Sexual dimorphism is a usual biological process that occurs in humans with regard to body parts. Height and all head dimensions measured in this study were significantly greater in males compared to females, except for the head width that was not significant among the Kalabari. Concerning height, it is known that males attain puberty 2 years later than their female counterparts, affording them additional time to grow (Eboh and Igbigbi 2017); this could be a reason for the gender difference observed in the present study. Significant greater body height in males compared to females have also been reported in previous studies (Marko et al. 2018; Eboh and Igbigbi 2017; Ukoha et al. 2015; Shah et al. 2015; Agarwal et al. 2014; Mansur et al. 2014; Agnihotri et al. 2011; Ilayperuma 2010; Pelin et al. 2010), which also reported that males are significantly taller than females.
Structurally, the general appearance of the male skull is such that it tends to be larger and heavier or thicker than that of the female which is smaller and lighter (Gleeson 2016). In the present study, the head circumference, length, and width observed to be significantly greater in males than females are to further emphasize and confirm the aforementioned with regard to the five ethnic groups studied.
The differences in the head circumference between males and females observed in all the ethnic groups in the present study are in tandem with previous studies (Marko et al. 2018; Eboh and Igbigbi 2017; Ukoha et al. 2015, Mansur et al. 2014; and Agnihotri et al. 2011). In these studies, they also reported that the head circumference is greater in males than in females. On the other hand, the result of the current study is at variance with Jervas et al. (2015), who reported otherwise. Genetic, environmental, and ethnic factors may be the reasons for the variation.
It was reported in some previous studies (Eboh and Igbigbi 2017; Ukoha et al. 2015; Shah et al. 2015; Agarwal et al. 2014; Ilayperuma 2010) that the head length in males is significantly greater when compared to females; the findings in the present study that males have a longer head than females support the aforementioned. Nevertheless, Jervas et al. (2015) observed no significant difference between males and females in the head length. This discrepancy can be attributed to factors that affect anthropometric measurements.
The significant differences in the head breadth observed between males and females among the Bekwara, Esan, Ogbia, and Urhobo in the present study agree with some previous reports (Jervas et al. 2015; Ukoha et al. 2015; Shah et al. 2015; Agnihotri et al. 2011; Ilayperuma 2010) that reported a significant greater head breadth in males than in females. The aforementioned observation also holds true in four of the five ethnic groups studied in a previous research (Eboh and Igbigbi 2017). On the contrary, among the Kalabari in the present study, there was no significant difference in head breadth between males and females, which is in line with the report on the Annang in a prior study (Eboh and Igbigbi 2017). Population differences may account for variations observed.
The result of the present study indicates in 36 of the 45 cases or data across the five ethnic groups that there was a significant relationship between height and head dimension (p < 0.05). The low correlation (R < 0.5) between height and head dimensions in 35 of the cases indicates a poor relationship. Agarwal et al. (2014) reported a significant low correlation between the height, and head length and width. Ilayperuma (2010) reported that the correlation between height, and head length and width was significant in both genders.
Marko et al. (2018) observed a significant correlation between height and head circumference. Jervas et al. (2015) observed no significant correlation between height and head circumference. Mansur et al. (2014) noted a significant correlation of stature with the head circumference in both genders and combined data. Ukoha et al. (2015) observed significant correlations between the height, and head circumference, length, and width in both genders, except for the head breadth in which the female data was not significant.
Agnihotri et al. (2011) noted that the head circumference and length showed a significant correlation with height, but the head width showed otherwise. Eboh and Igbigbi (2017) reported that the circumference and head length had a significant positive correlation with the height in all the five ethnic groups studied, except for the male data among the Ukwuani and Bini, in which the correlation of height with the head circumference and head length, respectively, was not significant. In addition, Eboh and Igbigbi (2017) also reported that only combined and female data among the Bini and Ikwere and combined data among the Izon showed a significant positive correlation between the height and head width.
Provided correlation (R) of head dimensions with height was significant, regression equations were formulated (Tables 3, 5, 7, and 9). Results showed in all head dimensions studied, in all the data across the five ethnic groups, that only in male data among the Bekwara, was the proportion of the variance in head circumference and width that could be explained by the model was up to 30%, which is low. Similar results were reported by Eboh and Igbigbi (2017), while Jervas et al. (2015) noted a very low value of R2 in female data between the height and head width.
The high error of estimates based on the two regression methods indicates that the level of reliability of the models is low. Variable values of the standard error of estimates have been reported in previous studies (Marko et al. 2018; Eboh and Igbigbi 2017; Ukoha et al. 2015; Krishan 2008; Krishan and Kumar 2007). Genetic, environmental, and nature of the data may be responsible for the variation.