Bateman's principle

In biology, Bateman's principle is the theory that females almost always invest more energy into producing offspring than males, and therefore in most species females are a limiting resource over which the other sex will compete.

Description

Typically it is the females who have a relatively larger investment in producing each offspring. A single male can easily fertilize all a female's eggs: she will not produce more offspring by mating with more than one male. A male is capable of fathering more offspring than any (one) female can bear, if he mates with several females. By and large, a male's reproductive success increases with each female he mates with, whereas a female's reproductive success is not increased nearly as much by mating with more males. This results in sexual selection, in which males compete with each other, and females become choosy in which males to mate with.

Bateman's observations came from his empirical work on mating behaviour in fruit flies. He attributed the origin of the unequal investment to the differences in the production of gametes: sperm are cheaper than eggs. Animals are therefore fundamentally polygynous, as a result of being anisogamous.
*"A female can have only a limited number of offspring, whereas a male can have a virtually unlimited number, provided that he can find females willing to mate with him. Thus females generally need to be much choosier about who they mate with." Harv|Hewett|2003
*"A male can easily produce sperm in excess of what it would take to fertilize all the females that could conceivably be available [...] Hence the development of the masculine emphasis on courtship and territoriality or other forms of conflict with competing males." Harv|Williams|1966
*"in most animals the fertility of the female is limited by egg production which causes a severe strain on their nutrition. In mammals the corresponding limiting factors are uterine nutrition and milk production, which together may be termed the capacity for rearing young. In the male, however, fertility is seldom likely to be limited by sperm production but rather by the number of inseminations or the number of females available to him... In general, then, the fertility of an individual female will be much more limited than the fertility of a male... This would explain why in unisexual organisms there is nearly always a combination of an undiscriminating eagerness in the males and a discriminating passivity in the females." Harv|Bateman|1948
*"among polygynous species, the variance in male reproductive success is likely to be greater than the variance in female reproductive success." Harv|Huxley|1938
*"The female, with the rarest exceptions, is less eager than the male... she is coy, and may often be seen endeavouring for a long time to escape." Harv|Darwin|1871

Exceptions and counter-examples

Some modern evolutionary biologists believe Bateman's principle is incorrect for such a large percentage of species that it should no longer be considered a valid principle. Olivia Harvcoltxt|Judson|2002|pp=10-13 argues that the formulation of Bateman's principle was limited by such things as short observation time in his experiments. Tim Harvcoltxt|Birkhead|2001 has also documented extensive examples of exceptions to Bateman's principle, with a focus on sperm competition.

Sex-role reversal

The most well-known exceptions to Bateman's principle are the existence of sex-role reversed species such as pipefish (seahorses), phalaropes and jacanas in which the males perform the majority of the parental care, and are cryptic while the females are highly ornamented and territorially aggressive (Harvcolnb|Emlen|Oring|1977;Harvcolnb|Knowlton|1982; Harvcolnb|Berglund|Widemo|Rosenqvist|2005).

Because females in these species display the behavior predicted for males by Bateman, some believe that such examples actually support, rather than undermine, his principle Harv|Flinn|2004.

Other examples of violations to Bateman's principle

Observation of many species, from rabbits to fruit flies, has shown that females have more offspring if they mate with a larger number of males. This seemingly contradicts Bateman's theory, specifically his conclusion that "while males had more children the more partners they mated with, females did not" Harvcol|Judson|2002|pp=11-13.

Research has also shown some species in which males will guard one female and mate only with her, attempting to prevent her from mating with any other males. Examples include stick insects and Idaho ground squirrels Harvcol|Judson|2002|pp=9-10. These observations also seem to challenge Bateman's theory, specifically the assertion that that "a male's reproductive success increases with each female he mates."

Females do not always have a relatively larger investment in producing offspring. In species that reproduce by spawning (releasing sperm and eggs into water), for example, each sex's investment is approximately equal. In animals with internal fertilization, many sperm must be produced for every egg; so, even though it takes less energy to create one sperm than one egg, males of many species spend more energy making gametes than do females Harvcol|Judson|2002|pp=29-33. The statement that the sex that invests the most in producing offspring will become a limiting resource is not always true. In flowers, for example, the female part of the flower invests more energy into making seeds than the male part of the flower does. The reproduction of most flowering plants, however, is limited by delivery of the male gamete - pollen - not by production of the female gamete Harvcol|Judson|2002|pp=197.

Bateman's statement "there is nearly always a combination of an undiscriminating eagerness in the males and a discriminating passivity in the females" and his assumption that anisogamous species would be polygynous have also been argued to be false, because females of most species mate with several males Harvcol|Judson|2002|pp=12-13.

In any given population, 80% of females are attracted to 20% of males. However, these 20% of top tier males "alpha males" are only attracted to 40% of females "alpha females". The remaining 80% of males "beta males" must find a way to then compete for females in an effort to pass on their genetic code to worthy mates. Beta males display more sexually aggressive behavior than alpha males, since sex is less available to them. Naturally, the alpha females will have to be selective and determine the status of the male. Alpha females will generally not compete over a beta male, although they will compete between other females for the more limited alpha male. This may account for why pre-selection may act as an attraction trigger in females. Beta males will compete both between other alpha and beta males for both alpha and beta females. Since the chances of survival for a beta male are linked to the degree of sexually aggressive behavior, the optimal mating strategy is to pursue as many females as possible. This would suggest that Bateman's principle holds for a beta male population. However, sex partners are not limited, in fact abundant, for the alpha male population. For the pursued alpha male population, Bateman's principle does not hold. [Ridley, Mat (2002), The Red Queen: Sex and the Evolution of Human Nature] [Baker, Robin (2003), Sperm Wars: The Science of Sex]

In humans, Bateman's principle appears to hold. Females, both alpha and beta, are more selective in their sexual partners. The data collected for this conclusion however is tainted due to cultural and social pressure. A woman is not free to act on her desires for a man the same way a man is uninhibited for his desires for a woman. Women may feel guilt associated with being labeled a "whore" or "slut" by society if they become sexually involved with many men. Men do not face the same social pressure. During courtship, beta males will try and impress females by token symbolic gestures or significantly more financial contribution in return for sexual interest. Beta males also show a greater preoccupation with an interest in sex than do alpha males, since alpha males have no shortage of sexual options available to them. Females display different behavior during courtship, waiting for the male to initiate. A woman will have the tendency to avoid feeling powerless or vulnerable. Part of female sexuality is trying to feel sexually powerful. Females want to feel desired and wanted, and so prefer for a male to initiate. Women have an additional emotional motive to be selective; to avoid feeling "easy". For this reason, most women take pride in being labeled "difficult, not easy, hard-to-get, picky, or a tease". Despite male initiation, even by an alpha male, women will still not allow themselves to outwardly express interest in the male before her insecurities are comforted (self esteem is reinforced, feels desired and wanted, etc.) and feels safe and secure (male's commitment, trust, etc.). While human data may show females being more sexually selective, there is no way to distinguish between how much is contributed from actual partner selection and how much is to be credited to female sexuality and maintaining good self esteem. Therefore, it is not possible to conclude with reasonable certainty that Bateman's principle applies to humans. [Thornhill, Randy (2008), The Evolutionary Biology of Human Female Sexuality]

ee also

*Coolidge effect
*Parental investment

References

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Further reading

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External links

* [http://www.nbb.cornell.edu/neurobio/emlen/Misc%20Info/SB_papers.html Popular articles on sex-role reversal]