Ellis, Reid, and Kramer offer compelling evidence for a two-tiered life history (LH) model in the developmental calibration of puberty and reproduction, distinguishing energetic stress from ambient extrinsic mortality (EM). Their synthesis promises to catalyze a new wave of empirical research. We suggest that one key pathway for advancing this work is to better account for how social context shapes both the experience and expression of energetic stress and EM. Nutrition behavior is not just about energy – it is embedded in human cooperative ecologies and our evolution as social animals. Social context may, therefore, calibrate behavioral and physiological mechanisms through which both tiers shape LH traits.

As Dr. Kramer has argued elsewhere, energy balance in humans is shaped more by cooperative ecology than individual foraging success. In many species, survival and reproduction are tied to an individual’s ability to independently acquire resources. But humans are unusual: food and caregiving responsibilities are widely shared across kin and non-kin, with juveniles, grandparents, and other group members contributing to a common pool of energetic support (Kramer, Reference Kramer2018; Kramer & Ellison, Reference Kramer and Ellison2010). This cooperative structure buffers individuals against scarcity and allows reproductive and developmental strategies to reflect community-level energy flows. Nutrition is deeply social, and when paired with labor sharing and cooperative child rearing, it has profound implications for LH strategies. In fact, cooperative breeding and energetic subsidies from allomaternal care were likely prerequisites for evolving large brains – and with them, the cognitive capacities that characterize slow LH strategies, such as behavioral inhibition, long-term planning, and flexible risk evaluation (Isler & Schaik, Reference Isler and Schaik2014). In highly cooperative groups, limited resources may still be strategically pooled to protect individual nutrition. There may be cases where apparently high energetic stress and EM cues are associated with both faster pubertal timing and fertility, but this could be due to lack of information on the use of social strategies to meet energy needs. Further, Tier 1-related delayed puberty may reflect either extreme deprivation that is insurmountable through cooperation or preemptive social breakdown before scarcity becomes critical. These two paths may not have the same consequences.

Because cooperative sociality has shaped both our survival and our brains, our social ecology itself may cue internal shifts in resource allocation. In social neuroscience, Social Baseline Theory (SBT) posits that the presence of conspecifics is the dominant niche to which humans have adapted to, and thus, bioenergetic spending is lowest in the context of non-threatening conspecifics, but especially in deep, healthy relationships (Beckes & Coan, Reference Beckes and Coan2011). Evidence is mounting that partner presence reduces neural responses to threat, pain, and cognitive load, and is associated with coping strategies and health outcomes (Beckes & Sbarra, Reference Beckes and Sbarra2022). Importantly, these shifts are not mediated solely by effortful control via central executive systems (Beckes et al., Reference Beckes, Medina-DeVilliers and Coan2021), suggesting that ecophysiological mechanisms – such as glucose regulation, cortisol attenuation, or vagal tone – may dynamically calibrate energy allocation in response to social cues. The most dramatic version of this may be the impact of skin-to-skin contact therapy for premature babies, “Kangaroo care,” which increases survival, clinical, and developmental outcomes (Campbell-Yeo et al., Reference Campbell-Yeo, Disher, Benoit and Johnston2015). This is, however, not a formal test of SBT.

How SBT extends to nutrition behavior and development is generally underexplored, but a few findings warrant further research. First, multiple studies indicate that social disconnection increases the consumption of energy-dense foods, hunger hormones, and fasting blood glucose – perhaps in preparation for navigating challenges independently (e.g., Ein-Dor et al., Reference Ein-Dor, Coan, Reizer, Gross, Dahan, Wegener, Carel, Cloninger and Zohar2015; Jaremka et al., Reference Jaremka, Fagundes, Peng, Belury, Andridge, Malarkey and Kiecolt-Glaser2015; Lin et al., Reference Lin, MacCormack, Boker, Coan and Stanton2024). This may be especially impactful in adolescence, where cultural and psychological factors increase social salience and a second phase of neural reorganization and neurochemical sensitivity emerges (Andersen, Reference Andersen2003). Together, this may create opportunities for social and nutritional calibration of pubertal timing (Villamor & Jansen, Reference Villamor and Jansen2016) and LH-relevant traits. Second, the presence of a reliable adult appears to guide better behavioral inhibition and reduce delay discounting in children – behaviors associated with slower life history strategies – but the physiological correlates of those behaviors will depend on their home context (e.g., Sturge-Apple et al., Reference Sturge-Apple, Suor, Davies, Cicchetti, Skibo and Rogosch2016). For a developing body, a cooperative social ecology may signal stability of social resources towards solving problems of energy (Tier 1) and EM (Tier 2). This may allow physiological systems to prospectively reallocate energy for maintenance activities towards growth. For example, shifting energy for persistent threat-vigilance towards growth of the energetically expensive brain and the flexible cognitive tools it provides.

However, much of this research has been conducted in western educated industrialized rich democratic samples, often under constrained laboratory conditions, and with adults. Expanding this work to include ethnographic and biological anthropological methods will be essential for understanding how these mechanisms operate across diverse ecologies and cultural models of cooperation. Evidence from more diverse sources on parental deprivation, pubertal timing, and reproduction absent energetic stress does exist. This research was alluded to in the main paper and expertise in cooperative ecology could not be better represented. The authors wisely chose to focus on energetics and EM for clarity. Still, given the importance of the social ecology to energetics and EM, research will inevitably need to incorporate social context into the two-tiered LH model.

We commend Ellis and colleagues for advancing a framework that invites interdisciplinary synthesis and opens doors towards innovation. Our laboratory has developed a set of studies that examine real-time social and nutritional context under neuroimaging, alongside metabolic panels and assessments of nutrition, social, and physical affordances, and behavioral measures of risk-taking and behavioral inhibition. These studies aim to test how social and nutritional resources shape behavioral traits consistent with LH theory via the ecophysiological mechanisms proposed by SBT. This research was created long before the two-tiered model, inspired by reading across disciplines on human behavioral ecology, as the authors did here. Yet the formalization of the two-tiered LH model has already enriched our thinking as we move to data analysis. No doubt others will be similarly impacted, and we hope that they too consider social context as a missing link between energetic and developmental approaches in life history research.