10.7 Correlation of Environmental Factors with Mating Systems

In the table below, we have a comparison of the 4 mating systems discussed in this chapter.

Each of these matings systems have many details to keep track of, which can be difficult to memorize. We’ve discussed both causes and effects of different mating systems. Instead of trying to memorize which causes and effects belong to which system, try to keep in mind the reasoning why some of these traits favor some mating systems over others, and why some phenomena result from certain mating systems but not others.

For example, one cause we discussed is the distribution of resources (Figure 10.9). When resources are aggregated (clumped), polygyny is more likely. When resources are distributed (spread out), monogamy and promiscuity are more likely. We also discussed how a high need for parental care will likely favor monogamy, because parental care will be shared between parents. Another cause not discussed here but that we will explore together in class is female fertility. As a teaser, if female fertility is visible to males, this will favor one mating system over others… Stay tuned for more information about the effects of several aspects of female fertility!

We also discussed effects resulting from different mating systems. For example, infanticide is a sad but common occurance for many species. In species where paternity may be unknown for offspring, such as promiscuity, there is less of a risk of infanticide as an individual will not want to risk killing their own child. If we compare this to polyandry or polygyny, where individuals of one sex compete for access to individuals of the other, we are more likely to see an increase in infanticide as individuals fight over mates, sometimes killing offspring under the care of a parent, in order to have as many mates as possible.

Each of these mating systems has certain costs and benefits for each of the sexes. Polyandry allows females to increase their reproductive success through more potential successful matings. This is often difficult for the sex who expends more energy on gamete production. Polyandry is, as discussed in 10.3, highly favorable when offspring mortality rates are high. In addition, polyandry has different effects on male fitness than in polygyny – in polyandry, males expend energy through parental care and are often guaranteed to have some offspring, as females will mate with multiple males.

Polygyny can be thought of as the inverse of polyandry — with females spending energy and time on gamete production and parental care, the males are left with energy to defend and compete for their mates and offspring, as well as having an opportunity to spread the highest quality genetics to future generations. In contrast to polyandry, most males will not have any offspring because females only choose certain males. Males with higher quality genes will more often be chosen by females to mate and protect the offspring. Polygyny also sets the stage for the evolution of sexual dimorphism and males having elaborate traits that can be costly for survival but beneficial for reproduction.

Promiscuity allows for almost all members of a species, both male and female, to mate and have offspring. The energy required for parental care is more limited in these species as offspring often require either very little, or in some cases a larger social group will all contribute to the offspring’s care. Additionally, there is higher genetic variability in the offspring as there is not one particular sex vying for the best mates.

Monogamy, similar to promiscuity, allows almost all members of a species to find a mate. The energy expended by parental care is then divided evenly between the parents, with both sexes taking on the same role of either intensive or absent care of the offspring.

Exercise

Use the flowchart below to practice connecting mating systems to sexual selection. The bullet points show different options for gamete investment, parental care, and so on. Starting at the left for a particular mating system, such as polygyny, determine which bulleted option is most likely. When you get to the final box, is dimorphism likely? If so, which sex will have elaborate sexually-selected traits? Or, will males and females be monomorphic? Complete this process for all four mating systems.