Population geneticists typically quantify genetic viability of a population using mean and variance of successful gametes per individual as a proxy for genetic variability (Lande, 1994). Mating systems provide a potentially unifying framework for integrating genetic and demographic perspectives on population viability. While effect of small population size on mating success has been considered in context of Allee effects for small populations, few viability models include behavioural variation (Anthony and Blumstein, 2000).
Extent to which biases in OSR determine population growth is even less clear. In monogamous animals, female biased sex ratios are clearly a demographic liability, whereas in polygynous animals, this may improve viability of a small population, relative to a population of same size with 1:1 sex ratio.
However, even for animals that have adopted mating systems with strongly biased sex ratios, social dysfunction and reproductive collapse may occur, when bias exceeds critical thresholds (Milner-Gulland, 2003).
This suggests a more complex relationship between demographic consequences of bias in sex ratios and viability of a population. While sex ratio is an important parameter in demography of animal population, influence of individual variation on population dynamics remains poorly understood.
Territorial behaviour exhibited by -male animals daring breeding season greatly influences the number of males copulating with females and thus siring young. Recent empirical evidence suggests that at extreme biases in male sex ratio and small population size, number of males in a population is related to female fecundity.
Garber (2006) has suggested that effects of sex ratio on viability of small populations depend strongly on mating system of local species. He also gave view of mating system theory. Mating system dynamics may influence a population’s vulnerability to harvest and may, therefore, determine recovery as well as extinction of populations. Exceeding upper and lower thresholds in a population’s sex ratio result in a reproductive collapse (Milner- Gulland, 2003).
Behavioural options for reproduction are diverse. Generally mating systems have been distinguished based on (1) number of individuals with which a male or female copulates; (2) whether a male and a female form a pair bond and cooperate in parental care and (3) how long the pair bond is maintained (Alcock, 2001). Number of combinations of these variables is substantial.
Among diversity of mating options, territoriality may have a profound impact on a population’s sex ratio (i.e., if only a small fraction of males are breeding). Territoriality may also influence other parameters that are important for population viability. For example, among yellow bellied marmots, there is considerable variation in number of females occupying a male’s territory.
Average annual reproductive success of females actually declines as group size increases (Downhower and Armitage, 1971). In polygynous mammals, such as sea lions, male reproductive success is largely determined by number of females present in their territory.
Males compete for males and females invest in parental care (Trivers, 1972). Territorial behavior is likely to be of particular relevance for population dynamics in polygynous populations, resulting in biased OSRs.