Female stress and birth seasonality in Tanzania

Delivery records from hospitals in Ikwiriri and Ifakara show that the number of births increases throughout the dry season (June-October) and declines throughout the rainy season (November), reaching a peak just before the rains and the lowest level at the end of the rainy season. This pattern does not correspond with the usual explanations of birth seasonality. Conceptions are most frequent at the period of highest temperature, which is contrary to the theory that predicts them to be more frequent during the dry season. The drop of the conception rate during the wet agricultural season suggests that stress on women may be the main cause of birth seasonality in Tanzania. Due to wet conditions and frequent staying on in the rice fields, exposure to malaria increases during the rainy season's latter part. The negative association of the number of births with rainfall in the months preceding conception indicates that almost half of the variation in the number of births may be due to the effects of malaria and physical exhaustion on fecundity. The remainder may be attributable to seasonal variations in pregnancy loss and sexual behavior. The absence of strong birth seasonality in nonholoendemic areas of Tanzania and the low birth rate in holoendemic areas provide further support for a critical role for malaria infection. The fact that the magnitude of seasonal variation in births increases with high parity and has decreased over the past decade results from recent changes in Tanzania's rural economy. Young people in Tanzania are progressively withdrawing from agriculture, especially when they have no children yet.     

滇南热带季节雨林和橡胶林对降雨侵蚀力的减缓效应

降雨是影响土壤侵蚀的主要因素,降雨侵蚀力通常用降雨动能和一段时间的最大雨强的乘积表示,简化的计算也可用日降雨量估算。本文选取昆-曼国际大通道滇南段旷地4个气象站(勐腊、勐仑、景洪、思茅)以及1个人工林(橡胶林)和1个热带季节雨林的日降雨量资料,计算了降雨侵蚀力以及随季节变化的规律,比较了森林破坏对降雨侵蚀力的影响。结果表明,昆-曼国际大通道滇南段旷地的降雨侵蚀力干季均较小,雨季逐渐增大,在7、8月达最大;旷地的降雨侵蚀力明显大于橡胶林和热带季节雨林,其差值在干季较小,雨季显著;旷地的年降雨侵蚀力可达到橡胶林的1.8倍以上,是热带季节雨林的2.3倍以上;而月降雨侵蚀力差异最大可达6~7倍;此外橡胶林和热带季节雨林降雨侵蚀力干季相差不大,而雨季橡胶林降雨侵蚀力明显大于热带季节雨林。说明了水土保持效果橡胶林和热带季节雨林比旷地要好,而热带季节雨林的效果要比橡胶林好;森林破坏将极大地增加降雨侵蚀力,加速水土流失。     

Rainfall and labile carbon availability control litter nitrogen dynamics in a tropical dry forest

N cycling in tropical dry forests is driven by rainfall seasonality but the mechanisms involved are not well understood. We studied the seasonal variation in N dynamics and microbial biomass in the surface litter of a tropical dry forest ecosystem in Mexico over a 2-year period. Litter was collected at 4 different times of the year to determine changes in total, soluble, and microbial C and N concentrations. Additionally, litter from each sampling date was incubated under laboratory conditions to determine potential C mineralization rate, net N mineralization, net C and N microbial immobilization, and net nitrification. Litter C concentrations were highest in the early-dry season and lowest in the rainy season, while the seasonal changes in N concentrations varied between years. Litter P was higher in the rainy than in the early-dry season. Water-soluble organic C (WSOC) and water-soluble N concentrations were highest during the early- and late-dry seasons and represented up to 4.1 and 5.9% of the total C and N, respectively. NH₄⁺ and NO₃⁻ showed different seasonal and annual variations. They represented an average 23% of soluble N. Microbial C was generally higher in the dry than in the wet seasons, while microbial N was lowest in the late-dry and highest in the early-rainy seasons. Incubations showed that lowest potential C mineralization rates and C and N microbial immobilization occurred in rainy season litter, and were positively correlated to WSOC. Net nitrification was highest in rainy season litter. Our results showed that the seasonal pattern in N dynamics was influenced by rainfall seasonality and labile C availability, and not by microbial biomass. We propose a conceptual model to hypothesize how N dynamics in the litter layer of the Chamela tropical dry forest respond to the seasonal variation in rainfall.     

Successional pattern of phytoplankton (>55 microm) in Lekki lagoon, Nigeria

Lagoons are dominant features along large stretches of the West Africa coast. These freshwater environments are very valuable areas where phytoplankton constitute the basis of aquatic food webs. In order to know the effects of environmental variables on phytoplankton, a study of the successional pattern of phytoplankton in Lekki lagoon was carried out monthly for two years (June 2003-May 2005). Phytoplankton samples were collected from 12 stations using a plankton net of 55 microm mesh, and samples preserved in 4% unbuffered formalin. Besides, surface water samples were taken for physico-chemical analysis. For each year, the seasonal distribution and succession of dominant phytoplankton followed different patterns. Phytoplankton abundance was higher during the dry season (November-April) for the two annual cycles. The diatoms (Aulacoseira granulata and A. granulata var angustissima) and blue green algaes, Microcystis aeruginosa, Merismopedia tennuissima and Trichodesmium lacustre showed this trend by being the abundant species in some of these months. For the rainy season, the green alga Mougeotia sp. dominated. The replacement of one form by another throughout seasonal cycles was probably controlled by the changes in environmental variables such as rainfall, nitrate-nitrogen and phosphate-phosphorus.     

Hormonal correlates of reproductive seasonality in wild female hanuman langurs (Presbytis entellus)

To date, it is not known whether the seasonal occurrence of sexual behavior and mating in free-ranging Hanuman langurs at Ramnagar, Nepal, is correlated with seasonal changes in female ovarian function, and, if so, which factor(s) triggers the onset of the reproductive period. Using noninvasive fecal hormone analysis in combination with behavioral observations, this study was carried out to: 1) investigate and characterize seasonal patterns of ovarian cyclicity and timing of conception in wild langur females living in a highly seasonal habitat; and 2) examine the relationship between seasonal patterns of ovarian cyclicity, behavioral estrus, and female physical condition. Behavioral data and fecal samples were collected during a total period of 14 months from nine females living in a multi-male group. Physical condition of the females was assessed monthly by visual inspection, using a seven-fold scale. Ovulatory cycles and timing of conceptions were identified by the measurement of immunoreactive pregnanediol glucuronide (iPdG) in extracted feces. Hormone profiles in individual females revealed a clearly seasonal distribution in the occurrence of ovulatory cycles, which were restricted to the period from July to October. The distribution of female estrus behavior showed a similar seasonal pattern, and in total 88.2% of all estrus periods observed in the focal females were accompanied by ovulation. Onset of ovarian cycles as well as mating activities were strongly correlated with the onset of the rainy season. Females conceived, on average, in their second ovulatory cycle (pregnancy length: 211.6 +/- 3.4 days), with timing of conception being confined to the months when animals showed an improved physical condition. Collectively the present data clearly suggest that in seasonally-breeding langurs at Ramnagar, ecological conditions (rainfall, food availability, and quality) influence the onset of ovulations and timing of conceptions.     

Composition and Structure of the Chironomidae (Insecta: Diptera) Community Associated with Bryophytes in a First-Order Stream in the Atlantic Forest, Brazil

This study describes the structure of the Chironomidae community associated with bryophytes in a first-order stream located in a biological reserve of the Atlantic Forest, during two seasons. Samples of bryophytes adhered to rocks along a 100-m stretch of the stream were removed with a metal blade, and 200-mL pots were filled with the samples. The numerical density (individuals per gram of dry weight), Shannon’s diversity index, Pielou’s evenness index, the dominance index (DI), and estimated richness were calculated for each collection period (dry and rainy). Linear regression analysis was employed to test the existence of a correlation between rainfall and the individual’s density and richness. The high numerical density and richness of Chironomidae taxa observed are probably related to the peculiar conditions of the bryophyte habitat. The retention of larvae during periods of higher rainfall contributed to the high density and richness of Chironomidae larvae. The rarefaction analysis showed higher richness in the rainy season related to the greater retention of food particles. The data from this study show that bryophytes provide stable habitats for the colonization by and refuge of Chironomidae larvae, mainly under conductions of faster water flow and higher precipitation.     

Seasonality of reproduction of wild black-and-white snub-nosed monkeys (Rhinopithecus bieti) at Mt. Lasha, Yunnan, China

Black-and-white snub-nosed monkeys (Rhinopithecus bieti) inhabit one of the harshest habitats by any nonhuman primate. Reliable predictive cues to initiate reproduction may be particularly critical for R. bieti because they inhabit such seasonally energetically challenging environments. To better understand the seasonal distribution of and predictive cues to reproduction, we collected breeding and birth data in a population of R. bieti at Mt. Lasha in Yunling Nature Reserve, Yunnan, China, from January 2008 to May 2010, using a combination of 10-min instantaneous scans and ad libitum observations. We examined variations in temperature, rainfall, and food availability, as well as photoperiod differences between Mt. Lasha and the more northerly Xiaochangdu to identify environmental influences on the timing of reproductive events. Our data show the area exhibited distinct seasonal fluctuations in rainfall, temperature, and food availability. Mating occurred year-round, but peaked in August, coinciding with the end of the period of highest temperatures and food availability, and during the peak rainfall. Copulation frequency peaked 1 month after corresponding peaks in staple foods, rainfall, and minimum temperatures, and 3-4 months after peaks in high-quality foods. Births were significantly seasonal, with a birth peak from mid-February to early April, and a mean birth date of 14 March. Eleven births occurred in 41 days in 2009, and 16 births occurred in 52 days in 2010. Births occurred during periods of increasing temperatures and food availability. Our findings are suggestive of at least one environmental control of conception timing, and support the notion that food availability during key reproductive stages is an ultimate factor for birth seasonality, but provide no supporting evidence for photoperiod during the conception season as a proximate cue to reproduction in R. bieti.     

Ovitrap surveys of dengue vector mosquitoes in Chiang Mai, northern Thailand: seasonal shifts in relative abundance of Aedes albopictus and Ae. aegypti

Aedes aegypti (L.) and Aedes albopictus (Skuse) were surveyed using ovitraps in residential areas in Chiang Mai, northern Thailand. Egg populations (both species inclusive) remained low in the dry season, but increased/decreased exponentially during the first/latter half of the rainy season, respectively. This seasonal pattern was similar to the seasonal distribution of dengue haemorrhagic fever cases in the area. During the dry season (November-March) Ae.aegypti was dominant in urban and indoor ovitraps. With onset of the rainy season in April, relative abundance of Ae.albopictus increased in rural and outdoor ovitraps. Ae.albopictus displaced Ae.aegypti in the latter half of the rainy season in the rural area. Possible mechanisms to account for this seasonal decline of Ae.aegypti and reciprocal fluctuations in relative abundance of Ae.albopictus are discussed in relation to food availability for larvae in container habitats.     

The relationship between outbreaks, rainfall and low density populations of the African armyworm, Spodoptera exempta, in Kenya

This study has demonstrated that low numbers of African armyworms (Spodoptera exempta) (Walker) (Lepidoptera: Noctuidae) can be found throughout the year in parts of the coastal and highland regions of Kenya where there is frequent rainfall. During the study period the numbers of moths caught in pheromone traps in these regions built up during the short and long rains, and decreased dramatically during the intervening dry seasons. There was a lag of one to two months between the peaks for rainfall and moth numbers. This contrasts with the situation at sites of the seasonal outbreaks of armyworm larvae, where a sudden preceding influx of moths coincides with the rainfall. A positive correlation was found, for the long rains seasons only, between the number of armyworm outbreaks throughout the country and the peak numbers of moths in coastal and highland regions.It is suggested therefore that the peak numbers of moths trapped in the eastern highlands and coastal regions during the rainy seasons arise principally from outbreaks in the extensive seasonal grasslands. These grassland areas diminish considerably during the prolonged dry periods between the two rainy seasons, and it is only the eastern highland and coastal regions that are likely to provide suitable habitats for breeding during the dry seasons. The significance of these populations for initiating the first outbreaks of the following season is discussed.     

Seasonal polyphenism in Bicyclus (Lepidoptera: Satyridae) butterflies: different climates need different cues

A comparison is made between northern and southern hemisphere populations of Bicyclus butterflies in Africa regarding their responses in wing pattern polyphenism to seasonal change in rainfall and temperature. In southern habitats where temperature and rainfall are often positively correlated, a high temperature during the larval period induces conspicuous wet season forms whereas a fall in temperature elicits cryptic dry season forms. In northern habitats, however, where temperature and rainfall usually are negatively correlated, a rise in temperature should not induce a wet season form because such a rise is correlated with the onset of the dry season. Here, wing pattern plasticity, as measured using museum material, was regressed on mean monthly values for rainfall and temperature. Rainfall appeared to be a frequent determinant of wing pattern plasticity whereas temperature was much less often a significant independent variate. We conclude that the wing pattern may only respond to seasonal change in temperature if rainfall and temperature are positively correlated; in other situations rainfall remains the only significant determinant for wing pattern plasticity.     

Reproductive phenology, life-forms, and habitats of the Venezuelan Central Plain

Reproductive phenology of 171 plant species belonging to 57 families of angiosperms was studied according to life-forms in four habitat types in a savanna-forest mosaic on the Venezuelan Central Plain. Flowering, unripe fruit, and mature fruit patterns were affected significantly according to life-forms and habitats respectively. Production of flowers, unripe fruits, and mature fruits showed marked seasonality for all habitats except for the forest. Flowering peaked during the rainy season, and fruiting peaked toward the end of the rainy season. The savanna and the disturbed area had similar proportions of species that flowered over the year. The percentage of species with unripe fruits produced throughout the year was more seasonal for the disturbed area than for the other habitats. Mature fruit patterns showed an increase during the late rainy season for the ecotone and savanna. A large number of herbaceous (annual and perennial) and liana species flowered during the wet season, and a smaller fraction flowered during the dry season; and trees, shrubs, and epiphytes increased flowering activity during the dry season. Unripe fruit patterns were similar to those of flowering for all life-forms, however, tree species were less seasonal. Mature fruit production by shrubs peaked in the period of maximum rainfall, while the peak for perennial herbs was in the late rainy season and the peak for annual herbs was during the transition between the rainy season and the dry season. The largest proportion of tree and liana species with ripe fruits occurred during the dry season. Differences among phenological patterns in habitats were caused mainly by life-forms and promote a wider distribution of reproductive events in habitats and overall community in the Venezuelan Central Plain.     

Evolution of African mole-rat sociality: burrow architecture, rainfall and foraging in colonies of the cooperatively breeding Fukomys mechowii

African mole rats (Bathyergidae) offer an excellent system with which to test theories relating to the evolution and maintenance of sociality in mammals. The aridity food distribution hypothesis (AFDH) suggests that, within the bathyergids, sociality has evolved in response to patterns of rainfall, its effects on food distribution, and the subsequent costs and risks of foraging and dispersal. Here, in the first detailed study of burrow architecture in a social mole-rat species, with data from 32 burrows, we show that in the giant mole-rat Fukomys mechowii burrow fractal dimension increases with colony size and is higher during the rainy season than during the dry season. The mass of food in the burrow increases with fractal dimension and is higher during the rainy season than during the dry season. These results link for the first time colony size, burrow architecture, rainfall and foraging success and provide support for two assumptions of the AFDH, namely that (1) in arid conditions burrowing may be severely constrained by the high costs of digging; and (2) the potential risks of failing to locate food may be mitigated by increases in colony size.     

The ecology of Biomphalaria sudanica in Lake Ziway, Ethiopia

A longitudinal study was made over a period of 24 months from January 2001 to December 2002 in Lake Ziway, Ethiopia, to elucidate factors influencing changes in the population density of Biomphalaria sudanica and to identify seasons of peak transmission of Schistosoma mansoni. Between January 2001 and December 2002, bimonthly visits were made to the lake to collect data on snail population density and schistosome infection in snails, vegetation and lake water level. Water samples were collected during the dry and rainy seasons and chemical analyses were made for major cations, anions, electrical conductivity, total hardness and pH using standard methods for the examination of water and wastewater. The seasonal fluctuation in the population density of B. sudanica was found to be highly associated with rainfall, water level of the lake and vegetation availability and abundance. Water chemical analysis showed that major cations and anions were all within the range reported for other African freshwaters. Both malacological and meteorological data show that snail population density in Lake Ziway peaks at the end of the long rainy season (June–August), coinciding with a seasonal transmission peak of schistosomiasis.     

Seasonality of insect abundance in an Australian upland tropical rainforest

Monthly light and Malaise trap catches, taken over 31 months, were used to examine seasonal and annual changes in the abundance of predominant orders of upland tropical rainforest insects. Insect numbers and biomass increased during the late dry season, reached a peak during the wetter months, and declined during the early dry period. Fluctuations in insect abundance appeared to relate to (1) climatic factors such as length and severity of the dry season, or amount and period of rainfall; and (2) food availability such as an increase in the production of new leaves, or flowering and fruiting periodicity.     

A seasonal survey of gastrointestinal parasites in captive wild impala antelope on a game facility south of Lusaka, Zambia

Faecal samples (n?=?1947) from captive wild impala (Aepyceros melampus melampus) were examined over a period of 14 months to determine quantitative seasonal helminth egg excretion patterns and qualitative protozoan oocyst excretion patterns. Geometric mean monthly faecal egg counts (FECs) ranged from 20 to 575 and coprocultures revealed three parasite genera, namely Trichostrongylus, Haemonchus and Strongyloides. Larvae of the Trichostrongylus spp. were most predominant from faecal cultures. No trematode eggs or lungworms were detected and eggs of the cestode Monezia were only seen in two samples during the entire study period. The nematode FECs showed a marked seasonal variation, being higher during the rainy season, moderate during the cool dry season and low during the hot dry season. The rainy season had significantly higher FECs than the dry season (P?0.05), but were significantly higher in pelleted faeces in the rainy season than the dry season (P?相似文献   

Diurnal resting in brown lemurs in a dry deciduous forest, northwestern Madagascar: implications for seasonal thermoregulation

Decreased activity has been reported in both nocturnal and diurnal primates during the prolonged dry season in western Madagascar, and this has been interpreted as a reaction to the severe environment, with its food scarcity and/or thermal stress. Several day-active lemurs rest more as trees defoliate, although the reason for this is unclear. To understand the mechanism underpinning the diurnal resting of lemurs in seasonal deciduous forests, I observed common brown lemurs (Eulemur fulvus fulvus) for one year in Ankarafantsika National Park, northwestern Madagascar. In Ankarafantsika, despite high fruit availability during the dry season, brown lemurs are known to engage in diurnal resting. To examine the effects of thermal factors and defoliation on lemur inactivity, I recorded the activity of a troop at 1 min intervals, hourly ambient temperature, daily rainfall, and weather during observations (06:00-18:00). I quantified the amount of leaves biweekly for 680 trees. I tested correlations between percentages of resting time and each factor across hours during the day and across seasons. During the rainy season, resting time did not differ between sunny and cloudy days, and lemurs were active throughout the daytime. At the hourly level during the dry season, lemurs rested exclusively at midday, apparently at peak sunlight intensity rather than at peak ambient temperature. At seasonal level, percentages of total resting time from 08:00 to 16:00 were greater during dry season (81.9%) than during rainy season (62.6%), and percentages increased as ambient temperatures increased. Defoliation was related to seasonal decrease in weekly rainfall, which served as an index of water retained in the forest. Defoliation probably reflected aridification as well as the penetration of sunlight into the forest. Diurnal resting increased as both the amount of leaves and weekly rainfall decreased seasonally. These results suggest that heat stress under dry conditions may promote resting. Diurnal resting may function as behavioral thermoregulation to avoid overheating and to minimize water loss via excessive evaporative respiration.     

SOME ASPECTS OF THE BEHAVIOUR OF THE WATTLED PLOVER AFRIBYX SENEGALLUS (LINNAEUS)

The question of how aridity might influence avian clutch size, through the influences of rainfall seasonality and environmental stochasticity (unpredictability), has received little attention. A marked east-west gradient in aridity across South Africa provides a unique opportunity to test for such influences. Using an extensive collection of nest records for 106 terrestrial bird species from the South African Nest Record Card Scheme, we tested three predictions related to rainfall seasonality and stochasticity. Analyses were conducted at two levels, the first examining each species independently, and the second grouping species into five dietary guilds. The first prediction, that clutch size should generally increase with higher rainfall seasonality (i.e. higher seasonal fluctuation of food availability), was supported, particularly in the most arid environments where food abundance is more closely linked to rainfall. Controlling for rainfall seasonality, the second prediction, that clutch size should generally decrease as a bet-hedging strategy in arid, stochastic environments, was also supported. Although the timing of the rainy season differs among regions in South Africa (winter, early summer, later summer, year-round), birds primarily nest during spring. The relative timing of rainfall and breeding is expected to have different consequences for seasonal variation in clutch size among rainfall regions. The third prediction, of different patterns of seasonal variation in clutch size between rainfall regions, was also supported. In the winter and early-summer rainfall regions, early-nesting birds (breeding with or soon after the rains) generally had a larger clutch size than late-nesting birds. In the late-summer rainfall region, early-nesting birds (breeding well before the rains) had a smaller clutch size than late-nesting birds.     

Seasonal trends in intestinal nematode infection and medicinal plant use among chimpanzees in the Mahale Mountains, Tanzania

A longitudinal study of nematode infection in chimpanzees was conducted between 1989 and 1994 on the M group chimpanzees of the Mahale Mountains National Park, Tanzania during two annual dry and rainy season periods and a third rainy season. Chemical and physical antiparasite properties of medicinal plant use against the strongyle nematodeOesophagostomum stephanostomum have recently been reported at Mahale. Here, the incidence of nematode infections were analyzed for seasonal trends to elucidate the possible influence of parasite infection on previously reported seasonality of medicinal plant use and to test the hypothesis that the use of these plants is stimulated byO. stephanostomum. The number of chimpanzees infected byO. stephanostomum was significantly higher in the rainy season than in the dry season of both 1989–1990 and 1991–1992. However, the incidence ofTrichuris trichura andStrongyloides fuelleborni showed no seasonality. Reinfection of individuals byO. stephanostomum occurred in synchrony with annual variation in rainfall: there was a sharp rise in the occurrence of new infections per individual within one to two months after the beginning of the first heavy rains of the season. This pattern coincides with the reproductive cycle of this nematode species.O. stephanostomum (95%) infections were associated significantly more frequently with medicinal plant use than eitherT. trichiura (50%) orS. fuelleborni (40%) infections. These observations are consistent with previous reports for the increased use of these plants during the rainy season and are consistent the hypothesis that medicinal plant use is stimulated byO. stephanostomum infection.     

Seasonal Patterns of Flowering and Fruiting in a Dry Tropical Forest in Jamaica1

Tropical dry forests occupy more area and are more endangered than rainforests, yet their regeneration ecology has received less study and is consequently poorly understood. We recorded the flowering and fruiting phenology of a tropical dry forest in Jamaica over a period of 26 mo within ten 15 × 15‐m plots. Community‐wide recruitment reached a maximum in the wet season, whereas no recruitment occurred during the dry season. We observed a unimodal peak in rainfall and fruit production, and the periodicity and intensity of seed production were significantly correlated with rainfall seasonality (the optimal time for germination). Flowering at the community and system levels lagged behind a significant increase and subsequent decrease in rainfall by 7 and 3 mo, respectively, indicating that the dominant factor controlling flowering periodicity is the passage of the major (4‐mo long) rainy season and changes in soil moisture conditions. Fruiting lagged behind flowering by 2 mo and a significant increase in fruiting occurred 2 mo prior to a significant increase in rainfall. At the population level, a correspondence analysis identified a major dichotomy in the patterns of flowering and fruiting between species and indicated two broad species groups based on their time of peak fruiting and the number of times they were in fruit. These were either individuals which were usually in peak fruit 1–2 mo prior to the start of the major rainy season or those that were in fruit more or less continuously throughout the year with no peak fruiting time. This study supports the view that seasonal variation in rainfall and hence soil water availability constitutes both the proximate and the ultimate cause of flowering periodicity in tropical dry forests.     

Seasonality of vital events in a Pacific Island population.

Analyses of vital data derived from a family record register for the native population of Guam reveal significant variations in births, deaths, and marriages over the period 1901-41. Although lacking marked photoperiod or temperature changes of temperate zones, the tropical island is subject to marked seasonal differences in rainfall characteristic of western Pacific islands. Marital patterns exhibit troughs associated respectively with the Lenten period and with Christmas celebrations. Infant and childhood deaths show close correspondence with rainfall patterns, consistently exceeding expected values during the rainy season (July-November) when conditions are optimal for the spread of communicable and gastrointestinal diseases. Births attain a peak in November, or at the beginning of the more advantageous season for infant health and survival. Seasonality in vital events, reported for many Euroamerican and some African and Asian populations of modern and historical periods, has rarely been documented for native populations of the tropical Pacific. Comparisons of differences in these patterns among different populations in varied environments provide unique opportunities to evaluate causal models of interactions among biological, sociocultural, and physioenvironmental factors.