Quality versus Quantity: Investigating Fundamental Trade-Offs in Social Evolution

• Applicant: Delphine De Moor, Ph.D.
• Subject Area: Evolution, Anthropology ; Sensory and Behavioural Biology
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 567690166

Across many social species, being more socially connected tends to enhance survival and reproductive success. Two key dimensions of social connection are driving these effects: the number of affiliative relationships an individual develops (i.e., relationship quantity) and the strength, stability, and reciprocity of those relationships (i.e., relationship quality). Notably, measures of relationship quantity and quality are not strongly correlated, suggesting that their positive effects on fitness operate independently. This raises a natural question: Can individuals successfully maximise both the quality and quantity of their social relationships, or are they constrained by fundamental trade-offs, compelled to prioritise either a few strong relationships or more numerous, weaker ones? Trade-offs are a cornerstone of evolutionary theory and occur whenever optimising one trait comes at the cost of another due to limited resources or competing functions. Trade-offs can manifest independently across biological levels: in the phenotype and in the genotype, within and across individuals and species. Each of these levels is important, as trade-offs that occur at one level may be masked at another, and because each level has its own evolutionary implications. Within-species trade-offs shape fitness landscapes and the capacity for future evolutionary change; comparisons across species complement this perspective by illustrating how animals have responded to historical selection pressures. Yet, no study to date has systematically investigated the trade-offs that shape the social strategies individuals adopt. Here, I propose a multi-level investigation to fill this gap: First, I will use detailed longitudinal data on female and male social relationships within an intensively studied baboon population to test for quality-quantity trade-offs within and across individuals. This will reveal whether investment in relationship quality comes at the expense of investment in relationship quantity, and whether individuals vary in the extent to which they are constrained by trade-offs. Secondly, I will integrate genetic kinship into these analyses to conduct the first investigation into whether genetic correlations between quality and quantity constrain their evolutionary response to selection. Finally, I will investigate trade-offs across 17 papionin primate species and test how environmental harshness shapes investment in quality versus quantity, by running a systematic comparative analysis of cross-species variation in social strategies using a single, shared analytical framework. Together, these studies will allow me to address fundamental questions about how social relationships evolve: whether relationship quality and quantity represent alternative evolutionary strategies, what predicts investment in one versus the other, and whether genetic constraints influence their ability to respond to natural selection.

• DFG Programme: Research Grants