Bateman's Principle

Bateman's Principle

1. A01

   • the sex which invests the most in producing offspring becomes a limiting resource over which the other sex will compete.
   • the origin of this unequal investment lies with the differences in the production of gametes.
     • human female is born with all the eggs she will ever produce. males produce small gametes while females produce fewer, larger gametes. females limited to 300, a single male ejaculation might contain between 40 to 600 million sperm.
   • since the female invests more in the production of an egg than a male does a sperm, she is going to be discriminating in her choice of mate. sperm are clearly plentiful and cheap to produce so a male is going to benefit more from mating freely and spreading more widely.
   • Fruit flies - observed that promiscuity benefited males much more than females. in effect, the reproductive success of a male increases with each female he mates with, whereas mating with more males does not increase the reproductive success of females.
     • As a consequence, sexual selection occurs, whereby males compete with each other for females and females become more choosy about which males they mate with.

2. A02

   • It is not necessarily the case that females expend more energy making gametes than males do. Although it is true that producing one egg is a greater investment than producing one sperm, many sperm are produced to ensure fertilisation of each egg; in some species with internal fertilisation this results in the male investing more in the process than females. In species that reproduce by spawning the investment from each sex is roughly equal.
   • The assertion that all a female requires is fertilisation from one carefully selected male is also challenged by observations of some animal species which indicates females have more offspring if they mate with a larger number of males. For example Taylor et al found that female fruit flies housed continually with males (thus with opportunities for multiple matings) produced more offspring than singly mated females housed alone or with virgin females.
   • There are many examples in the animal kingdom that contradict Bateman's assertion that a male's reproductive success will depend on the number of females he mates with. The clown shrimp for example, rather than mate with as many females as possible, will spend many weeks diligently guarding a single female. The macaw is a monogamous bird and the same pair remain together for the majority of their 40-60 lifespan.
   • Given that a single ejaculation provides more than enough sperm to fertilise an egg, Bateman's principle has difficulty explaining the extra energy costs incurred by species that engage in multiple copulations which tip the balance towards a more equal investment. Dewsbury found that female hamsters need to be stimulated by multiple ejaculations in order to release the hormone necessary for pregnancy.
   • Another problem for Bateman's principle arises with examples of sex-role reversal, where parental care in the main responsibility of the male. One example is the wattled jacana, a type of water bird where the female has the colourful plumage instead of the male. Emlen and Wrege noted that the only contribution that a female jacana makes to chick care is in defence against predators. Otherwise, females rarely intervene. Direct brood care was only observed when the male was not himself available to help.