Philip Denbaum | December 4, 2016

Darwin coined the famous phrase survival of the fittest. What is meant by “fittest” here is how well adapted to the environment an individual is. What is meant by survival is staying alive and producing offspring, as in, survival of your genetic lineage rather than your individual survival. This has led to some extraordinary adaptations which increase the fitness of its bearers through increasing the ability to stay alive and reproduce. When individuals have to compete for partners to be able to reproduce, evolution often leads to seemingly strange adaptations that have often left scientists confused over how such a trait could evolve.

Fig 1. Peacock courting peahen.

One of the most classic examples of this is the peafowl where the male, known as peacock has a dazzling plumage of colorful feathers so large that they seem to impair the movement and flight of the peacocks as well as make them very conspicuous to predators. However, the peahen love this plumage and the males with the biggest, brightest and most colorful feathers will get more opportunities to mate and reproduce than other males. This is a case of sexual selection through something called the handicap principle. Basically, the females are choosing a male who is showing his health and quality through wasting energy on producing his plumage rather than investing that energy in survival as well as handicapping himself by being poorly camouflaged. A less impressive plumage, lacking colour, brightness and fullness signals to the peahen that this male may be sick or weak and can’t afford the energetically expensive ornament. The fact that a male of poor quality can’t produce as an impressive plumage makes it an honest signal, males can’t fake their quality. This is sometimes compared to the purchase of fancy cars, clothes and other objects that people can only acquire when they are wealthy – it is things that they don’t need for survival but they are wealthy enough that they can “waste” their money on it rather than spending on something that increases their ability to survive. In general it is the males who compete for females by fighting over them or by impressing them with a fancy feather dress? Why is it not the females who try to compete for males?

The answer lies in parental investment. Often one sex does the majority of the parental care and that has a big part in determining who is picky. Usually, the females are the ones who invest more in parental care and thus are usually the sex which is more picky, relying on behavioral or physical cues to discern between high- and low-quality partners. But parental investment runs deeper than just taking care of offspring through feeding, protecting and teaching. It starts much earlier than that – in the gametes, in fact. Think about human sperm and eggs – the females’ gametes are much bigger than the other; that means it requires more parental investment. This mating strategy, involving gametes of unequal size, is called anisogamy. Every time a mating occurs, males and females pay a price through energy spent on producing those gametes. This is where it starts differentiating in cost. A female’s egg is big, it contains nutrients and costs substantially more in energy to produce than a male’s sperm. This evens out a bit, since at each copulation, the amount of sperm males ejaculate is a lot more than the amount of eggs that a female releases. But in each zygote, and therefore in each offspring formed, only one sperm and one egg is involved. Thus, in each offspring produced, the female always has a larger investment. Why does the female always invest more in the offspring? The answer is simply that the biological definition of male and female is that the female is the sex with the larger gamete. 

So, the female who has the bigger investment will naturally have more reason to be picky about her partners. This is not only due to investment in energy though, it is also an investment in time. When an individual is pregnant, it can’t reproduce again until it is no longer pregnant. This means that females, who tend to be the ones who are pregnant, become the limiting factor in how many offspring can be produced. Males tend to produce sperm constantly and can reproduce basically continuously and females, once pregnant, can’t reproduce for as long as their gestation period lasts, and sometimes even longer than that. This leads to a situation where males often will outnumber the females in a population. This drives males to compete for females and it usually ends up with a situation where certain males manage to reproduce more than others.

Figure 2. In the seahorse genus, males are generally the sex which get pregnant and cares for the developing offspring. On the right a male Weedy seadragon (Phyllopteryx taeniolatus) brooding eggs and on the left, a male pot-bellied seahorse (Hippocampus abdominalis) giving birth. Rudie Kuiter.

To every rule in biology there is at least one exception and the parental care is no different. There are many different arrangements in nature, both for determining who gets to mate, but also for who takes care of the offspring. One of the most famous examples of this is seahorses, a genus with more than 50 species, found in oceans all over the world. In this genus, the males carry the eggs. Males receive the eggs from the female during reproduction and carries them in a brood pouch on his stomach. Once he has received the eggs, he covers them in sperm and then carries the fertilized eggs, providing them with nutrients and fresh oxygen until they are ready to hatch. The effects of this is seen in the pickiness of males and the courtship efforts and physical ornaments that females have in some seahorse species. In the seahorse case however, studies have shown that even though the male does the majority of the parenting, energetically the investment turns out to be quite equal between mom and dad. 

So why then is the male pickier and the female not? Well, regardless of the energetic investment, the male needs to take care of the developing young for some time. During this time of gestation, it is very difficult for him to receive more eggs from other females. Females can keep reproducing with other males soon after the previous reproduction. This leads to the earlier mentioned skew in individuals of either sex that is available for reproduction at any one time. So because there will always be more females available than males, females will have to compete for the males attention. Males who will spend much time caring for the offspring will be picky to make sure they don’t waste time raising young that have genes from an low quality female.

Fig 3. Broadnosed pipefish (Syngnathus typhle) yellow/white in their preferred habitat of eelgrass.

Anders Berglund of Uppsala University has shown that males of the species Syngnathus typhle, commonly known as broadnosed pipefish, prefer to mate with larger females [1]. The females also here have more ornaments than the males. They have stripes along the sides of the body which help show the males how large she is. This has been tested both in pipefish and in humans where students have been given pictures of different pipefish-like objects with different types of stripes and it seems the stripes of S. typhle females are of significant help when judging their size. In pipefish males were more prone to pick a larger female when she had stripes than in the control experiment where the stripes had been covered up. So, here even physiological adaptations are seen in the females which shows how strong the effects of sexual selection can be.

Survival of the fittest is a quite straightforward concept. It is the term fitness which can be a bit misleading at times. It isn’t always the strongest, fastest or most well camouflaged that are the fittest, sometimes it is simply about being the sexiest.

References:

1. Berglund A. “Sex role reversal in a pipefish: female ornaments as amplifying handicaps”. ANNALES ZOOLOGICI FENNICI. 37.1 (2000): 1-13.