War of the Sexes – Part 2

October 18, 2009

Part 2: COST-BENEFITS or “Do-the Add-Ups & Take-Aways”or who pays the “cost”? and who gets the “benefit”?

From Part 1 “.. In summary, from primordial times, this binary form of reproduction has resulted in very different methods and strategies, for each to maximise its chances of successful offspring”.   

While there are numerous speculations on why sexual reproduction, and diploidy (ie double-stranded DNA) evolved, one of the more common lines of thought, is that asexual reproduction, or cloning, especially single-stranded,  is doomed to failure over time.

This is because of the “Xerox problem”. Take a photocopy of a photocopy of a photocopy enough times, and your image will fade. Errors creep into DNA, and over the generations, information is lost, the errors and blank spots become so many that the organism becomes unable to sustain itself.  If all the members of a population are copies of each other the whole population dies out. Doubling the DNA from absorption from an external source, allows a greater chance for the errors to be corrected by the presence of a ‘good copy’.

Another related reason, is based on the principle of conservation of energy.  All life, needs to absorb nutrients and convert those nutrients to energy for growth and survival.  Minimum energy input is enough for minimum maintenance, but reproduction, even at its most simple through doubling of a parent cell – needs much more energy, than mere survival and maintenance does.

Weak cells with limited nutrients, may still have enough energy stores to double, but ending up with two smaller, compacted cells each time it divides, sacrificing other biological mechanisms to maintain the survival of just its central core.  This way it minimises its energy needs, while still having a chance of its ‘core’ being picked up by a larger cell.  Motility and speed are the drivers of this reproductive method towards quantity, and self-sacrifice – often trading off longevity and survival (with higher energy costs) for the chance to pass on its genes. Quickly producing multiple small fast short-lived mobile copies of itself, increases the chance further that one or two will get picked up by larger nutrient-rich cells, and incorporate its core in the next generation.

Other benefits of sexual reproduction are assisting in the spread of advantageous genes, and assisting in the elimination of disadvantageous genes, hence the drive towards quality with increased energy investment.  Some species of plants and water-based creatures, alternate asexual reproduction with sexual methods in alternate generations depending on environment.  Asexual reproduction with high quantity/ minimum energy can fill ponds or other ecosystems very quickly with clones. As the population reaches maximum and some die off due to lack of nutrients, they switch to sexual reproduction to maximise quality of fewer offspring.

In flowering or sexually reproducing plants, many can self-fertilise being hermaphroditic, but this sacrifices quality for maintenance/survival. Others need to cross-fertilise with another individual, and some plants have distinctive separate male and female individuals. The male cell, or pollen in plants, is produced in larger numbers, is small and often motile with or without assistance from other species.  The female is larger again, forming fruit or other high nutrient casings for the seed(s) or embryo(s). 

The cost of sexual reproduction is time and energy and increased risk of failure, including death of the parent organism. The process of meiosis (formation of gametes), coupled with fusion of gametes is time-consuming, complex and takes far more energy than simply doubling. In larger organisms,  individuals must find and/or attract mates or for their reproductive cells, and this results in additional expenditures of time and energy.  At the same time, individuals may encounter predators, and the contact between individuals during mating is an ideal mechanism by which pathogens may be transmitted, even plants and trees have STDs.

The cost of sexual reproduction is most obvious in those species characterized by two distinct classes of gametes (e. g., eggs and sperm), often produced by two distinct classes or sexes of individuals – males and females.  This cost has been termed the “cost of meiosis”, “the 50% cost”, or the “cost of genome dilution”.  This disadvantage of sexual reproduction may be summarized by the phrase “males are useless” (some of you may have reached this conclusion already).

The sex which invests in the sustenance for the embryo (the egg producing females) invests and contributes far more, than the sex which only provides chromosomes (the sperm producing males).  In other words, females produce offspring, while males only produce genes.  The female contributes only 50% of her genes to the egg, and this contribution is “diluted” in its influence by the male genetic contribution to the offspring, despite her greater investment.

For example, asexual reproduction in trees allows a greater spread of offspring trees to create large forests of single species, all genetic copies of each other. On the other hand, sexually reproducing trees, as in the flowering trees, tend to form smaller groves and not spread so far.  In this case, the cost of sexual reproduction is seen in fewer offspring, but more variable in their capacity to adapt.

In the event of the arrival of a deleterious mutation, parasite or pathogen – the asexual trees will be far more affected and are more prone to species or local population, extinction.  The sexually reproducing trees, although smaller in number, are more likely to have adaptive individuals due to the diversity. The other main benefit of sexual reproduction, is the capacity to repair damaged DNA. By mixing with a strange copy of DNA the cell can repair any damaged sections in one, by copying from  the other.  This is why, advantageous genes tend to be dominant, while deleterious mutations tend to be recessive within populations and in-breeding often causes a proliferation of the damaged genes.

Sexual reproduction thereby has greater benefits for the population, which comes at the expense, or the “cost”, to the individual, and an even larger cost for the female in many species where female investment is particularly high.  This cost is often re-balanced by shifting the “cost” to shorter life-spans and lower numbers of males.

The amount of female investment in offspring does vary across genera, classes, families of organisms. One of nature’s major methods to re-balance the imbalance of cost-benefit between the sexes in sexual reproduction, is through sexual dimorphism (the degree of difference between sexes) and sex-ratios of adult members of populations.  The greater the degree of sexual dimorphism, the greater the adult sex-ratio is skewed to favour the survival of the sex which invests the most in offspring.

In some species, such as many reptiles, sexual dimorphism is quite minimal as the female does little for sustenance of the offspring once eggs are laid and fertilised. The female can just walk away and let nature take its course. In these species, the investment by the female is only marginally greater than the male. This is usually seen in high fertility species, where the females produce large numbers of small eggs which require no further care once expelled. Some species of fish, switch sex as they continue to grow. All start off as small highly mobile male fingerlings, but if/when they reach a certain size, they become female. In other species, as in some birds, male investment in offspring is equal. This is usually seen in low fertility species. When only one or two eggs can ever hatch and be brought to maturity, it is in the males and female’s equal interest to invest in its sustenance.  In these cases, sexual dimorphism, or the degree of difference between the sexes, are often minimal too – because the energy design is more evenly balanced.

The largest degree of sexual dimorphism is found amongst mammals. By definition, the bearing of live but immature young requiring continuing care for considerable periods of time, means mammals are forced to invest far more time and energy in the care of offspring.

To balance this cost to the individual over a species or population group, greater sexual dimorphism also leads to expendable, or excess, males, and a greater sex ratio of adult females to males. Nature over-produces males in many sexually reproducing species. Flowering plants produce huge amounts of pollen, most of which will be wasted. The small built-for-speed and short lifespan male cell, is the redundant sex. Survival of the female is more important to the survival of the species.

Mammals also tend to live primarily in groups, herds, pods, troops, packs etc. Safety in numbers. Bearing live, immature young, which need close care for long periods of time means most mammals live in social groups where the young are communally protected.  One adult female, or even two adults is not enough for such investment to be successful.  Most mammals are gregarious and group together for communal survival strategies. The higher natural death rate for males, and greater survival rates for females, and the need for greater female investment in reproduction, means that many mammals tend towards numerically larger female to male ratios.

In mammals – males, like pollen, are produced by nature in excess, and designed to compete with each other for the opportunity to reproduce and this also involves rights of female choice in mates.  Count them up. Do the sums on our sister mammals. In the overwhelming majority of mammal species, adult females outnumber males.

In some wolf species, and related canids like hyenas, adolescent males are expelled from the maternal pack on reaching maturity. The ‘Lone Wolf’ is usually a young male. It is difficult to survive alone, and most will not find a new home.  For the lone wolf must find a new pack to accept him as a pack-mate. He may be critically injured or ill before he has a chance. He will be submissive to strange pack leaders – nearly always females – and bring gifts and try to show off his skills to win acceptance. With some packs, which already have their quota of males, he may be attacked before he has a chance. If he is accepted, it will still be some time before he earns breeding rights from the females amongst his pack-mates.

If he is unsuccessful, he may be accepted back into his maternal pack, but as a non-breeding male. He often becomes “Den-Uncle”, and border scout, he will have a place, just not a breeding one eg caring for the pups while the other adults are off hunting.

Eurasian or Mongolian steppe horses in the wild, have a similar way of expelling adolescent stallions to make their way.  Some theorists have speculated, that when Man (sic) first moved into these areas, and started culling populations, these young males were finding it increasingly difficult to find new packs or herds, and homo sapiens first domesticated these young lone males. Capturing females from existing herds and building their own tamed herd in this manner by encouraging and re-inforcing male dominance.

Other mammalian species keep all males at a distance from the female-dominated central herd, others find adolescent males in groups on the fringes. These males are often less well-fed being fringe-dwellers, and injury or illness will also cull their numbers. The most well-known method for removing the excess male problem is male-male competition where males may even fight to the death for the opportunity to mate, or force hierarchical submission where lower-status males are more dependent on chance and luck.  In other species, female-choice operates over time to evolve weaker males that die off more easily, for example in some birds where the male is much more obvious to predators with neon-bright feathering, with less capacity to fight off predators or even feed themselves. The female feathering is more camouflaged making them less likely to become victims of prey.

Whatever method is used, the end-result is the same, a majority of males die off, or even if they don’t die — a majority of males are excluded from the reproductive pool and do not get the opportunity to mate.

Some species, like bonobos, are in reverse, it is the adolescent females who are exiled on maturity and must win acceptance in a new group.  Males remain with their maternal birth-group and their status and breeding-rights are based on their mother’s status. It is the females who must earn their own place, and that of any sons they have. However, unlike the male expulsion method where a majority of males will not win a place, in the female expulsion method, the majority of females do win a place.  Also, low status males within their maternal birth-group may still not get breeding-rights (as with the wolf) or not at the same frequency as higher-status males, because of the natural female-choice also operating.

In summary, in nature the biological sex-based form of reproduction is not necessarily a 50-50 equal split arrangement – its a variable “cost-benefit”  tradeoff – the cost to the individual, is balanced by benefits to the group, and in sexually dimorphic species where one sex has more investment to make, that sex is advantaged in other ways to make up for it – the costs and benefits are skewed. In mammals, including homo sapiens, the “50% cost” to the individual, is by necessity balanced by greater survival benefits and advantages to the females, and hence the population/species as a whole.

In some species, notably mammals and primates, where male-dominance is the norm, the first method is in shifting the “costs-benefits” is in reproduction, so the entire ‘costs’ are shifted to the female, the entire ‘benefits’ are shifted to the male.

In part 3, my focus will shift to the human war of the sexes, and ways in which the social constructions underpin the human male control of reproduction, for their benefit.