Bateman's principle
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In biology, Bateman's principle is the theory that females almost always invest more energy into producing offspring than males invest, and therefore in most species females are a limiting resource over which the other sex will compete. It is named for British geneticist Angus John Bateman (1919–1996).
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Description
Typically it is the females who have a relatively larger investment in producing each offspring. Bateman attributed the origin of the unequal investment to the differences in the production of gametes: sperm are cheaper than eggs. 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 if he mates with several females. By and large, a male's potential reproductive success is limited by the number of females he mates with, whereas a female's potential reproductive success is limited by how many eggs she can produce. This results in sexual selection, in which males compete with each other, and females become choosy in which males to mate with. As a result of being anisogamous, males are fundamentally promiscuous, and females are fundamentally selective.
Bateman's study
Bateman's observations came from his empirical work on mating behaviour in fruit flies (Drosophila melanogaster). Bateman hypothesized that male reproductive success increases with number of mates, whereas female reproductive success does not. He believed that this hypothesis could be supported by illustrating the variance in number of mates between females and males, and by plotting reproductive success versus number of mates.
To test this, Bateman crossed virgin parent fruit flies that were each heterozygous for a unique dominant mutant phenotype. He placed 3-5 flies of each sex in milk bottles for 3-4 days, allowed the females to lay eggs, then removed the parent flies and counted the offspring once hatched. Because most (75%) of the offspring expressed the phenotypes of one or both parents, Bateman deduced how many mates each individual had by observing the offspring’s mutations. He judged reproductive success by counting the relative number of offspring sharing each parental phenotype Template:Harv. Bateman concluded that 1) the variation in number of mates for males was greater than for females, and 2) "The males show direct proportionality between number of mates and fertility... The females, provided they have been mated with at least once, show absolutely no effect of number of mates" Template:Harv.
Conclusions
- "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." Template:Harv
- "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." Template:Harv
- "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." Template:Harv
- "among polygynous species, the variance in male reproductive success is likely to be greater than the variance in female reproductive success." Template:Harv
- "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." Template:Harv
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 Template:Harvcoltxt argues that the formulation of Bateman's principle was limited by such things as short observation time in his experiments. Tim Template:Harvcoltxt has also documented extensive examples of exceptions to Bateman's principle, with a focus on sperm competition.
Some flaws in Bateman’s study
Researchers have recently reconstructed Bateman’s original 1948 experiments and critiqued the results using a present-day understanding of genetics and statistics. They found that many of Bateman’s conclusions were based on poor genetic techniques or statistical errors and oversights Template:Harv.
- Bateman selected his mutant strains because they had visible morphological deformations (eyes, wings, bristles, etc.). These mutations were easily discernable, but may have interfered with mating, mate selection, or mobility. Strains therefore had different reproductive success.
- Pure-breeding mutant strains are often inbred, and inbred organisms typically have reduced reproductive success and fewer viable offspring. Also, many mutant strains are bred to eliminate most genetic variation other than the pure-breeding mutation, so individuals within a strain are nearly clones. Samples were therefore not completely independent, reducing the replicability and sample size of his study.
- Several of the mutant strains are homozygous lethal, and individuals heterozygous for more than one mutation may have reduced viability. This would result in the premature death of some offspring genotypes, biasing Bateman’s counts.
- Female flies take 4 days to reach sexual maturity, while males take only 1 day. Bateman ran experimental crosses for 3-4 days, using flies of different ages. This experiment window was not long enough for sexually immature females to mate more than once.
- Several calculations of the variance in number of mates had arithmetic errors. Modern software shows that most of his results were not statistically significant Template:Harv.
The reanalysis of Bateman’s original data shows that reproductive success is positively and significantly correlated with number of mates in both males and females (though the trend, as predicted, is greater in males). That is, both sexes benefit from mating with multiple partners Template:Harv.
Females can be promiscuous
Bateman’s principle implies that females are choosy because there is little advantage for her to mate with multiple males. 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" Template:Harvcol. Exceptions to Bateman's principle abound, as do hypotheses explaining the evolution of female promiscuity. Females in fact have a lot to gain, depending on the species:
Genetic compatibility
Some combinations of male and females genomes are incompatible, causing abortion or reduced offspring viability. Mating with multiple males can increase likelihood of finding a compatible partner. For example, in the pseudoscorpion Cordylochernes scorpioides, females that mate once with two mates tend to have more offspring surviving to adulthood than females that mate twice with one mate. This is presumably because the female is putting her proverbial eggs in multiple baskets (Template:Harvnb; Template:Harvnb)
Reduced risk of inbreeding
Incompatibility and low offspring fitness is common in inbred populations where genetic diversity is low. Splendid Fairy-wrens (Malarus splendis) are relatively sedentary and have a low dispersal rate. Though the species is socially monogamous, mating within the social group would lead to a highly inbred population. Females sneak copulations with males outside the group, resulting in more successful, genetically diverse offspring Template:Harv.
Protection against infanticide
Male chimpanzees, lions, and many other mammals, will kill unrelated offspring in order to bring the female into estrus, providing a mating opportunity for the male. By mating with multiple males, a female can confuse the paternity of her offspring, and males are less likely to commit infanticide if they may have sired the offspring (Template:Harvnb; Template:Harvnb)
Bet-hedging against sterility
In some species, like the shiner perch (Cymatogaster aggregata), copulation and fertilization are separated by several months. If the female mated once, and the male’s sperm was unviable, she would have to wait until the next breeding cycle to mate again. Instead, the promiscuous female mates with multiple males to ensure that her eggs will be successfully fertilized Template:Harv.
Males can be choosy
Bateman bases his hypotheses on the assumption that males are promiscuous because their gametes are inexpensive and unlimited. However, the male reproductive expense can be great, and often this results in males choosing mates carefully. Several hypotheses explain the evolution of these behaviors.
Sperm is not always cheap
Although a single sperm normally takes little energy to produce, sometimes it takes substantial energy to produce them, or at least enough to fertilize a female’s eggs.
- In one species of fruit fly, Drosophila bifurca, males produce sperm with flagella 58mm long, and require 17 days for the testes to fully develop. Several hypotheses may explain the evolution of giant sperm—the sperm act as immediate parental investment by providing nutrition for the developing zygote; they are large enough to block the reproductive tract of the female, preventing sperm competition; the sperm are large enough to individually transfer to the female, so the male only needs to deposit enough sperm to fertilize the available eggs. Because the sperm are so big, there is a greater chance of successful fertilization Template:Harv.
- A typical fairy-wren ejaculate may contain over 8 billion sperm (compared to about 180 million in humans). In these species, extra-pair copulation is common, so sperm competition is fierce. An individual can increase his chances of fertilizing the female’s egg by producing more sperm than his competitor. For this and many other species with a similar strategy, producing this many sperm requires a large resource investment that may take days or weeks to replenish Template:Harv.
Copulation can be demanding
Females of many species require multiple copulations in order to stimulate ovulation or to produce hormones to initiate pregnancy. This requires a high energy investment on the part of the males. Lionesses, for example, may require the stimulation from over 100 copulations to produce a single litter of cubs. This adaptation may ensure the strength and fitness of the lioness’ mate, which would result in higher quality offspring Template:Harv.
Sex-role reversal: males may invest a lot
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 (Template:Harvcolnb;Template:Harvcolnb; Template:Harvcolnb). 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 Template:Harv.
Other examples of violations to Bateman's principle
Research has 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 Template:Harvcol. These observations also seem to challenge Bateman's theory, specifically the assertion 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 Template:Harvcol.
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 gametophyte - pollen - not by production of the female gamete Template:Harvcol.
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 many species mate with several males Template:Harvcol.
See also
