoo Environment. Int J Primatol. 2011;32(1):99-117. https://doi.org/10.1007/s10764-010-9441-3

Pearson EL, Davis JM, Litchfield CA. A Case Study of Orangutan and Siamang Behavior Within a Mixed-Species Zoo Exhibit. J Appl Anim Welf Sci. 2010;13(4):330-46. https://doi.org/10.1080/10888705.2010.507125

Probst C, Matschei C. Mixed-species Exhibits With Mammals in Central European Zoos. Int Zoo News. 2008;55(6):324-47.

Hardie SM. Exhibiting Mixed-species Groups of Sympatric Tamarins Saguinus spp at Belfast Zoo. Int Zoo Yearb. 1997;35(1):261-6. https://doi.org/10.1111/j.1748-1090.1997.tb01218.x

Birke L. Effects of Browse, Human Visitors and Noise on the Behaviour of Captive Orangutans. Anim Welf. 2002;11(2):189-202. https://doi.org/10.1017/S0962728600028141

Hosey G. A Preliminary Model of Human-animal Relationships in the Zoo. Appl Anim Behav Sci. 2008;109(2-4):105-27. https://doi.org/10.1016/j.applanim.2007.04.013

Schapiro SJ, Bloomsmith MA, Laule GE. Positive Reinforcement Training as a Technique to Alter Nonhuman Primate Behavior: Quantitative Assessments of Effectiveness. J Appl Anim Welf Sci. 2003;6(3):175-87. https://doi.org/10.1207/S15327604JAWS0603_03

Maggioncalda AN, Weber B, Lester B, Schaefer B, Sodaro C, Cassella CM, Mallar C, Cossaboon C, Demitros C, Fogarty D, Schmidt D, Bredahl D, Frank E, Lyon E, Watts E, Singleton I, Porton I, Rafert JW, Franklin JA, Mellen J, Smith J, Killmar KS, Mayo LK, Cocks L, Perkins L, MacPhee M, Fernandez M, Elder M, Bond M, Bastian M, Sahw M, Czekala N, Lung N, Greenbaltt N, Boutelle S, Walker S, Husband S. Orangutan (Pongo) Care Manual [Internet]. Silver Spring; Assn Zoos Aq; 2017 cited 2023. 141 p. Available from: https://ams.aza.org/iweb/upload/Orangutan%20Care%20Manual%20DRAFT-0a71e9d2.pdf

Dezecache G, Bourgeois A, Bazin C, Schlenker P, Chemla E, Maille A. Orangutans’ Comprehension of Zoo Keepers Communicative Signals. Animals. 2019;9(6):300. https://doi.org/10.3390/ani9060300

Perea-García JO, Miani A, Alstrup AK, Malmkvist J, Pertoldi C, Jensen TH, Nielsen RK, Hansen DW, Bach LA. Orangulas: Effect of Scheduled Visual Enrichment on Behavioral and Endocrine Aspects of a Captive Orangutan (Pongo pygmaeus). J Zoo Aquac Res. 2020;8(1):67-72. https://doi.org/10.19227/jzar.v8i1.416

Zar JH. Biostatistical Analysis. 5th ed. New Jersey: Pearson; 2010. 944 p.

Gerstman BB. Basic Biostatistics. 2nd ed. Burlington: Jones & Bartlett Learning; 2014. 648 p.

Hastie T, Tibshirani R, Friedman J. The Elements of Statistical Learning: Data Mining, Inference, and Prediction. New York: Springer; 2009. 745 p.

Mery F, Burns JG. Behavioural Plasticity: An Interaction Between Evolution and Experience. Evol Ecol. 2009;24(3):571-83. https://doi.org/10.1007/s10682-009-9336-y

Rampon C, Tsien JZ. Genetic Analysis of Learning Behavior-Induced Structural Plasticity. Hippocampus. 2000;10(5):605-9. https://doi.org/10.1002/1098-1063(2000)10:5<605::AID-HIPO11>3.0.CO;2-3

Chapillon P, Patin V, Roy V, Vincent A, Caston J. Effects of Pre- and Postnatal Stimulation on Developmental, Emotional, and Cognitive Aspects in Rodents: A Review. Dev Psychobiol. 2002;41(4):373-87. https://doi.org/10.1002/dev.10066

Anderson JR, Ang MY, Lock LC, Weiche I. Nesting, Sleeping and Nighttime Behaviors in Wild and Captive Great Apes. Primates. 2019;60(4):321-32. https://doi.org/10.1007/s10329-019-00723-2

Dunn AJ, Berridge CW. Physiological and Behavioral Responses to Corticotropin-Releasing Factor Administration: is CRF a Mediator of Anxiety or Stress Responses? Brain Res Rev. 1990;15(2):71-100. https://doi.org/10.1016/0165-0173(90)90012-D

Sherwen SL, Hemsworth PH. The Visitor Effect on Zoo Animals: Implications and Opportunities for Zoo Animal Welfare. Animals. 2019;19(6):366. https://doi.org/10.3390/ani9060366

Dall SR, Houston AI, McNamara JM. The Behavioural Ecology of Personality: Consistent Individual Differences from an Adaptive Perspective. Ecol Lett. 2004;7(8):734-9. https://doi.org/10.1111/j.1461-0248.2004.00618.x

Sih A, Bell A, Johnsen JC. Behavioral Syndromes: An Ecological and Evolutionary Overview. Trends Ecol Evol. 2004;19(7):372-8. https://doi.org/10.1016/j.tree.2004.04.009

Réale D, Dingemanse NJ. Personality and Individual Social Specialisation. In: Székely T, Moore AJ, Komdeur J, editors. Social Behaviour: Genes, Ecology and Evolution. UK: Cambridge UP; 2010. p. 417-40.