Sperm variation in Great Tit males (Parus major) is linked to a haematological health-related trait, but not ornamentation
Abstrakt
The phenotype-linked fertility hypothesis (PLFH) proposes that both sexual ornaments and sperm traits are phenotypically plastic and co-affected by environmental factors through individual condition, resulting in a positive correlation between ornament expression and functional fertility. Ornaments may then serve females in the identification of the most fertile males.
Despite intense research on the relationship between sexual characters and male ejaculate quality, published results are not consistent with the PLFH. The aim of our study was to test if sperm morphology is associated with sexual ornamentation and several health/condition-dependent traits in Great Tit males (Parus major).
We evaluated the association between sperm morphology and two types of ornaments, carotenoid- and melanin-based ventral feather coloration, to evaluate predictions of the PLFH. As surrogates for condition and health/stress status, we used standardized male weight and the peripheral blood heterophil to lymphocyte ratio (H/L).
Also, we used the immature erythrocyte frequency as a trait linked to the rate of haematopoiesis, and presumably metabolism and pace of life. Our results support an association of sperm traits with health-related traits: the within-male variability in total sperm length was negatively related to the H/L ratio.
This either suggests that birds maintaining low sperm variability may afford to invest more into heterophil production or, in contrast to the PLFH, there could be a trade-off between individual investment in reproduction (ejaculate quality) and the avoidance of long-term physiological stress. Contrary to the predictions of the PLFH we were unable to identify any parameter of sperm morphology associated with either body condition or the expression of male sexual traits.
Thus, our study contributes to evidence rejecting the hypothesis of ornamental involvement in fertility selection, while giving weak support to the sperm competition theory.