Abstract
Testes size has been regarded as the trait of choice in males to genetically improve female reproductive performance. The physiology of testicular development, the factors affecting it and the genetic relationships between testes size and female reproduction in sheep are examined in this review. Testicular development in young rams follows a sigmoid curve with two distinct phases and is more closely associated with body weight than with age. Testicular size has been found to be significantly correlated with LH (luteinizing hormone), FSH (follicle stimulating hormone) and testosterone concentrations of the blood. Photoperiod and season of birth are the main environmental factors influencing testicular growth in ram lambs, but in harsh environments nutrition can be an important limiting factor. From the genetic perspective, testicular size would be a useful trait to base selection upon in order to improve female reproduction if it could be measured at a young age, if the heritability was high and if the genetic correlations with female reproductive traits were large. The average of forty estimates of heritability for various measurements of testicular size in sheep was 0.33 (range: 0 to 0.75 ). There is still some uncertainty with regard to the reliability of the genetic correlations between measurements of testicular size and female reproduction in sheep.
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