This is the conclusion of a three-part guest series by Dr Carin Bondar, author of The Nature of Sex, which is available now in the UK, Australia and New Zealand. Catch up with part one and two.

Although it seems like a minor miracle to have made it successfully through steps one and two of sexual activity, the fact of the matter is that most creatures do it with remarkable ease. Despite the hardships suffered along the way, the creation of offspring is the usually the outcome of successful sex, regardless of whether that sex was pleasurable or not. The next hurdle for any animal is to ensure that its spawn live long enough to reach sexual maturity and keep the cycle going.

It would seem logical that all offpsring are treated equally. And yet, that isn't always so. Sometimes, mothers pick favorites. They do so based on several factors, but a major one is the quality of the father that created them.  One of my favorite examples to illustrate this point is that of the blue-footed booby (Sula nebouxii). The gorgeous blue-green feet possessed by the males are sexually selected – girls prefer boys with vibrant colored feet to those with dull ones. Foot color is a dynamic, condition-dependent trait, meaning that the current health and nutrition status of a male is easy for a female to assess. Foot color can change in less than 48 hours if a male is deprived of food, so this characteristic is essentially a way of letting females know that he is healthy and ready to be the father of her babies.

^{Painting a male blue footed booby's feet gray (as imagined here) leads to his mate laying a smaller second egg. Konstantin Kalisko/123rf.com}

In addition to this clear visual indicator of a male’s fitness, the reproductive biology of blue-footed boobies provides an opportunity for biologists to manipulate it. You see, once a male and a female have copulated, the female will lay two eggs. The second egg will come along approximately four days after the first. Biologists interested in the ability of mothers to manipulate the level of resources to offspring have gone so far as to paint the bright blue feet of fathers to a dull gray color between the laying of the first and second eggs. When they did this, the second egg came along with an overall lower mass and volume than the first, indicating that the mother is not going to invest too heavily in offspring that come from a substandard father. Egg mass and size are directly related to chick survival.

Sometimes in the animal kingdom, it’s not just about favoritism, but about outright child abuse. Case in point: Nazca boobies from the Galápagos archipelago. Nazca boobies (Sula granti) are ground-nesting colonial seabirds that exhibit highly unique behavior. In any given year there are adults within the colony who do not breed. These are either males who have not found a mating partner (the sex ratio is generally skewed) or females whose nests have failed.

^{Adult Nazca boobies. Keith Levit/123rf.com}

While parents are away foraging, these nonbreeding adults pay “visits” to unattended chicks, hanging out with them for periods of between a minute and an hour. These visits can take a few different forms. Some are affiliative (friendly) and peaceful: Non-parent adult visitors (NAVs) will stand beside or preen young chicks, or in some cases even provide them with gifts of pebbles or feathers. Unfortunately, the bulk of visits involve aggression by the visitor, who will scratch or bite chicks – leaving them with bloody wounds on their necks. The scratches aren’t fatal, but they attract the attention of blood-sucking ectoparasitic birds like finches or mockingbirds. These ectoparasites will feed from the cuts and deepen them substantially, which often leads to the death of the chicks.

^{Nazca booby chick. Stephen Hill/123rf.com} 

The last, and least common, form of the visits is the most perplexing: adults (usually males) will engage in sexual copulations with chicks. Biologists have yet to come to any kind of consensus as to why this occurs. The bottom line is that all three kinds of NAV behavior in Nazca boobies are extremely common. They are observed across different colonies in different areas, and therefore cannot be dismissed as some form of behavioral mistake. Up to 24.6 percent of chicks die as a result of the direct or indirect effects of NAV behavior.

Now although the previous section paints a pretty dismal picture of the effects of non-reproductive adults, there are other scenarios where non-reproductive females are extremely helpful for the survival and well-being of offspring – specifically the well-being of their daughters’ offspring.

Humans and toothed whales represent an extremely specialized form of reproductive senescence. Here, females experience a complete and irreversible cessation of reproductive capabilities (menopause) at a mid-point in their life cycles, which means that they exist in a state of biological irrelevance for a good portion of their lives. Human females rarely give birth after the age of 45 following a fertility decline lasting two decades. Why would such a strategy evolve?

Picture it: as ancestral humans were expanding their populations across African savannas in the Plio-Pleistocene Era (between 1.7 and 1.9 million years ago), the increasingly arid conditions meant that food sources were changing. Juveniles that were once able to gather food were no longer able to forage for roots and tubers located deep beneath the ground because the strength of their small arms was insufficient to dig through the hard earth.

^{Were arid savannas a driver of human menopause development? Romas Vysniauskas/123rf.com}

This resulted in a conundrum for ancient mothers: remain on a nomadic path in order to follow food sources that their young children could manage, or stay in a constant area where food is tough to find, and provision the children for longer periods of time. If they chose the latter, this would require a greater expenditure of their maternal energy on their current offspring, leaving them with limited chances of further reproductive success. Mothers could not overburden themselves with extra offspring because the costs of their own death would be great – juvenile humans are unlikely to survive without direct parental provisioning, so an over-taxed mother could lose her entire brood if she fell ill or died.

This scenario presented a valuable opportunity for older females (already experiencing natural reproductive senescence) to increase their own inclusive fitness. You see, if an elder (grandmother) stepped in to help provision her grandchildren it would be biologically beneficial for both her daughter (who could have more offspring at a faster pace without having to sacrifice the well-being of current offspring) as well as herself (through the increased survival of her grandchildren and the increased prospect of further grandchildren).

More vigorous grandmothers could provide a higher level of care, which would be under a high level of selection through the direct fitness benefits conferred. In other words, selection may actually have favored the earlier occurrence of complete reproductive senescence because grandmothers could gain more net biological fitness through helping than through increased child rearing of their own.

Gestating, birthing and providing milk for a human infant represents a huge energetic and physiological cost to females. To put it simply: the massive costs of child rearing in our species are better faced by a younger body than an older one. However, by provisioning their grandchildren, older females can continue to increase their own biological worth. So in this unique situation (aptly termed the “grandmother hypothesis”) the end of fertility is not the end of reproduction.

So there you have it! I hope you have enjoyed this short series on the immense power of the sexual process. I’m happy to answer questions or queries through my website: www.carinbondar.com, and I hope that you will enjoy reading my book!

The Nature of Sex is now available in the UK, Australia and New Zealand. Stay tuned for release dates in the USA and Canada in spring 2016.