Animal Behavior - Biology 4200/5430 |
Bowling Green State University, Fall 2009 |
One half of the world cannot understand the pleasures of the other
-- Jane Austen (1775-1817)
Life is sexually transmitted and Death is an acquired trait.
Disruptive selection in gamete characteristics and evolution of <Anisogamy> condition in which one type of gamete (i.e., egg) becomes increasingly large to provide the zygote with storage for a head start in development. With increased size the gamete is less mobile. In turn selection produces another type of gamete (i.e., sperm) that is optimized for locomotion to find and fuse with the egg. The larger gamete is termed female, the smaller one is the male. Compared to isogamy this allows for both increased provisions for zygote and optimized mobility.
Bateman's rule: increased reproductive investment is accompanied by increased selectiveness for mating partners. While female gametes are energetically expensive, large and limited in number, male gametes are cheap, mobile and available in large numbers. This asymmetry leads to reluctant females as the choosy sex while ardent males are the competing sex. A.J. Bateman (1948) phrased this rule based on work where he had examined variation in reproductive succcess between male and female fruitflies. Most females had reproduced with little variation, while only a few males had accounted for all reproduction and most males had no success at all. Limits to reproductive success thus differ between sexes where males are generally limited by the number of successful matings, while that of females is primarily determined by the rate of egg production.
Trivers-Willard Hypothesis: Moreover, as only the most competitive males achieve mating success, a bias in sex ratio may emerge where dominant/healthy females produce a relative excess of males which stand a disproportionate chance to develop into the successful male.
Sex ratio describes the relative number of males to females in a population and is most commonly around 50:50. Such a sex ratio might not seem very efficient considering that a male can fertilize several females. Fisher's theoretical work illustrated that such a balance forms a general equilibrium point. If there are fewer males than females, then males face better odds in mating. An advantage for males favors females who produces extra sons. The same argument follows for deviations in the opposite direction. Thus, if a population ever comes to deviate from a ratio of 50:50, natural selection will tend to drive it back to that balance.
Mating systems are fluid, dynamic and highly optimized systems with many factors figuring into the equation. Selection will always foster the most successful tradeoffs for these parameters, but many strategies may lead to success. Mating partners are rarely selected at random. In most species it is strongly controlled of diverse factors. Although many individual strategies may prove successful, a variety of considerations provide predictive power for the type of mating system that is present:
If one sex invests a disproportionate amount into the success of the zygote, then the other sex will likely compete strongly for access to mating opportunities. Differential reproductive investment will polarize operational sex ratio (i.e., most females and only a few males get mating opportunities).
Sexspecific differences in reproductive success thus favors conditions for male-male competition and polygynous mating systems where a small number of males account for a disproportionate amount of the breeding. Size matters - Selection for sex differences in body size.
When resources (e.g., nesting sites, food resources) can be dominated by a small subset of males, a polygynous mating system will likely emerge:
In contrast, when resources are difficult to dominate (e.g., a scattered renewable food source) things get more complicated. Monogamy is a possible outcome which exists either over a lifetime (e.g., greylag geese) or serially (e.g., ducks). Under a variety of such conditions, female choice will encourage shifting part of the burden of parental care to the male. Alternatively, in situations where social groups are able to form, females may mate with any male member of the group leading to Polygynandry (Promiscuity). Sperm competition will in this case most likely determine paternal identity (e.g., elephants, chimpanzees). In situations where food is scattered and limited, both parents need to provision the young. When multiple helpers are needed for successful reproduction, females specialize in egg production while pairing with multiple males in polyandry (e.g., Jacanas).
Consider mammals, birds, and fish and phrase your prediction as to the sex that you would be expected to invest more into the offspring.
Discuss the significance of a cryptic reproductive cycle in primates living in small social groups?