Comparative Renal Physiology and Excretory Adaptations in Ruminants and Poultry: Implications for Nutrition, Health, and Environmental Management

• This manuscript provides a detailed synthesis of the comparative physiology and functional adaptations of the urinary system in ruminants and poultry. • Highlights the fundamental physiological difference between ruminants primarily excrete nitrogen as urea while birds excrete it as uric acid. • Points to the need for more detailed histological studies of avian kidneys and integrated research linking ruminant nutrition directly to on-farm emission metrics. The urinary systems of ruminants and poultry represent an evolutionary adaptation to distinct ecological niches, digestive strategies, and water conservation needs. This critical review employs a systematic approach to compare these systems, analyzing their anatomical, physiological, and functional divergences with emphasis on contemporary agricultural challenges. Ruminants, as ureotelic mammals, possess a urinary system where nitrogen excretion is dominated by urea, heavily influenced by rumen microbial metabolism and the urea nitrogen recycling loop. This physiology underpins both the use of urinary purine derivatives as markers for microbial protein synthesis and the significant environmental burden of nitrous oxide (N₂O) emissions from urine patches. In contrast, the avian uricotelic system, characterized by a mixed population of loopless (reptilian type) and looped (mammalian type) nephrons and a cloacal terminus, prioritizes water conservation and eggshell calcification but presents unique pathologies like gout. By synthesizing scattered literature, this review provides a novel, integrated framework linking core physiology to applied outcomes in nutrition, disease management, and environmental sustainability. This review identified critical knowledge gaps, particularly in avian renal microanatomy and the precise modeling of dietary interventions on nitrogen loss and proposes specific, actionable research directions to advance sustainable animal production.