I know the title may seem equivocal and the purpose is precisely to be that. Today we will deal scientifically with how the size of the penis can be determined by various factors and how these can influence the social dynamics of entire populations of marine organisms. The topic may seem futile but in reality, after this reading you will understand how extraordinary the world of animals is and how much instinct and environment can modify and make a genital structure adaptable. I would begin by explaining the 2 types of competition utilized during the reproduction of the organisms:

• SPERMATIC COMPETITION: it is one of the post copula sexual selection mechanisms that involves all those competition processes between the sperms of two or more males to fertilize the same egg. On a practical level this concerns three distinct factors which are:
  • Sperm quantity
  • Sperm quality (sperm speed and vitality)
  • Order of copula (which, depending on the species, is relevant or not, and is fundamental especially in those species where the female is shown to be “unfaithful” and therefore promiscuous).

The selective pressure generated by this mechanism has led to the evolution of techniques such as the elimination of sperm deposited by previous rivals (with spermicidal substances for example) or the affixing of structures that prevent new copulas from the female (copulation caps).

• MALE-MALE OR SEXUAL SELECTION COMPETITION: for this mechanism Darwin said:

“In such case sexual selection must have come into action, for the males have acquired their present structure, not from being better fitted to survive in the struggle for existence, but from having gained an advantage over other males, and from having transmitted this advantage to their male offspring alone.”

The essence of sexual selection lies in the differential of access to reproductive partners which involves a differential in the coupling success of various individuals: this can depend both on a superior ability to compete with individuals of the same sex and from being preferred by the individuals of opposite sex. Sexual selection therefore leads to the appearance of structures used as “weapons” (e.g. tooth of the narwhal) in the competition between males and the choice of some males over others because they appear in the eyes of the female those healthier.

After a summary of the dynamics that regulate the reproductive act, let’s move on to some practical examples to understand how these 2 types of competition can affect the male sexual organ.

We will talk about pinnipeds (seals, sea lions and walruses) that are an ideal model system to contrast investment in pre- and postcopulatory traits as differences in social structure.  Pinnipeds can be categorized into two groups species with harems and without harems.

In species without harems we can observe the so call sperm competition that takes to an increase in size of baculum and of all sperm components with an elongation of the head, which results in faster swimming speeds.

In contrast, in species with harems the association between body size corrected baculum length and sexual size dimorphism remained significantly negative.

In harem-holding species, where sexual size dimorphism and variance in male mating success is extreme, a small number of territorial males secure most of reproductive acts. With such high reproductive success, the risk of sperm competition may be low and therefore selection to increase testes size above the naturally selected testes size in species with harems could be absent. So, for these types of pinnipeds BIGGER IS NOT BETTER.

A general prediction is that expenditure on precopulatory traits limits expenditure on postcopulatory traits and vice versa.

It is therefore clear that animal nature and biology (not only the case of pinnipeds) have evolved adaptively to the social structure and habits of a species. This shows how the preservation of the species remains the only fundamental pivot and how the animal instinct is therefore regulated by laws that are not random. These laws have led to modifications of the male genital apparatus and of the spermatic product (in sperm competition) and should not be underestimated, the modification of secondary sexual characteristics (in male-male competition).

We are animals and the environment is able to change even biologically over large time scales. Only in this way it is possible to become perfect machines suitable for the generation of a vital offspring and equally suitable for life on this earth.

Nothing is left to chance in this random combination of molecules.

Maria Bruno