Mate Selection

Evolutionary theory popularly suggests the idea of natural selection. Natural selection is a process by which evolution occurs. In natural selection, individuals with slightly different traits are born. If one trait makes an individual more likely to die or less likely to reproduce, those traits will not be passed down. As a result, hereditary genes that increase chances of survival or reproduction are passed down to offspring, occasionally affecting large scale change. An example of a hereditary trait that does not help with survival but does help with reproduction is male peacock plumage. Darwin, who famously proposed natural selection wrote, “The sight of a feather in a peacock’s tail, whenever I gaze at it, makes me sick.” This is because excessive plumage doesn’t help with flight or food collection, but it is attractive to female peacocks. 

Mammals reproduce in a variety of ways. These variant reproductive strategies stem from various conditions and create noticeable effects on populations. 

• Solitary – one male occupies a large territory and interacts with other females, but not other males.
  • Interactions among males can lead to fights
  • Seen in Orangutans
• Polygyny – two types
  • One-male – one male, many females 
    • Male and females form a social group
    • Leads to high level of sexual dimorphism
    • Seen in Gorillas
  • Multi-male – many males, many females
    • Complicated interactions to decide who mates with whom.
• Polyandry – one female, many males. 
  • Males help with child raising
  • Very rare
• Monogamy – one male, one female
  • Gibbons (Hylobates)

Polygyny leads to a high level of sexual dimorphism because of the competition it creates between males. Males compete for high rank in order to have better reproductive success. This process can be seen in baboons and chimpanzees (Pan troglodytes). Successful males tend to have larger bodies and canines than their competition. Thus, fossils and skeletons found with large canines and bodies can be assumed to have a reproductive system of one-male/many-females. In fission-fusion systems, the social groups that are formed continuously change, leading to more similarities between males and females, or reduced sexual dimorphism.

The above video demonstrates gorilla mating and the male hierarchy in gorillas.

In groups with alpha males and many females, females generally stay with their group and raise children. In order to raise children, females must invest heavily in their offspring. That investment can come in the form of energy or time. Because females are not able to reproduce while they are nursing, infanticide is a common occurrence when an alpha male is replaced. Counteracting infanticide thus becomes a female reproductive strategy. Infanticide is counteracted by females’ promiscuity. Additionally, physically weaker males can gain social power by befriending females. After infancy, as children grow, most leave. Female children only stay if there is a new alpha male.

Male mammals do not typically invest in their offspring unless doing so is necessary for the survival of the offspring. Males are interested in increasing the number of their offspring, and their limiting resource is typically access to females, creating the systems we see. Successful males are typically larger in body size and canine size in order to win competition. Males must also be reproductively attractive, which is where male peacock plumage helps.

Because of the competition, it’s difficult for male primates to transition to pair-bonding. Pair-bonding is the most frequent form of reproductive system seen in humans. But this transition obviously occurred. However, before we understand human reproduction, let’s look at primate reproductive systems and the reproductive systems of our oldest ancestors.