Comparative analysis of body composition and bone microarchitecture in wild-type and restricted ovulator hens

Summary: The laying hen biology for egg production reshapes how hens allocate nutrients, often at the expense of their bone integrity. To form an eggshell each day, hens mobilize large amounts of calcium from their bones as well as a portion form the feed, using medullary bones as a temporary reservoir. Although this system sustains continuous egg production, the repeated cycles of resorption and incomplete replenishment of medullary bone progressively weakens structural bones, leaving them thinner, more porous, and prone to fracture. In the current study, restricted ovulator (RO) hens, a non-laying genetic variant of White Leghorns, were used. By comparing non-laying RO hens with their egg-laying wild-type (WT) siblings across major laying phases, we quantified the direct effects of egg production on bone integrity and whole-body composition. Non-laying hens maintained denser cortices, thicker trabeculae, and greater whole-body mineral reserves, while also storing more body fat. However, laying hens diverted resources to egg formation, resulting in limited cortical bone development, loss of trabecular connectivity, and continuous turnover of medullary bone without net gain. Structural bones were affected to varying degrees due to egg production, with the femur being the most vulnerable and keel the least. The effects of egg production extended beyond bone, reshaping whole-body composition toward leaner tissue profiles. In summary, our results emphasize that egg production is the primary driver of structural bone loss and altered body composition in laying hens. Future research should focus on developing better nutritional, genetic, and management strategies to prevent bone fragility and welfare issues in commercial layer production.