Increased, Differentiation to Testis leads to Increased, Male Biased Sex Ratio

A major uncertainty for this KER involves what would be defined as "normal" for degree of testis differentiation and by extension sex ratio. There needs to be knowledge as to baseline expectations for testis differentiation for a given species in a given habitiat (or lab setting) to ascertain whether increases are occurring. Baseline information of this type is available or can be inferred for some species but certainly not for all that might be considered.

A second significant uncertainty involves situations where the gonad cannot be clearly defined as either testis or ovary. This can occur in some fish and amphibian species, where the gonad has cell types indicative of both testes and ovaries (Abdul-moneim et al. 2015). In these instances classification of individuals as male versus female may not be possible, requiring a third category related to an intersex condition. There are seemingly multiple underlying causes of intersex, one of which appears to be exposure to estrogenic chemicals during gonad differentiation (Jobling et al. 1998; Norris et al. 2018; Grim et al. 2020).

A third uncertainty involves whether all individuals defined as males based on gonad phenotype will have the same degree of function in terms of producing viable gametes. It is possible, for example, that genotypic females which develop a male phenotype due to an environmental factor such as exposure to an endocrine-active chemical may not be functionally equivalent to a genetic male relative to sperm production/viability. This could be an important consideration relative to the types of predictions attempted based on a male-biased sex ratio in a population.

Not applicable.

Timescales will vary based on species-specific developmental rates, but since one KE often will define the second (i.e., an animal is defined as a male based on the presence of testis) timescale may not be a relevant consideration.  

Not applicable.