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?.