Sibling effects on nutritional status: Intersections of cooperation and competition across development

Examination of sibling effects on nutritional status is complicated by siblings being both alloparents and resource competitors, as well as the extensive changes children undergo across development. To evaluate sibling nutritional effects with consideration of these opposing roles, we use an evolutionary framework rooted in human ontogeny and cooperative breeding.

Anthropometric data were collected from 113 Ngandu horticulturalist children (birth-<18 years old).We generated weight-for-age (WAZ), height-for-age (HAZ), and BMI-for-age (BMIZ) z-scores. Nutritional status across developmental risk periods (<2.5, 2.5-<5, 5-<10, and ≥10 years) was assessed using one-way ANOVAs. OLS regression was used to examine interactions between presence of siblings and risk periods.

A considerable percentage of Ngandu children are suffering from stunting, underweight, or caloric deficiencies, but many fall within healthy ranges for WAZ (65%), HAZ (60%), and BMIZ (88%). Siblings have significant impacts on children's nutritional status, but their impacts vary by risk period. In general, older siblings (≥5 years old) have positive effects on their same-aged siblings, while younger siblings (<5) negatively affect those between 2.5 and <10 years old.

Closely aged siblings do not always represent competitors. In this ecocultural context, those aged ≥5 years old appear to cooperate to improve their siblings' nutritional status. By evaluating the effects of siblings during specific risk periods, we offer a strategy to untangle the intersections of cooperation and competition. Our results add to the growing body of evidence showing benefits to allocare and further demonstrate that even children target their assistance toward vulnerable periods in development.