The evolution of laterality, that is the biased use of laterally paired, morphologically symmetrical organs, has attracted the interest of researchers from a variety of disciplines. It is, however, difficult to quantify the fitness benefits of laterality because many organs, such as human hands, possess multimodal functions. Males of the earwig Labidura riparia (Insecta: Dermaptera: Labiduridae) have morphologically similar laterally paired penises, only one of which is used for inseminating the female during a single copulation bout, and thus provide a rare opportunity to address how selection pressure may shape the evolution of population-level laterality. Our population studies revealed that in 10 populations, located at 2.23-43.3° north, the right penis is predominantly used for copulating (88.6%). A damaged penis was found in 23% of rare left-handers, suggesting that the left penis can function as a spare when the right one is damaged. By pairing L. riparia females with surgically manipulated males, we found that males forced to use the right penis outperformed left-handed males in copulation (the probability of establishing genital coupling during the 1-hr observation period: odds ratio [OR] of 3.50) and insemination (probability of transferring a detectable amount of sperm: OR of 2.94). This right-handed advantage may be due to the coiled morphology of the sperm storage organ with a right-facing opening. Thus, female genital morphology may play a significant role in the evolution of handedness and may have acted as a driving force to reduce penis number in related taxa.
In the initial clinical examination of a child with ambiguous genitalia an accurate measurement of the corporeal length is needed. Most often the corporeal length is measured with a ruler from the symphysis pubis to the tip of the glans of a stretched penis. More recently, ultrasound has been successfully used to measure corporeal length. This study aimed to (i) establish normal values for corporeal length in normal male newborns using ultrasound measurement, (ii) compare these measurements to stretched corporeal measurements, (iii) compare the corporeal length of newborns of different races, and (iv) determine the relationship between corporeal length and birth weight, birth length and head circumference. This was a prospective study of 141 newborns. Ultrasound imaging was done in an oblique parasagittal plane such that the corpus could be included in a single image and measured, Stretched corporeal length was measured with the penis stretched alongside a wooden spatula and the length from the pubic bone to the tip of the glans was marked on the spatula and measured. By ultrasound measurement the mean corporeal length of the normal newborn was 3.18 +/- 0.56cm. There was no significant difference in the mean corporeal length when determined by ultrasound and by stretched corporeal measurement. There was no significant difference in the mean corporeal length of the different races when the length was determined by either ultrasound or stretched corporeal measurement. There was a positive correlation between ultrasound length and birth weight and birth length. However, there was no correlation between ultrasound length and head circumference. There was no significant correlation between the stretched length and either birth weight, birth length or head circumference.
Micropenis is an important sign in congenital hypopituitarism and various disorders. Documented norms for penile length exist only for babies of Caucasian and Middle-Eastern origin. This study was carried out to establish such norms for Asian newborns. We studied 228 male live births within their first three days of life. Stretched penile lengths were marked off on unmarked wooden spatulas, which were placed vertically along the dorsal aspect of the penis, with one rounded end on the pubic bone. The mean penile length +/- S.D. for the full-term Asian baby was 3.6 +/- 0.4 cm. Race had a significant effect: Chinese 3.5 cm, Malay 3.6 cm and Indian 3.8 cm. Penile length correlated with birth weight and gestational age. Asian babies thus have similar norms to Caucasian babies. An Asian newborn whose penis measures less than 2.6 cm has micropenis and may need prompt investigation for underlying endocrine disorders.
Micropenis may be an important sign of underlying hypogonadism or pituitary hypofunction in the neonatal period. Penile lengths of normal newborns have been reported in many Western populations. However, the data may not be applicable in the Asian or the multiracial Malaysian population. Our study aimed to establish the normal penile length and testicular volume in term newborn infants in the major ethnic groups in Malaysia.