Females Have Already Invested the Most (anisogamy)

Females usually contribute more to the physical development their offspring. In all animal species, the egg is considerably larger than the sperm that fertilise it. In humans the ovum is the largest cell in the human body, while the sperm is the smallest. This anisogamy (difference in the size of the sex cells) means that eggs are more costly to produce than sperm.

In the case of humans, females are burdened with nine months of gestation followed by a period of lactation, which means that they provide almost all of the energy required for the infant to grow from conception to an age where it can weaned onto other food. This means that if offspring do not survive because of lack of care it is more costly to females because they have already invested so much. Due to this difference in the cost of reproduction, females tend to be choosier than males, and males compete for the opportunity to mate (Trivers, 1972).

The optimal strategy for females, therefore, is to be choosy in selecting mates who have high quality genes and/or lots of resources and to nurture the young that they produce. For males the optimal strategy, because sperm are inexpensive to produce, is to invest most of their efforts in reproduction rather than parental care.

Parental Certainty (Mother's Baby - Father's? Maybe!)

Another explanation for sex differences in parental care is that females are more certain that they are the parent of offspring than males are. Concealed ovulation, internal fertilisation and sexual infidelity mean that males cannot be sure that they are the father of their partner's offspring. DNA studies have shown that extra-pair paternity (the father is someone other than the official father) range from 1% in rural areas to 30% in dense urban areas and it is estimated that approximately 1 in 10 live births are the result of female infidelity (Platek & Shackelford, 2006). This means that if a male were to invest in his children, there is a 1 in 10 chance that he is actually using his time and resources to invest in children who are not his own.

Males who invest their resources in the children of other males are not reproducing themselves and, therefore, any genes that predispose men to care indiscriminately for any of the mother's children would not be favoured by natural selection. On the other hand, natural selection would favour genes that predispose males to recognise and respond to cues that signify paternal certainty. Two examples of these types of cue include the circumstances regarding the mother's behaviour around the time of conception and the resemblance of the child to its father.

Daley and Wilson (1982) made recordings of spontaneous conversations that took place in a maternity ward and found that relatives were much more likely to comment on the resemblance of the baby to the father than the mother or any other family member. When fathers expressed doubt over the resemblance they were quickly reassured by the mother. In one of the cases recorded by Daley and Wilson the father informed the hospital staff that if the child looked like his partner's former boyfriend (of a different race) he would not support the child. It is difficult to reach firm conclusions from this study; however, as it was a naturalistic observation with many uncontrolled extraneous variables and the sample of participants was unlikely to be representative.

There is also evidence that paternal care is given in proportion to how certain the father is of his paternity, for example Burch and Gallup (2000) interviewed convicted spouse abusers and found a significant positive correlation between how much fathers thought they resembled their child and the quality of the relationship with that child. There was also a significant negative correlation between the child's resemblance to the father and the severity of the mother's injuries.

Other evidence shows that step fathers invest less in step children than their own children (Anderson, Kaplan, Lam, & Lancaster, 1999) and are more likely to abuse step children (Daly & Wilson, 1996). This suggests that stepfathers conserve their resources for investment in their own children.

Williams's Principle

Williams's principle [sic] is based on the idea of Life History Tradeoffs: that reproductive success comes from trade-offs between reproductive effort and other survival traits. This is because reproductive effort and survival traits such as bodily maintenance and growth compete for the same pool of energy. This means that the optimal strategy of energy allocation is different at different times in an organism's life; for example, it is optimal to invest energy in growth and development early in life and delay reproduction until maturity.

Gross and Sargent (1985) applied Williams's principle to parental care. They claim that Lifetime Reproductive Success (the number of copies of genes that an individual leaves to future generations across its entire lifespan) depends on the organism finding a balance between the energy it devotes to reproduction and caring for offspring now, and how much energy it devotes to maintaining itself now and to producing and caring for future offspring.

According to Gross (2005) Investment in the present comes at a cost to investment in the future. So an organism must weigh up the benefits of continuing to invest in current offspring or invest in future progeny by devoting energy to bodily maintenance and producing more offspring later. These benefits will be different for each species and will have an effect on the sex that provides most of the parental care.

In 90% of mammals the female is the sole provider of parental care and in 10% both parents care for the young. In fish, however, most species do not care for the young at all, but of those that do, most parental care is provided by the male. Gross (2005) argues this is because larger female fish produce more eggs, so it is advantageous to the female to devote its energy to its own growth rather than to parental care. The male, on the other hand, has no reproductive benefit to gain from growth and has more to gain by ensuring that offspring are protected from predators.

Gross and Sargent (1985) also argue that other factors are important:

Brood Size

More investment is given to larger broods (Coleman et al. 1985) because there is more value in investing energy in many offspring than investing it in just a few.

Past Investment

If an animal has previously expended energy defending its brood it will continue to invest heavily to avoid having wasted previous effort.

Genetic Relatedness

Neff & Gross (2001) found that male blue gilled sunfish adjust the amount of paternal care they give depending on their confidence of paternity. They will even abandon the brood when the eggs hatch if they discover, perhaps through odour, that they have been cuckolded. They sometimes even cannibalise the brood. There is also evidence that humans adjust the amount of care given, based on how certain they are of paternity (Burch and Gallup, 2000).

Mating Opportunities

There is evidence in cichlids that increasing the availability of mating opportunities with females affects males' parental care decisions. Keenleyside (1983), for example found that increasing the ratio of female to male cichlids resulted in a corresponding increase in the rate of the males deserting their mates and their young. There is evidence that this is also the case in humans (Gangestad and Simpson, 2000).

Applying Williams's Principle to Humans

In humans, female fertility declines with age, whereas male fertility does not. This means that female's reproductive success benefits from the provision of parental care, while males can afford to invest more in their reproductive futures. However, it is also important to consider other factors, such as the relative value of current offspring, genetic relatedness and the availability of extra-pair mating opportunities. These have all been demonstrated to be important factors in research on humans (Gross, 2005).



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