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.
REFERENCES
Baegert, J.J. (1952). Observations in Lower California .
Translation, Introduction, and Notes by M.M. Brandenberg and C.L.
Baumann. University of California Press, Berkeley, CA.
Bakhtsiyarava, M., Grace, K., & Nawrotzki, R.J. (2018). Climate, birth
weight, and agricultural livelihoods in Kenya and Mali. Am J
Public Health 108 , S144-S150.
Barron, J.A., Metcalfe S.E., & Addison J.A. (2012). Response of the
North American monsoon to regional changes in ocean surface temperature.Paleooceanography 27 , PA3206.
Berkes, F., Colding, J., & Folke, C. (2000). Rediscovery of traditional
ecological knowledge as adaptive management. Ecol Appl10 , 1251-1262.
Bobak, M. & Gjonca, A. (2001). The seasonality of live births is
strongly influenced by socio-demographic factors. Hum Reprod16 , 1512-1517.
Bronson, F.H. (1995). Seasonal variation in human reproduction:
environmental factors. Q Rev Biol 70 , 141-164.
Casey, J.A., James, P., Rudolph, K.E., Wu, C.D., & Schwartz, B.S.
(2016). Greenness and birth outcomes in a range of Pennsylvania
communities. Int J Env ResPub He 13 , 311.
Clavigero, F.J. (1937). The History of [Lower] California .
translated by Sara E. Lake and A.A. Gray. Stanford University Press,
Stanford, CA.
Condon, R.G., & Scaglion, R. (1982). The ecology of human birth
seasonality. Hum Ecol 10 , 495-511.
Cowgill, U.M. (1965). Season of birth in man, contemporary situation
with special reference to Europe and the Southern Hemisphere.Ecology 47 , 614-623.
Crosby, H.W. (1981). Last of the Californios . Copley Books, La
Jolla, CA.
Crosby, H.W. (1994). Antigua California: Mission and Colony on the
Peninsular Frontier, 1697-1768 . University of New Mexico Press,
Albuquerque, NM.
Dadvand, P., Sunyer, J., Basagaña, X., Ballester, F., Lertxundi, A.,
Fernandez-Somoano, A., et al. (2012). Surrounding greenness and
pregnancy outcomes in four Spanish birth cohorts. Environ Health
Persp 129 , 1481-1487.
Dahlberg, J., & Andersson, G. (2018). Changing seasonal variation in
births by sociodemographic factors: A population-based register study.Hum Reprod Open 2018(4) , hoy015.https://doi.org/10.1093/hropen/hoy015
Dahlberg, J., & Andersson, G. (2019). Fecundity and human birth
seasonality in Sweden: A register-based study. Reprod Health16 , 87.
Davis, B.G., Trimble, C.S., & Vincent, D.R. (1980). Does age of
entrance affect school achievement? Elem Sch J80 ,133-143.
de Grenade, R., G.P. Nabhan, & M. Cariño Olvera. 2016. Oases of the
Baja California peninsula as sacred spaces of agrobiodiversity
persistence. Agriculture and Human Values 33:455-474.
Des Lauriers, M.R., Davis, L.G., Turnbull III, J., Southon, J.R.,
&Taylor, R.E. (2017). The earliest shell fishhooks from the Americas
reveal fishing technology of Pleistocene maritime foragers. Am
Antiquity 82 , 498-516.
Diaz, S.C., Touchan, R., & Swetnam, T.W. (2001). A tree-ring
reconstruction of past precipitation for Baja California Sur, Mexico.Int J Climatol 21 , 1007-1019.
Didan, K. (2015). MOD13Q1 MODIS/Terra Vegetation Indices 16-Day L3
Global 250m SIN Grid V006. NASA EOSDIS Land Processes
DAAC. https://doi.org/10.5067/MODIS/MOD13Q1.006
Drent, R.H., & Daan, S. (1980). The prudent parent – energetic
adjustments in avian breeding. Ardea 68 , 225-252.
Forchhammer, M.C., Post, E., & Stenseth, N.C. (1998). Breeding
phenology and climate. Nature 391 , 29-30.
Fujita, H. (1995). Prehistoric coastal adaptations in the Cape Region,
Baja California Sur. Pac Coast Archaeol Soc Q 31 , 4-19.
Galor, O. (2012). The demographic transition: Causes and consequences.Cliometrica 6 , 1-28.
Gerhard, H. E., & Guilick, P. (1964). Lower California Guidebook:
A Descriptive Traveler’s Guide . The Arthur H. Clark Company, Glendale,
CA.
Gilbert, J.P. (2019). Temperature directly and indirectly ifluences food
web structure. Sci Rep UK 9 , 5312.
Hamel, S., Garel, M., Festa-Bianchet, M., Gailard, J.M., & Côté, S.D.
(2009). Spring Normalized Difference Vegetation Index (NDVI) predicts
annual variation in timing of peak faecal crude protein in mountain
ungulates. J Appl Ecol 46 , 582-589.
Hastings, J.R., & Turner, R.M. (1965). Precipitation regimes in Baja
California, Mexico. Geogr Ann A 47 , 204-223.
Herteliu, C., Ileanu, B., Ausloos, M., & Rotundo, G. (2015). Effect of
religious rules on time of conception in Romania from 1905 to 2001.Hum Reprod 30 , 2202-2214.
Instituto Nacional de Estadística, Geografía e Informática. 2020.Archivo Histórico de Localidades Geoestadísticas . Available at:https://www.inegi.org.mx/app/geo2/ahl/default.html#Catalogos_predefinidos.
Last Accessed 15, May 2020.
Kaplan, H. (1996). A theory of fertility and parental investment in
traditional and modern human societies. Am J Phys Anthropol101 , 91-135.
Kaplan, H., Hill, K., Lancaster, J. & Hurtado, A.M. (2000). Theory of
human life history evolution: diet, intelligence, and longevity.Evol Anthropol 9 , 156–185.
Kaplan, H., Hill, K., Hurtado, A.M., & Lancaster, J. (2001). The
embodied capital theory of human evolution. In: Reproductive
Ecology and Human Evolution , [ed. Ellison, P.T.]. Aldine
Transaction, New Jersey. Pp. 293-318.
Kennish, R. (1997). Seasonal patterns of food availability: influences
on the reproductive output and body condition of the herbivorous crabGrapsus albolineatus . Oecologia 109 , 209-218.
Krutch, J.W. (1986). The Forgotten Peninsula: A Naturalist in Baja
California . University of Arizona Press, Tucson, AZ.
Lam, D.A., & Miron J.A. (1994) Global patterns of seasonal variation in
human fertility. Ann NY Acad Sci 709 , 9-28.
Lam, D.A., & Miron J.A. (1996). The effects of temperature on human
fertility. Demography 33 , 291-305.
Lawlor, D.A., Leon, D.A., & Smith, G.D. (2005). The association of
ambient outdoor temperature throughout pregnancy and offspring
birthweight: findings from the Aberdeen Children of the 1950scohort. Obstet Gynecol 112 , 647-657.
Li, W., & Li, Y. (2012). Managing rangeland as a complex system: How
government interventions decouple social systems from ecological
systems. Ecol Soc 17 , 9.
Lien, L., Tambs, K., Oppedal, B., Heyeradahl, S., & and Bjertness, E.
(2005). Is relatively young age within a school year a risk factor for
mental health problems and poor school performance? A population-based
cross-sectional study of adolescents in Oslo, Norway. BMC Public
Health 5 , 102.
Lindsey, R. (2009). Climate and Earth’s energy budget. NASA Earth
Observatory. Available at:https://earthobservatory.nasa.gov/features/EnergyBalance/page1.php.
Last Accessed on 8 July 2020.
Lluch-Cota, S., Parés-Sierra, A., Magaña-Rueda, V.O., Arreguín-Sánchez,
R., Bazzino, G., Herrera-Cervantes, H., & Lluch-Belda, D. (2010).
Changing climate in the Gulf of California. Prog Oceanogr87 , 114-126.
Macfarlan, S.J., &
Henrickson,
C.N. (2010) Inferring relationships between indigenous Baja California
Sur and Seri/Comcáac populations through cultural traits. J Calif
Gt Basin Anthropol 30 , 51-68.
Macfarlan, S.J., Schacht, R., Foley, C., Cahoon, S., Osusky, G., Vernon,
K.B., et al. (2020). Marriage dynamics in old Lower California:
Ecological constraints and reproductive value in an arid peninsular
frontier. Biodemogr Soc Biol 65 , 156-171.
Maldonado-Enriquez, D., Ortega-Rubio, A., Breceda-Solís Cámara, A.M.,
Díaz-Castro, S.C., Sosa-Ramírez, J., & Martínez-Rincón, R.O. (2020).
Trend and variability of NDVI of the main vegetation types in the Cape
Region of Baja California Sur. Rev Mex Biodivers 91 ,
e913213.
Martinez, P.L. (1960). A History of Lower California . Trans. E.
D. Turner. Editorial Baja California, La Paz, BCS.
Martinez, P.L. (1965). Guía Familiar de Baja California:
1700-1900 . Instituto Sudcaliforniano de Cultural, La Paz, BCS.
Moran, E. (2018). Human Adaptability: An Introduction to
Ecological Anthropology . 3rd Edition. Routledge, New
York, NY.
Nolan, J.E., & Howell, G. (2010). Hockey success and birth date: The
relative age effect revisited. Int Rev Sociol Sport 45 ,
507-512.
Plard, F., Gaillard, J.M., Coulson, T., Hewison, A.J.M., Delorme, D.,
Warnant, C., & Bonenfant, C. (2014). Mismatch between birth date and
vegetation phenology slows the demography of Roe Deer. PLoS Biol12 , e1001828.
Platt, L.D. (1998). Census Records for Latin America and the
Hispanic United States . Genealogic Publishing Company, Baltimore, MD.
Rebman, J.P., & Roberts, N.C. (2012). Baja California Plant Field
Guide . 3rd Ed. Sunbelt Publication, San Diego, CA.
Rubenstein, D.R., & Wikelski, M. (2003). Seasonal changes in food
quality: a proximate cue for reproductive timing in marine iguanas.Ecology 84 , 3013-3203.
Ruiz, J.A., Diaz, G., Reza, R., Serrano, V., & Medina, G. (2006).Estadisticas climatológicas básicas del estado de Baja California
Sur (Período 1961-2003) . Libro Técnico Núm. 2. INIFAP-CIRNO: Ciudad
Obregon, Sonora, México.
ORNL DAAC. (2018a). MODIS and VIIRS Land Products Global Subsetting and
Visualization Tool. ORNL DAAC, Oak Ridge, Tennessee, USA. Accessed July
10, 2020. Subset obtained for MOD13Q1 product at 23.072N,109.74W, time
period: 2001-01-01 to 2019-12-31, and subset size: 6.25 x 6.25
km. https://doi.org/10.3334/ORNLDAAC/1379
ORNL DAAC. (2018b). MODIS and VIIRS Land Products Global Subsetting and
Visualization Tool. ORNL DAAC, Oak Ridge, Tennessee, USA. Accessed July
10, 2020. Subset obtained for MOD13Q1 product at 23.4495N,110.2261W,
time period: 2001-01-01 to 2019-12-31, and subset size: 6.25 x 6.25
km. https://doi.org/10.3334/ORNLDAAC/1379
ORNL DAAC. (2018c). MODIS and VIIRS Land Products Global Subsetting and
Visualization Tool. ORNL DAAC, Oak Ridge, Tennessee, USA. Accessed July
10, 2020. Subset obtained for MOD13Q1 product at 23.8098N,110.057W, time
period: 2001-01-01 to 2019-12-31, and subset size: 6.25 x 6.25
km. https://doi.org/10.3334/ORNLDAAC/1379
ORNL DAAC. (2018d). MODIS and VIIRS Land Products Global Subsetting and
Visualization Tool. ORNL DAAC, Oak Ridge, Tennessee, USA. Accessed July
10, 2020. Subset obtained for MOD13Q1 product at 23.4794N,109.7189W,
time period: 2001-01-01 to 2019-12-31, and subset size: 6.25 x 6.25
km. https://doi.org/10.3334/ORNLDAAC/1379
ORNL DAAC. (2018e). MODIS and VIIRS Land Products Global Subsetting and
Visualization Tool. ORNL DAAC, Oak Ridge, Tennessee, USA. Accessed July
10, 2020. Subset obtained for MOD13Q1 product at 24.14N,110.46W, time
period: 2001-01-01 to 2019-12-31, and subset size: 6.25 x 6.25
km. https://doi.org/10.3334/ORNLDAAC/1379
ORNL DAAC. (2018f). MODIS and VIIRS Land Products Global Subsetting and
Visualization Tool. ORNL DAAC, Oak Ridge, Tennessee, USA. Accessed July
10, 2020. Subset obtained for MOD13Q1 product at 26.0152N,111.363W, time
period: 2001-01-01 to 2019-12-31, and subset size: 6.25 x 6.25
km. https://doi.org/10.3334/ORNLDAAC/1379
ORNL DAAC. (2018g). MODIS and VIIRS Land Products Global Subsetting and
Visualization Tool. ORNL DAAC, Oak Ridge, Tennessee, USA. Accessed July
10, 2020. Subset obtained for MOD13Q1 product at 26.0593N,111.8232W,
time period: 2001-01-01 to 2019-12-31, and subset size: 6.25 x 6.25
km. https://doi.org/10.3334/ORNLDAAC/1379
ORNL DAAC. (2018h). MODIS and VIIRS Land Products Global Subsetting and
Visualization Tool. ORNL DAAC, Oak Ridge, Tennessee, USA. Accessed July
10, 2020. Subset obtained for MOD13Q1 product at 26.8899N,111.983W, time
period: 2001-01-01 to 2019-12-31, and subset size: 6.25 x 6.25
km. https://doi.org/10.3334/ORNLDAAC/1379
ORNL DAAC. (2018i). MODIS and VIIRS Land Products Global Subsetting and
Visualization Tool. ORNL DAAC, Oak Ridge, Tennessee, USA. Accessed July
10, 2020. Subset obtained for MOD13Q1 product at 27.283N,112.8967W, time
period: 2001-01-01 to 2019-12-31, and subset size: 6.25 x 6.25
km. https://doi.org/10.3334/ORNLDAAC/1379
ORNL DAAC. (2018j). MODIS and VIIRS Land Products Global Subsetting and
Visualization Tool. ORNL DAAC, Oak Ridge, Tennessee, USA. Accessed July
10, 2020. Subset obtained for MOD13Q1 product at 27.337N,112.284W, time
period: 2001-01-01 to 2019-12-31, and subset size: 6.25 x 6.25
km. https://doi.org/10.3334/ORNLDAAC/1379
Osei, E., Agbemefle, I., Kye-Duodu, G., Binka, F.N. (2016). Linear
trends and seasonality of births and perinatal outcomes in Upper East
Region, Ghana from 2010-2014. BMC Pregnancy and Childb16 , 48.
Pettorelli, N. The Normalized Difference Vegetation Index . Oxford
University Press, Oxford, UK.
Salinas Zavala, C. A., Leyva Contreras, A., Lluch Belda, D., & Diaz
Rivera, E. (1990). Distribución geográfica y variabilidad climática de
los regimens pluciomeétricos en Baja California Sur, México.Atmosfera 3 , 217-237.
Secretaría de Desarrollo Social. (2020). Catálogo de Localidades.
Available at:
http://www.microrregiones.gob.mx/catloc/Default.aspx?tipo=clave&campo=mun&valor=03.
Last Accessed 15 May 2020.
Shigeoka, H. (2014). School entry cutoff date and the timing of births.
NBER Working Paper No. 21402 NBER. Available at
SSRN: http://dx.doi.org/10.2139/ssrn.2297711.
Sinclair, A.R.E. (2000). What determines phenology and synchrony of
ungulate breeding in Serengeti? Ecology 81 , 2100-2111.
StataCorp. (2019). Stata Statistical Software: Release 16 .
StataCorp LLC, College Station, TX.
Torche, F., & Corvalan, A. (2010) Seasonality of birth weight in Chile:
Environmental and socioeconomic factors. Ann Epidemiol20 , 818-826.
Trejo Barajas, D. (1994). La Población de La California peninsular en el
siglo XIX. In Población y Grupos de Poder en La Península de Baja
California , [ed. Trejo Barajas D & Landavazo Arias, M.A.].
Universidad Autónoma de Baja California Sur, La Paz, BCS, Mexico. pp.
14-69
Trejo Barajas, D. (2005). Declinación y crecimiento demográfico en baja
California, siglos XVIII y XIX. Una perspectiva desde los censos y
padrones locales. Hist Mexicana 54 , 761-831.
Trejo Barajas, D., & González Cruz, E. (2002). Historia General
de Baja California Sur: I. La Economía Regional . Universidad Autónoma
de Baja California Sur, La Paz, BCS, Mexico.
Verachtert, P., De Fraine, B., Onghena, P., & Ghesquiere, P. (2010).
Season of birth and school success in the early years of primary
education. Oxford Rev Edu 36 , 285-306.
Villerme, P. (1831). De la distribution par mois des conceptions.Ann. Hyg. Publ. Industr. Soc. 5 , 55-155.
Willigan, J.D., & Lynch, K.A. (1982). Sources and Methods of
Historical Demography . Academic Press, New York.
Wittemyer, G., Rasmussen, H.B., & Douglas-Hamilton, I. (2007). Breeding
phenology in relation to NDVI variability in free-ranging African
elephants. Ecography 31 , 42-50.
Wu, Z., Dijkstra, P., Koch, G.W., Penuelas, J., & Hungate, B.A. (2011).
Responses of terrestrial ecosystems to temperature and precipitation
change: a meta-analysis of experimental manipulation. Glob Change
Biol 17 , 927-942.
Yan, H., Liang, C., Li, Z. Liu, Z., Miao, B., He, C., & Sheng, L.
(2015). Impact of precipitation patterns on biomass and species richness
of annuals in a dry steppe. PLoS One 10 , e0125300.
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.