Increased, Male Biased Sex Ratio

Sex ratio is the ratio of males to females in a population. A male-biased sex ratio for a given species is defined as a significant increase in the number of males, relative to the average ratio found in most populations of that species.

While simple in concept, the “normal” sex ratio for a given species can be challenging to define.

• In organisms with genetic sex determination (GSD) such as mammals and birds, as well as many poikilothermic vertebrates, the male to female ratio often is 1:1. In these instances it is easy to define a deviation from normal in terms of either a relatively greater number of males or females.
• When considering organisms with environmental sex determination (ESD), such as many reptiles and some amphibians and fish, deviations from a 1:1 relationship can and do occur that nonetheless may be normal in the context of the organism’s life history. For example, some reptile species have temperature-dependent sex determination where differentiation of developing organisms to males versus females predominates at different temperatures (Norris and Carr 2020).
• Further complicating a generalized definition of normal sex ratios are situations where sexual differentiation is determined by a combination of genetic and environmental variables, such is the case in many fish species.

Even in species potentially requiring fewer males than females to maintain a viable population, at some point a female-biased population could become problematic in terms of having an adequate number of males to fertilize eggs produced by females or, in the longer term, ensure a robust level of genetic diversity in a population. Further, in situations where a population is male-biased relative to conditions considered normal for a given species, overall productivity may be negatively impacted due to fewer females being available to produce eggs.

A variety of external factors can produce populations that would be characterized as abnormally male-biased based on analysis of phenotypic sex ratios (examples, not comprehensive):

• Differential mortality can occur in males versus females. This might include situations where predation or harvest techniques geared toward larger individuals, which could be either males or females depending upon species may effectively skew the apparent male to female ratio higher.
• Endocrine disruption during early development, most prominently, during gonadal differentiation. For example, in some fish species, exposure during gonadal differentiation to androgen receptor agonists or inhibitors of cytochrome P450 19a1 (aromatase), an enzyme involved in the synthesis of 17β-estradiol, can caused male-biased populations (Delbes et al. 2022).  

Fundamentally, determination of sex ratio (and consequently male-biased sex ratio) is based on counts of the number of males and/or non males in a population, or some statistically representative sub-sample of a population.

• For mature animals that are sexually dimorphic, direct observation of phenotypic secondary sex characteristics is a common method for assessing sex ratios.
• In animals that are not sexually dimorphic or those in pubertal/juvenile stages examination of the gonad, either via gross observation or histological examination is required to determine phenotypic sex.
• There can be instances where gonads cannot be clearly identified histologically as either testis or ovary because cell types indicative of both are simultaneously present. This type of intersex condition has been observed in some amphibians and fish, and may require a third classification category (Abdul-moneim et al. 2015).
• For animals with GSD, genotyping or the use of genetic markers can also be employed to determine genotypic sex ratio.  However, it is noted that there are cases where genotypic sex ratio and phenotypic sex ratio may not be equivalent.

Considerations when evaluating measurements of sex ratio:

• Care needs to be taken to collect an adequate number of animals to ensure that statistical power of the sex ratio point estimates is sufficient to address whether true deviations from normal conditions exist. It is not uncommon for published papers to report skewed sex ratios based on sample sizes far too small to result in environmentally meaningful conclusions.
• Determination of sex ratios is generally straight-forward in a laboratory environment where all (or a defined proportion of) animals from a particular experimental treatment of interest can be collected and examined. Under such conditions, determination of a male bias relative to normal is a simple matter of a statistical comparison between the treated and control groups.
• Determination of sex ratios in the field/wild can often be quite challenging as variables such as sampling gear used, or time and location of collection could bias samples toward one sex versus another. Additionally, often more difficult than ascertaining phenotypic male to female ratio is determining whether observations deviate from what would be considered normal for a particular species of interest. As discussed above (Key Event Description), the relative number of males normally expected will be taxa-dependent, and in some cases may also vary by region and/or environmental conditions. In cases where a male bias is being proposed for a population in the field, compelling scientific support for the “normal” sex ratio expected in the field and for the unbiased nature of the sampling should be made.

Any sexually reproducing organism can theoretically experience a male-biased population, although the phenomenon certainly has not been demonstrated empirically in all species of potential concern.

Abul-moneim, A, DP Coulter, CT Mahapatra and MS Sepulveda. 2015. Intersex in fishes and amphibians: Population implications, prevalance, mechanisms and molecular biomarkers. J Appl Toxicol 35:1228-1240.

Delbes, G, M Blázquez, JI Fernandino, P Grigorova, BF Hales, C Metcalfe, L. Navarro-Martín, L Parent, B Robairee, A Rwigemera, G Van Der Kraak, M Wade and V Marlatt. 2022. Effects of endocrine-disrupting chemicals on gonad development: Mechanistic insights from fish and mammals. Environ Res 204B,  https://doi.org/10.1016/j.envres.2021.112040

Norris, DO and JA. Carr. 2020. Vertebrate Endocrinology, 6^th Edition. Elsevier.