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Despite the presence of nuptial gifts and the known benefits to females and offspring, some females are still monogamous. There are multiple reasons in why other species are monogamous like access to males, low sperm population, and age. Some Pieris napi are likely monogamous due to variation in egg production between polygamous and monogamous females. Polygamous females rely on male mates in order to increase their reproductive output, but polygamous females that only mate twice may not have as high of reproductive output as monogamous females do.

Fecundity benefits of polyandry
Fecundity in P. napi varies with the number of eggs produced as well as the lifespan of the female, and is positively influenced by body mass , other genetic factors independent of body size , and with the degree of polyandry. Fecundity has a positive correlation with the amount of spermatophore material and ejaculate from males received. Thus, mating with recently unmated males can increase overall female reproductive output, though small females are unable to compensate for the negative effects of size on fecundity by mating multiply. Material benefits derived from males means that polyandrous females have overall higher lifetime fecundity, produce more offspring, and have faster-developing offspring compared to monandrous females.

Polyandry versus monandry is a genetically-based mating strategy
Despite the benefits of mating multiply, many female P. napi exhibit monandry and mate only once, irrespective of the number of quality of mates available. Monandry is more common in northern populations of P. napi, and sperm competition is correspondingly lower in the north. In addition to whether or not they mate multiply, monandrous and polyandrous females exhibit different life history strategies: at the start of reproduction females that are monandrous produce more eggs than polyandrous females. It is hypothesized that this life history difference is why monandry is more common in the most northern parts of the species’ range, as early investment in reproduction can be more beneficial with shorter mating seasons.

Maintenance of the monandry/polyandry genetic polymorphism
The different mating strategies follow different heritable reproductive tactics with monandrous females relying on larval derived resources to realize their fecundity; whereas, polyandrous females rely on male donations to realize their reproductive output. The maintenance of the two mating systems has been hypothesized to be due to the availability of male nutrients, which can vary with how male-biased the operational sex-ratio is. Males who withhold nutrients are considered to be less fit for survival in the P. napi population. This is because there are more males than females within the population; therefore, the males are less likely to have recently mated and would have a larger nuptial gift. Moreover, males are unlikely to find another receptive female within the mating season. Polyandrous females increase lifetime fecundity with the promise of direct benefits they receive from males and mate sooner after eclosion than the previous generation. However, higher mating rates in females comes with a cost, which can explain the presence of monandrous females in primarily polyandrous populations. Polyandrous females have a higher cost of mating characterized by an increase in time spent looking for mates and time spent mating. This results in a decreased time spent looking for food. Suboptimal mating conditions can result in forced monandry and a reduction in their life span.