Signaling Between Mother and Infant During Lactation: Evolutionary, Anthropological,
Biological, and Practical Considerations
Lactation is an ancient process that long
predated the emergence of mammals or
placentation. It most likely evolved primarily
as a source of fluid and immune factors in our
egg-laying ancestors, with a nutritional role
developing later. Many nutritional components
of milk evolved from immune components. Milk
provides young mammals with nutrition and
immune protection, but also allows “signaling”
between mother and offspring during critical
early developmental phases of life. Lactation
allowed mammals to shape their offspring’s
growth and development over a much longer
period than was possible in egg-laying animals.
Lactation presents a wide range of signaling
options. The mother can exert influence by
providing or limiting access to the breast,
via altered volume and nutrient content
of the milk, and via myriad non-nutritive
components, including hormones, microbes,
oligosaccharides, microRNAs, and cells (eg,
milk stem cells). The infant can signal to the
mother by vocalization, feeding demands,
and non-nutritive suckling. Defining the role
of these signals is complicated, especially in
humans, by the complex inter-relationships
between factors and by methodological issues
with milk sampling, processing, and assays. An
experimental approach, including standardized
protocols for milk sampling and analysis, is
therefore preferable to investigate causality.
Primate lactation is characterized by long
periods of lactation with dilute, low-fat, and
low-protein milk, which has the advantage
of spreading the energy cost of investment
and prolonging the potential signaling period.
However, it also creates greater potential for
conflict between mother and infant, since
lactation is energetically expensive. The
mother needs to invest in all her offspring and
benefits from early weaning and another
pregnancy, while the infant prioritizes itself
over its siblings and benefits from longer
breastfeeding. Lactation is therefore not a
“one-way” process, and conflict should be
anticipated: lowering the tension between
mother and infant may provide benefits for
both parties. Applying an anthropological
perspective, combined with improved
understanding of signaling mechanisms, may
thus help to explain some breastfeeding
problems and define solutions.