"Month of birth effect" does not alter longitudinal growth in an experimental animal model

Previous cross-sectional human and animal data collected in the southern hemisphere suggest that month of birth influences growth throughout life; individuals born between August and January (spring/summer) were larger than individuals born between February and July (fall/winter) throughout their life cycle. It has been suggested that there is some form of annual global variation which may be caused by extraterrestrial factors. The present study, utilizing an experimental animal model, investigated the "month of birth effect" in the northern hemisphere on the somatic and skeletal growth of rabbits. The study sample consisted of 135 laboratory bred and reared New Zealand white rabbits (Oryctolagus cuniculus) from 62 different litters; spring/summer rabbits (n = 75) were born between February and July; fall/winter rabbits (n = 60) were born between August and January. Serial longitudinal body mass and craniofacial radiographic growth data were collected at 10, 25, 42, 84, and 126 days of age. Results from a two-way ANOVA showed no significant (P > 0.05) differences in the month of birth main effect or in the age × month of birth interaction for either somatic or skeletal growth. Results from the present study do not support the "month of birth effect" hypothesis. It was concluded that variations in growth patterns are more likely controlled by local environmental factors such as hemispheric seasonality in photoperiod or temperature, and not by more exotic factors such as the earth's orbital trajectory around the sun or global energy fields. Am. J. Hum. Biol. 9:481-486, 1997. © 1997 Wiley-Liss, Inc.