Bateman suggested that, since males are capable of producing millions of sperm cells with little effort, while females invest much higher levels of energy in order to nurture a relatively small number of eggs, the female plays a significantly larger role in their offspring's reproductive success.
Bateman's paradigm thus views females as the limiting factor of parental investment, over which males will compete in order to copulate successfully.
Bateman initially saw his study on Drosophila to be a test of Charles Darwin's doctrine of sexual selection,[2] which he saw not as flawed, but as incomplete.
Analysis of the data collected in sets one through four showed that the males' reproductive success, estimated as the number of sired offspring, increased at a steady rate until a total of three mates were reached.
She began doing so by running monogamy trials between different strains of flies and found that 25% of the offspring died due to becoming double mutants.
In reality, many factors were left out of the equation when calculating reproductive success as a function of the number of mates, which had the ability to completely dislodge the accuracy behind Bateman's results.
[4][3][6][5] Nevertheless, some modern experiments between the relationship of number of mates and the reproductive success of males and females support Bateman's principle.
In 2013, Fritzsche and Arnqvist tested Bateman's principle by estimating sexual selection between males and females in four seed beetles.
They suggested that the Bateman gradient is typically the most accurate and informative measure of sexual selection between different sexes and species.
They argued that there are cases in which males can be more selective than females, whereas Bateman suggested that his paradigm would be “almost universal” among sexually reproducing species.
Birkhead wrote a 2000 review arguing that since Bateman's experiments lasted only three to four days, the female fruit fly, Drosophila melanogaster, may not have needed to mate repeatedly, as it can store sperm for up to four days; if Bateman had used a species in which females had to copulate more often to fertilize their eggs, the results might have been different.
Since lower degrees of selection during times of stable environment allows genetic variation to build up by random mutations and allow some individuals in a population to survive environmental change while strong constant selection offsets the effect and increases the risk of the entire population dying out during catastrophic environmental change due to less genetic variation, constant loss of genetic variation caused by sexual selection have been suggested as a factor contributing to higher extinction rates in more sexually dimorphic species besides the nutrient, immunity and other costs of the ornaments themselves.
While the ornament cost risk would only be removed when the ornaments have been eliminated by selection, the genetic variation model predicts that the species ability to survive would improve significantly even at an early stage of reduction of sexual dimorphism due to other adaptive mutations arising and surviving due to minimal selection during times of stable environment while the genes causing sexually dimorphic anatomy have only in small part been affected by the mutations.
Applied to human evolution, this model can explain why early Homo sapiens display a significantly increased adaptability to environmental change already at its early divergence from Homo erectus that had a high muscular sexual dimorphism, as well as why human anatomy through the history of Homo sapiens show a diversification during times of stable climate and a selective loss of the more robust male forms during environmental change that does not recover during later stability, continuing through the loss of many robust characteristics in regional bottlenecks as recent as the end of the Ice Age and the time around the agricultural revolution.
DNA paternity testing, however, has shown that in nearly 90% of bird species, females copulate with multiple males during each breeding season.
[14][16] Even female waved albatrosses, which typically mate for life, are sexually promiscuous, with 17% of young fathered by extra-pair males.
[19][20] Females of the pseudoscorpion species, Cordylochernes scorpioides, have been shown to have higher reproductive success when mated with more than one male.
[9] 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.
[24] Consequently, reversals in sex roles and reproductive variance are consistent with Bateman's principle, and with Robert Trivers's parental investment theory.