Why Does Sexual Reproduction Exist?

Sexual reproduction, and everything surrounding it, is one of the most dangerous activities that an individual can engage in. In many species, sexual reproduction ends in death.

Given the dangers inherent in sexual reproduction, why in the world does most of the animal kingdom engage in it?

Simply put, sexual reproduction is the best biological strategy yet devised for maximizing fitness. Much of this has to do with tissues like the nervous system and immune tissues, where having a huge degree of variability ensures that a wide variety of organisms are created, where at least a few will succeed beyond their wildest dreams in the environment they find themselves in.

In general, sexual reproduction involves the sharing of genetic material between two parents: one male and one female. When the male sperm injects its DNA into the female egg, an exquisite series of biochemical reactions occurs, which results in the activation and inactivation of particular genes in a particular sequence.

The first “scrambling” of DNA, then, occurs between chromatids that arise from the male and homologous chromatids that arise from the female. In this context, homologous means the same length and with the same general sequence. For example, in humans, the largest chromosome, chromosome 1, is made up of two chromatids, joined at the centromere. Each chromatid from chromosome 1 is a single molecule which consists of 249 million base pairs of DNA. (A base pair is a combination of an A from one strand, and a T from another strand; or a C from one strand, and a G from another strand. Base pairs are held together by hydrogen bonds.) Offspring inherit one chromatid representing half of chromosome 1 from the mother and one chromatid representing chromosome 1 from the father.

But that’s not the end of the DNA scrambling which is the driving force behind sexual reproduction. Meiosis is the mechanism of cell division and DNA sorting which occurs in the formation of gametes (male sperm and female eggs). It has two phases; during the first phase, aptly named Meosis I, there is an event called crossing over. This results in a chromatid which is a medley of both of mother’s chromatids making up both halves of a chromosome. Crossing over is illustrated once per chromosome in this diagram, but in reality, there are on average 2-3 crossing over events on a large chromosome like chromosome 1. Thus, chromosome 1 usually is made up of 3-4 pieces derived from both chromatids. Each chromosome must create at least one crossing-over point (chiasma); this is called the obligate crossover. Without this physical connection, chromosomes do not properly sort into the daughter cells of meiosis I and fatal chromosomal abnormalities will occur.

Estrus vs Menses

Most female animals are only receptive to sexual reproduction during a narrow window of time, generally twice a year. This period is called estrus (or more commonly, we say the animal is “in heat”).

A few species, including humans, use a different mating strategy called menses (from the Latin word for “month”, since human menstrual cycles last about a month).

The neuroscience of reproduction, and therefore reproductive behavior, differs between animals with estrous cycles vs menstrual cycles. There are other differences in reproductive physiology which are beyond the scope of this book.

Females with estrous cycles are only sexually receptive for a relatively short period of time, perhaps 5% of the year or less. In neuroscience terms, that means that the chemical and anatomical features that support reproduction are only temporarily operational.

Females with menstrual cycles are sexually receptive throughout their lifetime. In neuroscience terms, that means there is more stability in the chemical and anatomical features that support reproductive behavior.