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 Table of Contents  
Year : 2022  |  Volume : 12  |  Issue : 3  |  Page : 271-276

Trends and seasonal variations in human secondary sex ratio in Southwest Nigeria: A 10-year survey

Department of Cell Biology and Genetics, University of Lagos, Lagos, Nigeria

Date of Submission25-Nov-2021
Date of Acceptance04-Aug-2022
Date of Web Publication15-Sep-2022

Correspondence Address:
Mr. Samson Taiwo Fakorede
Department of Cell Biology and Genetics, University of Lagos, Lagos
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/aihb.aihb_165_21

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Introduction: Secondary sex ratio is the number of live male births per 100 female births. In humans, the primary sex ratio, which is the sex ratio at conception, is expected to be 1:1 in natural populations based on the law of independent assortment of the X and Y chromosomes. This is not the case with the sex ratio at birth as it is largely affected by various social and cultural practices that it tends to bias towards one sex, with profound implications for population and demographic dynamics. Materials and Methods: We collected retrospective data of records of live births from three Southwestern Nigerian states, covering 10 years. The data were obtained from different Hospitals/Birth Centres from three states, i.e., Lagos, Ondo and Osun, between 2006 and 2017. Data analysis was performed to determine the monthly, quarterly, yearly and seasonal variation in sex ratios. Chi-square analysis was used to determine the significance of differences in sex ratios at P < 0.05. Results: Annual average sex ratios of 99.7, 105.6 and 106.0 were obtained for Lagos, Osun and Ondo, respectively, while the pooled data had a sex ratio of 104.2, indicating male preponderance. Sex ratios differ significantly according to season. Differences in sex ratio were significant during the dry season in Lagos and Osun states and in Ondo during the rainy season. Conclusions: Sex ratios from Southwest Nigeria are as diverse as other populations, with ratios ranging between 99.7 and 106.0.

Keywords: Demography, live births, seasonal variation, sex ratio, sex

How to cite this article:
Fakorede ST, Ojo SD, Shonde KM, Adekoya KO, Ogunkanmi LA, Oboh B. Trends and seasonal variations in human secondary sex ratio in Southwest Nigeria: A 10-year survey. Adv Hum Biol 2022;12:271-6

How to cite this URL:
Fakorede ST, Ojo SD, Shonde KM, Adekoya KO, Ogunkanmi LA, Oboh B. Trends and seasonal variations in human secondary sex ratio in Southwest Nigeria: A 10-year survey. Adv Hum Biol [serial online] 2022 [cited 2023 Feb 7];12:271-6. Available from: https://www.aihbonline.com/text.asp?2022/12/3/271/356110

  Introduction Top

The human secondary sex ratio (SSR) or sex ratio at birth (SRB) which conventionally means the number of live male births per 100 female births, continues to get attention from economic planners, demographers, cell biologists, biostatisticians and epidemiologists.[1],[2] This is because understanding the distribution of sexes in certain populations can help to proffer solutions to socio-economic problems, especially in the areas of planning and implementation of interventions as well as in the management and distribution of resources. In humans and some other sexually reproducing organisms, the SSR is expected to be similar to the primary ratio of 1:1 because of the segregation of the sex chromosomes; X and Y in humans. However, the SSR in humans is somewhat biased towards the male child as the natural SSR is believed to be 105–107, indicating that there are about 107 males for every 100 females.[3],[4],[5]

The imbalance or inequality in sex ratio, which is more prevalent in eastern Europe and Asia, has been attributed to several factors, chiefly of which are preferences for a male child. This trend is aided by several practices such as sex-selective abortions, infanticide and sex selection technology.[6],[7],[8],[9] On the other hand, unfavourable environmental conditions such as economic decline, earthquakes as well as excess male deaths as a result of war can lead to a low sex ratio.[10],[11],[12],[13] Other factors that are believed to affect the SRB include coital rate and parental age,[14] birth order,[15] maternal weight,[16] environmental toxins,[17],[18] hormonal imbalances,[19] chronic stress[20] and parental occupations.[21],[22],[23],[24]

SSR varies not only from one country to another but also among ethnic populations within a country. Mathews and Hamilton[25] reported sex ratios ranging from 103-105 for different populations groups in the United States. Among the Asian countries, the SRB was as high as 113.5 in China in 2015, largely due to government policy that significantly reduced fertility and the bias in the reporting of female births to state authorities. In South Korea, the sex ratio was 106.2 in 2014, and it was 111.3 in India between 2014 and 2016.[26],[27] In Nigeria, different values of sex ratios have been reported in different ethnic populations and geographical locations. Sex ratio values of 1.07 have been reported among the Igbo population in Eastern Nigeria[28] and 112 among the Hausas in the North.[29] Among the Yorubas in the Southwest, different values of male-to-female births were observed by several authors over a wide range of time, including 107.8[30] and 106[31] in Ibadan. Azeez et al.[3] and Oyeniyi[32] reported sex ratios of 102.7 and 114, respectively, for the region. Moreover, Eneni et al.[33] obtained a value of 120 in a South-Southern Nigerian population. The most recent study on the human SSR in the Southwest region was conducted by Azeez et al.[3] 2007. The study, which covered from 1995 to 2004, gave a combined ratio of 102.7 male births to 100 females. Our study was aimed at determining the trend as well as seasonal variation in sex ratios in Southwest Nigeria from 2007 to 2016.

  Materials and Methods Top

Population of study

Southwestern Nigeria [Figure 1] is occupied by the Yoruba ethnic individuals. Yoruba is one of the major ethnic populations in the country, in addition to the Igbos in the East and the Hausa-Fulani in the North. Other ethnic groups such as the Tiv, Efik, Ibibio, Ijaw and Kanuri constitute the minority. The Yorubas are found predominantly in 9 out of 36 states, including Ekiti, Lagos, Ogun, Ondo, Osun, Oyo and some parts of Kogi, Kwara and Edo states.
Figure 1: Map of Nigeria showing South-Western States (Modified from Fasasi and Alabi 2020.[34]

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Sources of data and statistical analysis

The retrospective data used for the study were collected from the birth records of selected hospitals from three Southwestern Nigerian states as a representative of the Yoruba ethnic population. These are General Hospital, Badagry, Lagos state, (GH_BDG); Obafemi Awolowo University Teaching Hospital, Ile-Ife, Osun state, (OAUTH_IFE); and General Hospital, Iju-Itaogbolu, Akure, Ondo state (GH_AKR). The data consisted of the total number of live single births for both sexes from 1 January to 31 December of each year for 10 years, that is, 2007–2016. The number of live births from each hospital was analysed for monthly, quarterly, yearly and seasonal variation in sex ratios using the formula:

For seasonal variation, the months of the year were classified into two, i.e., rainy and dry seasons. The rainy season in Southern Nigeria includes the months of March to July (long rainy period) and September to October (short rainy period), while November to February (long dry period) and the month of August (short dry period) constitute the dry season. Chi-square analysis was performed to determine the significance of differences in sex ratios at 0.05 significant levels. All statistical analyses were performed using IBM SPSS Statistics for Windows, Version 26.0. Armonk, NY: IBM Corp. Data of multiple births were excluded from the study.

  Results Top

The total number of single births recorded in each of the study locations covered by this study were 11,680 for GH_BDG, 11,212 for OAUTH_IFE and 22,819 for GH_AKR. The monthly SSRs in these locations over 10 years are presented in [Table 1]. The highest monthly SSR (108.7) was recorded for GH_BDG in October, while the lowest ratio (83.3) was in July, with statistically significant deviations. For OAUTH_IFE, the month of October had the highest sex ratio of 120.8, while April recorded the lowest ratio of 93.3 over the 10 years covered by the study. The sex ratios for February and October were statistically significant in this location. The highest monthly SSR of 116.2 was recorded in January for GH_AKR, with the lowest being 90.1 in June. There were significant deviations of the sex ratios in January, February, May, June, October, November and December across the 10 years. Monthly analysis of data from all three studied locations showed that the highest (114.1) and lowest (95.3) sex ratios was recorded in October and June, respectively.
Table 1: Monthly secondary sex ratios in all study locations and combined, from 2007 to 2016

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[Table 2] shows the annual sex ratio of live births from 2007 to 2016. It was observed that GH_BDG that year 2016 had the highest sex ratio of 115.4 males to 100 females, while the lowest ratio was in 2008 with 78.2 males to 100 females. There were statistical differences in the annual sex ratios of the years 2008, 2014 and 2016. The highest SSR of 117.4 was recorded for OAUTH_IFE in 2009, whereas the lowest sex ratio (96.2) was recorded in 2013, with values deviating significantly in 2009 and 2012. Annual analysis of the data from the GH_AKR showed that the sex ratio was highest (123.8) in 2009 and lowest (98.2) in 2012. Sex ratios were statistically significant in 2009, 2010 and 2013 for this location. Annual analysis of the combined data showed that year 2009 had the highest SRB of 116.3, while the lowest value (94.5) was in 2008.
Table 2: Annual secondary sex ratios in all study locations and combined, from 2007 to 2016

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Results of the quarterly variation in sex ratios from the three locations for the 10 years covered by the study are presented in [Table 3]. For GH_BDG, the highest quarterly sex ratio of 107.3 was in the fourth quarter, while the lowest ratio of 94.6 was in the first quarter. OAUTH_IFE had the highest (111.5) and lowest (100.6) quarterly sex ratios in the first and second quarters, respectively. The highest and lowest sex ratios of 111.3 and 99.6 were recorded for General Hospital, Iju-Itaogbolu, Akure in the first and second quarters, respectively. Results further showed that there were no significant differences (P > 0.05) in quarterly sex ratios for all study locations.
Table 3: Quarterly sex ratio of live births in Southwest Nigeria from 2007 to 2016

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[Table 4] presents the annual sex ratio for all three study locations. This includes an annual sex ratio of 99.7 for GH_BDG, 105.6 for OAUTH_IFE and 106.0 for General Hospital, Iju-Itaogbolu, Akure. The annual human SSR in Southwest Nigeria for the 10 years was 104.2 signifying more male births than females. There was no significant difference in sex ratios at a 0.05 significant level for the 10 years covered by this investigation.
Table 4: Sex ratio of live births in Southwest Nigeria over the 10 years

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The results obtained of annual seasonal variation in sex ratios [Figure 2] shows that the highest sex ratio of 147.1 was recorded in 2008 for GH_AKR, and the lowest (77.7) was recorded in 2009 for GH_BDG during the dry season. The sex ratios differ significantly (P < 0.05) in GH_BDG and GH_AKR during the period under study. During the rainy season, the highest SSR at birth (120.4) was recorded in OAUTH_IFE in 2015, while the lowest ratio (78.5) was recorded in GH_BDG in 2008. A significant difference in sex ratios was only observed in GH_AKR during the rainy season (P < 0.001).
Figure 2: Seasonal variation in the sex ratio of live births in Southwest Nigeria from 2007 to 2016 during rainy (a), and dry (b) seasons. GH_BDG: General Hospital, Badagry, OAUTH_IFE: Obafemi Awolowo University Teaching Hospital, Ile-Ife, GH_AKR: General Hospital, Akure.

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  Discussion Top

The human SSR varies from population to population and from one geographical location to another. This variation can be attributed to several factors, including cultural practices such as preference for a male child and patriarchal family system, government policy of small family size in some countries, sex-selection technology, environmental pollution, parental age, stress, war, natural disasters and genetics. Analysis of the SRB data published over several decades shows that regardless of race, ethnicity, nationality and geographic location, the SRB tends to go through periods of highs and lows regardless of genetic or environmental interplay. In the present study, we evaluated monthly, annually, quarterly and seasonal trends in human SRB in selected Yoruba populations of the Southwestern states of Lagos, Osun and Ondo, Nigeria. The study covers 10 years, spanning 1 January, 2007, to 31 December, 2016. Our findings revealed that the sex ratios of the selected locations differ monthly, quarterly and annually, with more than half of the months in each year having a high sex ratio, i.e., more male live births than females. On average, annual sex ratios in the three towns covered by this investigation are 99.7 for GH_BDG, Lagos state, 105.6 for OAUTH_IFE, Osun state and 106.0 for General Hospital, Iju-Itaogbolu, Akure, Ondo state. There was no statistical difference in the average sex ratios of studied locations (χ2 = 0.231; P = 0.891). Our finding is in agreement with several studies that have established that there are more male live births than females except for Lagos, with a slight female preponderance. The incidence of more female births than males has also been previously reported by Mosuro.[35] The low sex ratio recorded in Lagos, as revealed by this study and that of Mosuro,[35] which are over two decades apart, signifies that the sex ratio of Lagos is generally low, and this may be attributed to environmental factors such as pollution and increased stress among the adult males, Lagos being a highly industrialised and densely populated metropolitan area with poor transportation systems, unlike the Ile-Ife and Akure that are less industrialised. There are established relationships between environmental pollution and sexual differentiation.[36],[37],[38] The timing and/or extent of exposure to toxicants significantly affects the rate of cell division. Rapidly dividing cells, like the foetal gonadal tissue (FGT), are more susceptible to replication error than the slowly dividing ones. Hence, exposure of the FGT to environmental assault may lead to abnormalities that could compromise sex determination. Furthermore, somatic mutations in the sex-determining gene on the Y chromosome (SRY gene, a gene which initiates the determination of male sex in mammals) resulting from environmental pollutants as well as damage to the Y chromosome might lead to the predominance of female offspring.[39],[40] stress, on the other hand, has been implicated in increased levels of an adrenocorticotrophic hormone known as corticotrophin. Corticotrophin affects men and women differently. In men, elevated levels of corticotrophin bring about a reduced amount of testosterone and increased oestrogen leading to more female offspring. In women, however, high levels of corticotrophin cause the adrenal cortex to synthesise more testosterone, thereby creating hormonal balance for male offspring.[41] Variation in the sex ratios of the study locations is also ascribable to differences in living standards, but more evidence is needed for a valid argument.

The reported overall sex ratio of 104:100 for the entire study period showed that Southwestern Nigeria had more male birth to a female and falls in the 1.03–1.07 range for humans, especially the African populations.[42] There are several data from Nigeria and other parts of the world that agree with this finding. While Ayeni[30] observed a sex ratio of 107.85:100 for Ibadan, Southwestern Nigeria, Effiong et al.[31] reported a sex ratio of 1.06 among Yoruba children in Ibadan. Furthermore, sex ratio values of 107:100[43] and 1.12[29] have been reported among the Northern Hausas. Furthermore, for the sex ratio of Igbo births, Egwuatu[28] reported a value of 107. A 2007 study by Azeez et al.[3] revealed a sex ratio value of 102.7:100 in South West, Nigeria, while Oyeniyi[32] reported a value of 1.14 another Southwestern Nigerian population and, more recently, Eneni et al.[33] gave a high ratio of 120:100 in their study of a South-South Nigerian Teaching Hospital. According to a 2001 survey, Belgium's SRB was 1.04, while Italy, Switzerland and Ireland had a joint sex ratio of 1.07. A 1970–2002 report of sex ratios in the United States gave ratios 1.03, 1.04 and 1.05 for the African Americans, Mexican Americans and the white non-Hispanic populations, respectively. In China, there was an upsurge in the observed SSR ranging from 106 in 1979, 111 in 1990-117 in 2001 and as high as 130 in some rural communities.[7],[44] The last population census in Nigeria conducted in 2006 put the sex ratio of the entire population at 1.05, while the CIA World Factbook[44],[45] estimated the country's SRB to be 1.06. It is interesting to note that these figures align with the results obtained in the present study.

There were incidences of geographical variations in SRB in our sampled locations. The monthly ratio in GH_BDG was highest in October and lowest in July, whereas the sex ratio at General Hospital, Iju-Itaogbolu, Akure, was highest in January and lowest in June. The highest and lowest monthly sex ratios for OAUTH_IFE were recorded in October and April, respectively. Overall, the SRB for combined data from all locations during the period covered by the study was highest in October and lowest in June. The quarterly analysis showed that the incidence of sex ratio in OAUTH_IFE and General Hospital, Iju-Itaogbolu, Akure followed a similar pattern and differently from GH_BDG. There were more male births, i.e., high sex ratios recorded in either the first (GH_BDG) or third quarters (OAUTH_IFE and GH_AKR), while the lowest sex ratios were seen in the first quarter for GH_BDG and fourth quarter for OAUTH_IFE and GH_AKR. The highest and lowest annual sex ratios were obtained in GH_BDG in 2016 and 2008, respectively. OAUTH_IFE and GH_AKR both had their highest sex ratio in 2009. However, while the lowest sex ratio was observed in OAUTH_IFE in the year 2013, it was 2012 for GH_AKR. Geographical variations in SRB have been documented by previous studies,[46],[47],[48] and it is suggested that ambient temperature might skew the sex ratio in favour of a particular gender.[49],[50] For instance, studies have revealed that warmer temperatures tend to favour the birth of the male child.[50],[51],[52],[53] The underlying mechanism for this phenomenon remains unclear. This may, however, be attributed to the events occurring within the testis. A study by van Zelst et al.[54] showed that increased temperatures affect the disjunction of meiotic X and Y chromosomes in mice. In addition, Lerchl[49] demonstrated that the sex ratio of humans is affected by temperatures, with higher proportions of boys resulting from increased environmental temperatures. On the other hand, Kelsey et al.[55] showed that low ambient temperature affects the SRB by triggering stress responses in individuals that are protected against it. Furthermore, low temperatures can lead to increased stress in populations by disrupting resources, for example, economy, that sustain them.[56] Nevertheless, temperatures may not be the sole factor responsible for the differences observed in our findings because the study locations fall in the same geographical altitude and share relatively the same climatic conditions. Hence, other socio-economic and environmental factors might play significant roles.

The seasonal variation in SRB revealed significant differences (P < 0.05) in sex ratios during the rainy season for GH_AKR only, and GH_BDG and GH_AKR in the dry season, but there was no specific pattern in the monthly distribution of sex ratios in across the studied locations. Results of Azeez et al.[3] and Eneni et al.[33] from South-West and South-South, Nigeria, also agrees with our finding that sex ratios do not follow a general trend month-wise. Our findings further agree with several other reports indicating that sex ratios vary from one geographical location to another.[46],[48] Furthermore, in ex-Yugoslav nations, a study showed that sex ratios vary with little change in latitude.[57] Apart from geography, several factors such as socio-economic development as well as levels of fertility, industrialisation and health-care systems may together affect SRB within specific countries or regions.

  Conclusions Top

Sex ratios from Southwest Nigeria are as diverse as other populations from the literature, with ratios ranging between 99.7 and 106.0. These figures may not represent the accurate SRB from this region; however, since records of births occurring at home and abandoned infants were not captured by this study.

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Conflicts of interest

There are no conflicts of interest.

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  [Figure 1], [Figure 2]

  [Table 1], [Table 2], [Table 3], [Table 4]


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