Testis Size and Asymmetry in the Guinea Fowl (Numida Meleagris): A Test of the Compensation Hypothesis

Abstract

Despite numerous studies on testicular asymmetry in birds, there are still inconsistent reports on the pattern of asymmetry in species belonging to various phylogenetic groups and exhibiting diverse lifestyles. In addition, there is a lack of clarity on whether functional differences exist between the left and right testes despite differences in size, as well as no evidence of naturally occurring compensation in testis size in terrestrial and galliform birds. A study involving 400 Guinea Fowl (Numida meleagris) cocks was conducted to determine asymmetry and compensatory growth in the testes as well as age-related changes in absolute and relative testicular asymmetry. The left testis had significantly higher weight, volume and relative weight than the right testis. Similarly, populations of round spermatids, type I spermatocytes, total germ cell, Sertoli efficiency, actual and apparent seminiferous tubular diameters and testicular sperm production were higher in the left than the right testis. Correlations between the left and right testicular gross anatomical and histological biometric traits were strong and positive. Left biased asymmetry was recorded in the Guinea Fowl. Absolute testicular asymmetry increased with age, but tended to stabilise from 20 weeks of age, while relative testicular asymmetry remained stable throughout. Both absolute and relative testicular asymmetries were higher in breeding than non-breeding males. Compensatory growth was recorded in both testes with no difference in the degree of compensation. The left biased asymmetry observed in the Guinea Fowl was reflected in functional differences between the two testes.