Part 1. Introduction -- Chapter 1. Why is it Important to Continue Studying the Anatomy, Physiology, Sensory Ecology, and Evolution of Howler Monkeys? -- Part 2. Taxonomy, Genetics, Morphology and Evolution -- Chapter 2. Fossil Alouattines and the Origins of Alouatta: Craniodental Diversity and Interrelationships.- Chapter 3. The Taxonomy of Howler Monkeys: Integrating Old and New Knowledge from Morphological and Genetic Studies -- Chapter 4. Cytogenetics of Howler Monkeys -- Chapter 5. Hybridization in Howler Monkeys: Current Understanding and Future Directions -- Chapter 6. Morphology of Howler Monkeys: A Review and Quantitative Analyses -- Part 3. Physiology -- Chapter 7. Hematology and Serum Biochemistry in Wild Howler Monkeys -- Chapter 8. Endocrinology of Howler Monkeys: Review and Directions for Future Research -- Chapter 9. The Howler Monkey as a Model for Exploring Host-Gut Microbiota Interactions in Primates -- Chapter 10. Ecological Determinants of Parasitism in Howler Monkeys -- Part 4. Ontogeny and Sensory Ecology -- Chapter 11. An Ontogenetic Framework for Alouatta: Infant Development and Evaluating -- Chapter 12.The Sensory Systems of Alouatta: Evolution with an Eye to Ecology -- Chapter 13. Production of Loud and Quiet Calls in Howler Monkeys -- Chapter 14. Function of Loud Calls in Howler Monkeys -- Part 5. Conclusions -- Chapter 15. New Challenges in the Study of Howler Monkey Anatomy, Physiology, Sensory Ecology, and Evolution: Where we are and where we need to go?.