Socially transferred materials: why and how to study them.

Animal behavior, physiology, and fitness are influenced by materials transferred between conspecifics (e.g., milk or ejaculate), even in solitary taxa. Such socially transferred materials are currently studied separately, without acknowledgment that they are instigators of evolutionary change and potent mechanisms for indirect genetic effects. Recent methodological advances allow the linkage of socially transferred gene products to their physiological, behavioral, and fitness impacts, yet such integrative approaches are rarely used. This conceptual framing and classification of socially transferred materials bridges proximate and ultimate scales and connects molecular biology to evolution. The concept of socially transferred materials has the potential to bring about transformative innovation in fundamental research and also in applied fields, such as medicine and agriculture. When biological material is transferred from one individual's body to another, as in ejaculate, eggs, and milk, secondary donor-produced molecules are often transferred along with the main cargo, and influence the physiology and fitness of the receiver. Both social and solitary animals exhibit such social transfers at certain life stages. The secondary, bioactive, and transfer-supporting components in socially transferred materials have evolved convergently to the point where they are used in applications across taxa and type of transfer. The composition of these materials is typically highly dynamic and context dependent, and their components drive the physiological and behavioral evolution of many taxa. Our establishment of the concept of socially transferred materials unifies this multidisciplinary topic and will benefit both theory and applications. [ABSTRACT FROM AUTHOR]