Honey, I shrunk one of my gonads! Does ‘feminization’ of behaviour cause asymmetric testis development in coucals?

Wolfgang Goymann 

In most birds, females develop only a single ovary. This asymmetry is usually explained as an adaptation to reduce weight for flight, or to prevent damage to hard-shelled eggs by a second egg in the other oviduct. By contrast, males retain paired testes with the left one typically being larger. Coucals break this pattern: as sex-role reversal intensifies across the genus, males show a progressive shift towards a dominant right testis, culminating in complete loss of the testis in the most sex-role reversed species. The prevailing explanation — that testis loss lowers testosterone to enable male-only care — has been rejected. I propose instead, that testicular asymmetry in coucals is a non-adaptive developmental by-product of estrogen-mediated feminization of the male brain. Estrogens normally drive the development of the left ovary. Transient estrogen production in males could prevent further development of the left gonad into a testis. In contrast, because of its innate testicular predisposition the right gonad can grow into a functional testis, once estrogen production stops. Hence, reduction of the left testis may be a developmental by-product of behavioural feminization in coucals and could be an excellent model for studies on sexual differentiation.