Energetic costs of testosterone in two subsistence populations.

OBJECTIVE: Testosterone plays a role in mediating energetic trade-offs between growth, maintenance, and reproduction. Investments in a high testosterone phenotype trade-off against other functions, particularly survival-enhancing immune function and cellular repair; thus only individuals in good condition can maintain both a high testosterone phenotype and somatic maintenance. While these effects are observed in experimental manipulations, they are difficult to demonstrate in free-living animals, particularly in humans. We hypothesize that individuals with higher testosterone will have higher energetic expenditures than those with lower testosterone. METHODS: Total energetic expenditure (TEE) was quantified using doubly labeled water in n = 40 Tsimane forager-horticulturalists (50% male, 18-87 years) and n = 11 Hadza hunter-gatherers (100% male, 18-65 years), two populations living subsistence lifestyles, high levels of physical activity, and high infectious burden. Urinary testosterone, TEE, body composition, and physical activity were measured to assess potential physical and behavioral costs associated with a high testosterone phenotype. RESULTS: Endogenous male testosterone was significantly associated with energetic expenditure, controlling for fat free mass; a one standard deviation increase in testosterone is associated with the expenditure of an additional 96-240 calories per day. DISCUSSION: These results suggest that a high testosterone phenotype, while beneficial for male reproduction, is also energetically expensive and likely only possible to maintain in healthy males in robust condition.

The influence of age- and sex-specific labor demands on sleep in Namibian agropastoralists.

OBJECTIVE: To understand the basics of sleep quality in a pastoralist population and to explore predictors of this variation. DESIGN: Cross-sectional. SETTING: Northern Namibia, dry seasons of 2016 and 2017. PARTICIPANTS: The Himba, a nonindustrial seminomadic agropastoralist population without access to the electrical grid. MEASUREMENTS: Using Actiwatch-2 devices, 75 participants completed 721 days of wear. We calculated sleep duration, efficiency, and activity counts before and after sunset/sunrise and onset/offset. Participants were also interviewed about sleeping arrangements and nighttime disruptions. RESULTS: Himba show lower sleep duration and efficiency than other populations studied, and men had substantially lower duration and efficiency than women. Sex differences were not attenuated when napping was included with total sleep time. Age predicted longer sleep duration and lower evening and morning activity levels. Number of adult co-sleepers predicted increased sleep duration and efficiency in women. Livestock disturbance was not a commonly reported cause of nighttime waking. CONCLUSIONS: These findings support predictions that pastoralist groups may have lower sleep quality than other subsistence populations, but this does not appear to be a consequence of noise from livestock. Instead, lower sleep quality appears to be the result of subsistence and social activities, particularly for men and young adults overall.

Sleep variability and nighttime activity among Tsimane forager-horticulturalists.

OBJECTIVES: A common presumption in sleep research is that "normal" human sleep should show high night-to-night consistency. Yet, intra-individual sleep variation in small-scale subsistence societies has never been studied to test this idea. In this study, we assessed the degree of nightly variation in sleep patterns among Tsimane forager-horticulturalists in Bolivia, and explored possible drivers of the intra-individual variability. METHODS: We actigraphically recorded sleep among 120 Tsimane adults (67 female), aged 18-91, for an average of 4.9 nights per person using the Actigraph GT3X and Philips Respironics Actiwatch 2. We assessed intra-individual variation using intra-class correlations and average deviation from each individual's average sleep duration, onset, and offset times ( ɛ¯). RESULTS: Only 31% of total variation in sleep duration was due to differences among different individuals, with the remaining 69% due to nightly differences within the same individuals. We found no statistically significant differences in Tsimane sleep duration by day-of-the-week. Nightly variation in sleep duration was driven by highly variable sleep onset, especially for men. Nighttime activities associated with later sleep onset included hunting, fishing, housework, and watching TV. CONCLUSIONS: In contrast to nightly sleep variation in the United States being driven primarily by "sleeping-in" on weekends, Tsimane sleep variation, while comparable to that observed in the United States, was driven by changing "bedtimes," independent of day-of-the-week. We propose that this variation may reflect adaptive responses to changing opportunity costs to sleep/nighttime activity.

High resting metabolic rate among Amazonian forager-horticulturalists experiencing high pathogen burden.

OBJECTIVES: Resting metabolic rate (RMR) reflects energetic costs of homeostasis and accounts for 60 to 75% of total energy expenditure (TEE). Lean mass and physical activity account for much RMR variability, but the impact of prolonged immune activation from infection on human RMR is unclear in naturalistic settings. We evaluate the effects of infection on mass-corrected RMR among Bolivian forager-horticulturalists, and assess whether RMR declines more slowly with age than in hygienic sedentary populations, as might be expected if older adults experience high pathogen burden. MATERIALS AND METHODS: RMR was measured by indirect calorimetry (Fitmate MED, Cosmed) in 1,300 adults aged 20 to 90 and TEE was measured using doubly labeled water (n = 40). Immune biomarkers, clinical diagnoses, and anthropometrics were collected by the Tsimane Health and Life History Project. RESULTS: Tsimane have higher RMR and TEE than people in sedentary industrialized populations. Tsimane RMR is 18 to 47% (women) and 22 to 40% (men) higher than expected using six standard prediction equations. Tsimane mass-corrected TEE is similarly elevated compared to Westerners. Elevated leukocytes and helminths are associated with excess RMR in multivariate regressions, and jointly result in a predicted excess RMR of 10 to 15%. After age 40, RMR declines by 69 kcal/decade (p < .0001). Controlling for lean mass and height accounts for 71% of age-related RMR decline, and adding indicators of infection minimally affects the age slope. The residual level of age-related decline from age 40 is 1.2% per decade. CONCLUSION: High pathogen burden may lead to higher metabolic costs, which may be offset by smaller body mass or other energy-sparing mechanisms.

Response to de la Iglesia et al.

We wish to respond to the commentary of de la Iglesia et al. [1]. Studies comparing sleep in different communities have different goals. One frequent goal has been to determine how sleep is affected by manipulating specific 'modern' conditions. Many studies have investigated the effect of artificial light and electronic entertainment. Such studies have clearly shown that light, particularly blue light, delays sleep onset [2]. Studying the effect of artificial light on sleep was not a goal of our study.

What is segmented sleep? Actigraphy field validation for daytime sleep and nighttime wake.

OBJECTIVE: To compare different scoring parameter settings in actigraphy software for inferring sleep and wake bouts for validating analytical techniques outside of laboratory environments. DESIGN: To identify parameter settings that best identify napping during periods of wakefulness, we analyzed 137 days on which participants reported daytime napping, as compared with a random subset of 30 days when no naps were reported. To identify settings that identify periods of wakefulness during sleep, we used data from a subsample of women who reported discrete wake bouts while nursing at night. SETTING: Equatorial Tanzania in January to February 2016. PARTICIPANTS: The Hadza-a non-industrial foraging population. MEASUREMENTS: Thirty-three subjects participated in the study for 393 observation days. Using the Bland-Altman technique to determine concordance, we analyzed reported events of daytime napping and nighttime wake bouts. RESULTS: Only 1 parameter setting could reliably detect reported naps (15-minute nap length, ≤50 counts). Moreover, of the 6 tested parameter settings to detect wake bouts, the setting where the sleep-wake algorithm was parameterized to detect 20 consecutive minutes throughout the designated sleep period did not overestimate or underestimate wake bouts, had the lowest mean difference, and did not significantly differ from reported wake-bout events. CONCLUSION: We propose operational definitions for multiple dimensions of segmented sleep and conclude that actigraphy is an effective method for detecting segmented sleep in future cross-site comparative research. The implications of such work are far reaching, as sleep research in preindustrial and developing societies is documenting natural sleep-wake patterns in previously inaccessible environments.

Natural sleep and its seasonal variations in three pre-industrial societies.

How did humans sleep before the modern era? Because the tools to measure sleep under natural conditions were developed long after the invention of the electric devices suspected of delaying and reducing sleep, we investigated sleep in three preindustrial societies [1-3]. We find that all three show similar sleep organization, suggesting that they express core human sleep patterns, most likely characteristic of pre-modern era Homo sapiens. Sleep periods, the times from onset to offset, averaged 6.9-8.5 hr, with sleep durations of 5.7-7.1 hr, amounts near the low end of those industrial societies [4-7]. There was a difference of nearly 1 hr between summer and winter sleep. Daily variation in sleep duration was strongly linked to time of onset, rather than offset. None of these groups began sleep near sunset, onset occurring, on average, 3.3 hr after sunset. Awakening was usually before sunrise. The sleep period consistently occurred during the nighttime period of falling environmental temperature, was not interrupted by extended periods of waking, and terminated, with vasoconstriction, near the nadir of daily ambient temperature. The daily cycle of temperature change, largely eliminated from modern sleep environments, may be a potent natural regulator of sleep. Light exposure was maximal in the morning and greatly decreased at noon, indicating that all three groups seek shade at midday and that light activation of the suprachiasmatic nucleus is maximal in the morning. Napping occurred on <7% of days in winter and <22% of days in summer. Mimicking aspects of the natural environment might be effective in treating certain modern sleep disorders.

Patterns of senescence in human cardiovascular fitness: VO2 max in subsistence and industrialized populations.

OBJECTIVES: This study explores whether cardiovascular fitness levels and senescent decline are similar in the Tsimane of Bolivia and Canadians, as well as other subsistence and industrialized populations. Among Tsimane, we examine whether morbidity predicts lower levels and faster decline of cardiovascular fitness, or whether their lifestyle (e.g., high physical activity) promotes high levels and slow decline. Alternatively, high activity levels and morbidity might counterbalance such that Tsimane fitness levels and decline are similar to those in industrialized populations. METHODS: Maximal oxygen uptake (VO2 max) was estimated using a step test heart rate method for 701 participants. We compared these estimates to the Canadian Health Measures Survey and previous studies in industrialized and subsistence populations. We evaluated whether health indicators and proxies for market integration were associated with VO2 max levels and rate of decline for the Tsimane. RESULTS: The Tsimane have significantly higher levels of VO2 max and slower rates of decline than Canadians; initial evidence suggests differences in VO2 max levels between other subsistence and industrialized populations. Low hemoglobin predicts low VO2 max for Tsimane women while helminth infection predicts high VO2 max for Tsimane men, though results might be specific to the VO2 max scaling parameter used. No variables tested interact with age to moderate decline. CONCLUSIONS: The Tsimane demonstrate higher levels of cardiovascular fitness than industrialized populations, but levels similar to other subsistence populations. The high VO2 max of Tsimane is consistent with their high physical activity and few indicators of cardiovascular disease, measured in previous studies.