Do precipitation, temperature, and energy availability affect monthly birth pulses?
After controlling for year of birth, our analytic model shows that NDVI is significantly associated with monthly birth pulses (IRR=15.2; p<.001). In fact, for every one unit increase in NDVI the incident rate for monthly birth pulses increased by a factor of fifteen. However, neither precipitation nor temperature were associated with monthly birth pulses (Table 3). Consistent with population growth over time, we find that monthly birth pulses increased substantially over time (IRR=1.02; p<.001).
Table 3 approximately here
DISCUSSION
Leveraging historical data, here we explored the relationship between birth seasonality and three measures of energy availability (precipitation, temperature, and NDVI) in the physically demanding, hot and arid Baja California peninsula. We find, consistent with the animal ethology and evolutionary ecology literature, that local energy availability evaluated by way NDVI to be positively associated with monthly birth pulses. Although a relationship between precipitation and temperature on monthly birth pulses in humans has been found in past studies (Condon & Scaglion 1982; Lam & Miron 1998; Lawlor et al. 2005; Torche & Corvalan 2010; Osei et al. 2016), here we find that they have no significant effect on birth seasonality; however, both are indirectly related to it through their combined impact on NDVI.
Taken together, our findings suggest that human birth seasonality represents a locally-sensitive adaptive strategy designed to improve child health and survival outcomes and, ultimately, parental reproductive success (Ellison et al. 2005). Much of the Baja California peninsula is a hot, arid desert and, historically, people were dependent on seasonal rains for subsistence and economic livelihoods (Crosby 1981). Given our results, it appears that, at least during the 19th century, parents timed their births to coincide with seasonal variation in local energy availability. We do not suggest that individuals necessarily maintained and followed an explicit cultural rule stating, “give birth in the Fall and Winter”. Rather, we suggest that this patterning emerged by way of a form of embodied traditional ecological knowledge – that is, knowledge that is acquired and transmitted through lived experience within an ecosystem (Berkes et al. 2000). During the time period under study, economic livelihoods were nearly completely tethered to local ecology because of limited infrastructure development. Consequently, individuals were acutely aware of seasonal changes to both climate and energy availability (Crosby 1981). Thus, given similar results across animal taxa, individuals who timed their births to seasonal pulses in energy likely improved their children’s survival prospects (and their own lifetime reproductive success) (Dadvand et al. 2012; Case et al. 2016; Bakhtsiyarava et al. 2018). Today, subsistence ranchers residing in mountain ranges that lack infrastructure in BCS provide anecdotal evidence regarding this phenomenon. The lead author’s (SJM) conversations with women in these populations suggest that while they lack explicit cultural rules regarding the timing of birth, they unequivocally state that having a baby during the September through December growing season, makes life easier. During this time period their livestock gain weight rapidly, provide more milk and meat, and can also be sold in local markets for a better price per animal. Thus, maternal and child condition, as well as household economic wellbeing, are all improved with the advent of the rains.
In industrialized settings, however, humans are increasingly decoupled from the constraints of their local bio-physical ecology as resources are extracted and manufactured using extra-somatic energy sources (e.g., fossil fuels) and distributed through globally integrated market supply chains and (Li & Li 2012; Moran 2018). Under these conditions, in modern, developed populations, cultural institutions that influence social, economic, and reproductive success may have a greater influence on seasonal birth pulses than local energy availability (Dahlberg & Andersson 2018, 2019). For example, evidence from modern Japan show that births spike just after the cut-off date for school enrollment, which causes these children to be the oldest among their school cohort (Shigeoka 2014). Being oldest among an educational cohort promotes educational and economic prospects relative to those who are younger (Davis et al. 1980 Lien et al. 2005; Nolan & Howell 2010; Verachtert et al. 2010). One goal of research in the near future is to adjudicate the relationships between birth pulses, infrastructure development, climate, and energy in BCS by examining modern birth records.
Readers should be mindful of the limitations of this study. First, the data for births emanate from the 19th century, while our data on climate and local energy availability come from the mid-20th through early-21stcenturies. While temporal overlap between variables is ideal, it is impossible to obtain yearly precipitation, temperature, and NDVI data for the 19th century at the scale of individual community. Nonetheless, paleo-climatological, historical, and ethnographic data all indicate relatively similar environmental trends between today and the 19th century. Certainly, that we detect a relationship between seasonal pulses in NDVI and birth patterns supports our analytical approach and suggests that this association may in fact be stronger if our data were temporally matched. Second, as is typical with historical vital records, our data represent a sample of all births that occurred on the peninsula during the 19th century. While missing data can produce biased estimates (Willigan & Lynch 1982), there is no reason to assume that data loss in these vital records was biased in any particular direction. Lastly, the El Niño Southern Oscillation (ENSO) is a significant source of inter-annual variability for both the North American Monsoonal (Barron et al. 2012) and Baja Californian climate (Diaz et al. 2001; Lluch-Cota et al. 2010). Our analyses do not account for these events. During El Niño events, precipitation increases substantially increasing local energy availability, while during La Niña events they decrease (Diaz et al. 2001; Maldonado-Enriquez et al. 2020). Understanding the relationship between ENSO events, precipitation, and monthly birth pulses may shed light on additional sources of traditional ecological knowledge and methods for investigating ENSO events in the historic period. Because monthly birth pulses and NDVI are correlated and ENSO events and NDVI are correlated, it may be possible to identify historical ENSO events by examining inter-annual historical fluctuations in birth pulses, similar to how dendrochronologists use tree-ring width as a proxy for estimating historic ENSO events (Diaz et al. 2001).
In conclusion, we find that seasonal variation in local energy availability predicts birth seasonality across 10 communities in Baja California Sur, Mexico in the 19th century. Although social scientists have alluded to the climatological factors that influence seasonal variation in female energetics driving birth seasonality in humans (Ellison et al. 2005), our analysis represents the first attempt to adjudicate between various measures of energy availability. While past work has used precipitation and temperature as proxies, here we use a more direct measure of energy availability by way of NDVI. In doing so, we clearly show that birth seasonality covaries with green biomass, which is of particular importance to the subsistence livestock producers under study here. While climatological phenomena (precipitation and temperature) were not found to have direct effects on birth seasonality, we find support for their indirect effect through NDVI. Ultimately, our work further highlights the responsiveness of individuals to the biophysical properties of local environments and insight that can be gleaned from a historical and comparative approach.
ACKNOWLEDGEMENTS
Portions of this work was financially supported through the Center for Latin American Studies and the Office for Undergraduate Research at the University of Utah.
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Figure 1. Map of Baja California Sur and the ten communities identified in the text.
Table 1. Descriptive statistics associated birth records by community, organized by number of records in descending order.