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Introduction to Parental Investment

One of the most important things that an animal can do is to pass it’s genes on to the next generation, and the most effective way of doing this is through reproduction.  Every species is biologically driven to reproduce, and each has developed strategies to optimize their reproductive output while insuring the survival of their offspring (Williams 1966).  These strategies for successfully rearing offspring vary widely among different species in several ways.  One of the most drastic ways in which they differ is in the type of parental investment they provide (Gross 2005).  Parental investment differs in many ways, including which parent invests more into the offspring, and how much or for how long the parents invest into the offspring in total-if they invest any time at all.  Parental investment includes providing a mate with food, providing a mate with a place to nest, feed, or raise young, protection of a mate, and help in the birthing process.  It also includes providing protection, food, resources, or other knowledge to offspring (Trivers 1972).  Additionally, parental investment can be limited by things that would have a seriously negative effect on the parent’s own fitness (Williams 1966).  In many cases, parental investment has to do with a cost-benefit relationship; the cost to the parent of investing heavily into offspring, compared to the benefit of the survival of the offspring (Williams 1996, Trivers 1972).  Several scientists from many backgrounds have studied parental investment to look for, analyze and compare trends and general rules regarding parental investment.

Influential Scientists and Their Hypotheses

It was not until the mid 1960’s that scientists truly began to analyze parental investment (Williams 1966), and it wasn’t until Robert Trivers wrote his widely acclaimed scientific paper Parental Investment and Sexual Selection (1972) that the discipline became worthy of consideration and analysis.  George C. Williams (1966) analyzed parental care from a cost-benefit standpoint, asserting that parents will alter the amount of care they provide their offspring, maximizing their offspring’s ability to survive while minimizing the cost to their future reproductive success. Robert Trivers (1972) took William’s ideas and expanded upon them greatly.  He defined parental investment as the total amount of investment from a parent into each of its offspring which serves to increase the survival of the offspring while simultaneously decreasing the parent’s ability to have more offspring.  In many species, the greatest that males will invest in offspring is their sperm (Trivers 1972, Diamond 1992, Gross 2005), while females invest an egg, pregnancy, and additional child rearing (Trivers 1972). In other species, males will invest in offspring by providing the female with resources and protecting during pregnancy, or helping to provide for, protect and raise the offspring after birth (Trivers 1972, Gross 2005).

Trivers used this idea of relative parental investment and outlined the effect it has on sexual selection among different species.  He theorized that in a given species, the sex that invests the most in its offspring will also be the most selective when it comes to choosing potential mates.  Conversely, the sex who invests least in the offspring will be less choosy about who they mate with (Trivers 1972).  In the case of animals where parental investment is divided roughly equally between both parents-such as humans-both sexes will be roughly equally as selective when it comes to choosing mates (Trivers 1985).  Trivers also theorized that the relative parental investment of each sex would dictate restrictions in mating.  For the sex that invests most heavily in rearing the offspring, the main limitation to mating is the availability of resources. On the other hand, the main limitation to mating for the sex that invests least heavily in offspring is the availability of the sex which invests most heavily (Trivers 1972).

Robert Trivers:

from: http://www.crafoordprize.se/press/pressphotos/2007.4.61632b5e117dec92f478000104288.html

Parental Investment Trends and Strategies in Animals

There seem to be trends within groups of animals regarding types of parental investment.  For example, roughly ninety percent of all mammals display female-only parental care, while the remaining ten percent display both male and female parental care.  There is no example of male-only parental care among any mammal species. Unlike mammals, only about eight percent of birds demonstrate female-only care, while ninety percent of them exhibit care from both parents, and the remaining two percent display male-only care.  Fish are the most variable in their parental investment strategies.  About eighty percent of fish provide no parental care, ten percent demonstrate male-only care, six percent exhibit female-only care, and a minority of four percent display care from both parents (Gross 2005).

A Parental Investment Study in Fish

In their study in parental care in fish, Mart R. Gross and R. Craig Sargent (1985) explain why a majority of fish have evolved male-only parental care strategies.  They use a cost-benefit analysis, identifying one benefit of parental care: the survival of the young and the expression the parent’s genes; and three costs of parental care: the cost of finding a mate, the cost of their own survival, and the cost of being able to produce more offspring in the future.  The benefit-the expression of their genes-is roughly the same for both males and females, as they both have roughly half of their genes expressed by their offspring.  For the male, the only real cost is the cost of finding a mate, but since many fish are territorial, this cost is not very heavy, and often nonexistent.  For females, the main cost is future fertility, which is a greater than the male’s cost of finding a mate. Gross and Sargent theorize that because the benefit of parental care is equal and because males have lower costs than females, males stand to increase their inclusive fitness by providing parental care and ensuring the survival of their young (Gross & Sargent 2005). It is important to note that a parental strategy like this would be hard for mammals because fertilization and pregnancy are internal as opposed to external, as in fish.

image from: http://www.flmnh.ufl.edu/fish/Gallery/Descript/SergeantMajor/SergeantMajor.html

Parental Investment in Mammals

Mammals are another large group that emerges with regard to parental care strategies, and obviously the group that Humans fall into.  The vast majority of mammals display female-only parental care, with roughly ten percent providing bi-parental care, and no instances of male-only care (Gross 2005).  For mammals, female parental investment is initially greater than male parental investment.  Females produce a relatively large egg which is in and of itself a greater investment than the small male sperm.  Additionally, if that egg is fertilized, the absolute minimum investment of a female is the term of her pregnancy-which is by no means a small investment.  This investment is much greater than the initial male investment, which at a minimum is his sperm (Trivers 1972).  For example, the only investment male orangutans-primates like ourselves-provide is sperm (Diamond 1992).  While it is true that the majority of mammals follow this pattern where males invest only their sperm and females invest in an egg, pregnancy and child rearing, humans fall into the minority of mammals which share parental investment between the two sexes (Gross 2005, Woodward and Richards 2005).

Image from: http://asb.brain.riken.jp/research.html

Parental Investment in Humans

Although the amount of male parental investment is quite variable across different cultures, human males do tend to invest more than just gametes to their offspring (Woodward & Richards 2004).  In some cultures, like the Sambia in Papa New Guinea, males seldom provide any care for babies or children at all, but when boys reach age ten, the males collectively take care of them, while prohibiting females from investing any more parental care (Herdt 1999).  On the other hand, studies of cultures like that of the United States have shown that males invest more in their offspring from birth to adolescence, although not nearly as much as females do (Geary 2000).  In order for males to invest in their offspring, they must have a high degree of certainty that the offspring they are investing in are theirs (Diamond 1992), a high degree of certainty that their investment will improve their offspring’s chance of survival, and opportunities to mate with other females which are not restricted by their parental investment (Geary 2000).

Image from: http://www.theprosperouspeasant.com/2007/12/10/the-risk-of-happiness/

Humans have evolved to ensure that these criteria are fairly well met.  Although perhaps the last one-opportunities to mate with other females-is not traditionally accepted in Western-European culture, studies of hunter gatherers and groups like the Mormons show that opportunities to mate with other females are important and result in increased fitness for males (Diamond 1992).  This is important because heavy male parental investment along with female parental investment is necessary for human children to survive.  Human babies are not born with the necessary motor or mental skills to be able to survive on their own, and need to learn these skills from their parents.  Through most of their childhood and early adolescence, humans need to be provided with food, shelter, and protection, as well as training and socialization so that they can learn to be able to provide such things for themselves and their own future offspring.  As a result, humans have evolved to provide their offspring with far more parental investment from both males than most other animals (Diamond 1992, de Waal 2005).

Evidence and Support for Parental Investment Hypotheses

The hypotheses regarding parental investment seem to be well established and supported by scientific evidence.  The original hypotheses asserted by George Williams (1966) and Robert Trivers (1972) regarding the cost-benefit structure of parental investment seem to be supported by several scientific investigations. In Kevin Woodward and Miriam H. Richards (2005) relatively recent study on parental investment and mate choice in humans, they find Trivers’ original hypotheses to be supported by their data, but with one small incongruity.   In their experiment, they had men and women rate how choosy they would be towards selecting members of the opposite sex for five situations: a single date, a one night stand, casual dating, steady dating, and marriage. The general trend they found was that as the risk of parental investment increased, so did the average choosiness of both the males and females in their study.  The incongruity they found was that male and female average choosiness score for a one night stand was less than their average choosiness score for a single date.  The risk of parental investment for a one night stand (which involves sexual intercourse) is much greater than the risk of parental investment for a single date, so according to Trivers (1972) the choosiness score for a one night stand should be greater than the choosiness score for a single date.  However, they hypothesize that the perceived risk of parental investment for a one night stand is less than the perceived risk of parental investment for a single date.  Their critique on Trivers’ hypothesis was that choosiness for men and women will increase as the risk of perceived parental investment increases (Woodward & Richards 2005).

Another investigation, Mart Gross and Craig Sargent’s (1985) study on parental investment in fish, found there to be a relationship between the amount of parental investment that fish provide and the benefit that the fish gain in their inclusive fitness as a result of providing the investment.  Specifically, they found that the male-only parental investment in fish can be explained by the fact that the male fish serve to increase their inclusive fitness, with a very low cost (Gross & Sargent 1985).  This directly relates to George William’s (1966) assertion that animals will alter their parental investment to insure the survival of their offspring at a minimum cost to their future reproductive success.  While most scientific investigations serve to back up these two men’s hypotheses, additional experiments would always be beneficial.  It is also important to note that while data may support Robert Trivers’ (1972) assertion that parental investment plays key role in sexual selection, there are other factors that play perhaps a larger role as well, such as attractiveness, health, and masculinity (Diamond 1992).

Implications and Extrapolations

It is very important to understand the biology of parental investment.  While the investment of fish and birds may not be that fascinating, it is important to note that humans are not the only animals in which both parents invest in their young.  We, as well as other species, have experienced several evolutionary adaptations that have caused us to develop the kind of parental strategies that we exhibit.  Deeper than cultural practices, our biological roots have evolved us to provide substantial care for our offspring from both males and females.  The skills we acquire and our socialization are in large part a result of our parents’ investments of protection, time, resources, and knowledge into us as we develop.  If more people understand this fundamental need for investment from both parents, perhaps they would be more likely to spend an appropriate amount of effort in raising their children; effort that goes a long way in helping their children as well as their genes to survive.

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