Further studies on the month-of-birth effect on body size: rural schoolchildren and an animal model

A previous study (Henneberg and Louw: Am. J. Hum. Biol. 2:227-233, 1990) showed that urban schoolchildren born between August and January were taller and heavier by 13 to 17% of the standard deviation (S) than those born in February to July. The study is now extended to 1,522 impoverished rural schoolchildren from the Little Karoo district of S. Africa and to 804 German Shepherd dogs kennelled at a breeding station in the Transvaal. The dogs provided a quasi-experimental model because for all of them nutrition and living conditions were practically identical throughout the year. The month-of-birth effect in the body weight of dogs was found to be stronger than that in urban children: dogs born between August and January were heavier by 22% of S than those born in February through July. In rural children the effect is present, but its magnitude is somewhat less and the peak is shifted by 2-3 months. Children born in May through October are 11% of S lighter than those born in November through April. It seems that living conditions characteristic of extreme poverty tend to diminish the effect. In both the animal model and rural children the month-of-birth effect decreases with advancing age and is more pronounced in males than in females. These findings are in agreement with previous observations in urban children from Cape Town. Since the living conditions of dogs and humans differ, as do the climatic conditions of Cape Town, Little Karoo and the Transvaal, it is surmised that the similarity of the effect may result from factors influencing the entire planet in a uniform way.