Beate Gade Katrud

How can sexual selection result in reproductive advantage?

 

In many species individuals actively competes for mates. Typically in a population, there is an abundance of males competing for a limited number of receptive females. For a male, reproductive success depends on how many females he can impregnate. Females may have the opportunity to select a sexual partner from among several males. For a female, reproductive success depends on how many eggs she can produce during her reproductive lifetime, the quality of the sperm that fertilises them, and on the survival of her offspring to reproductive age. An animal’s reproductive success is a measure of its direct fitness. Sexual selection, a type of natural selection, occurs when individuals vary in their ability to compete for mates.

Dominance and ornamental traits influence choice mate

In many species, success of a male in dominance encounters with other males indicates his quality to the female, and she allows the victorious male to court her. Many studies confirm that males that rank higher in a dominance hierarchy mate more frequently than males that rank lower. 

Many exceptions demonstrate the complexity of reproductive behaviour among some species. For example, among baboons with a definite dominance hierarchy, males lower on the hierarchy copulated with females as frequently as males of higher status. Investigators observed, though, that dominant males copulated more frequently with females who were in oestrus, their fertile period. In addition, some lower ranking males develop alternative strategies for attracting females. For instance, a male might win a female’s interest by protecting her baby. 

Females may select their mates based on ornamental displays. Male fishes are often brightly coloured. Among many bird species, males exhibit bright colours and dramatic plumage, and male deer display elaborate antlers. The expression of ornamental traits may give the female important information about proper condition and good health.

Males can gather in a small display called lek in some species where they compete for females, and they try to get her attention. The dominant male may occupy a central position and be chosen by most of the females. 

Courtship rituals ensure that the male is indeed a male and is a member of the same species, and also provide the female further opportunity to evaluate him. In some species courtship may also be necessary as a signal to trigger nest building or ovulation. They can last seconds or hours and often involve a series of fixed action patterns before the pair is physiologically ready for copulation. 

An extreme courtship ritual has been described for red back spiders (Latrodectus hasselti). During copulation, the female eats her suitor because the male is able to copulate for a longer period and fertilise more eggs than non cannibalised males. 

 

Sexual selection favors polygynous mating system 

Males make little parental investment in their offspring apart from giving sperm, in most species. They ensure reproductive success by impregnating many females, that is called polygony.

In the mating system called polyandry, the female mates with several males. Benefits may include receiving gifts from several males or enlisting several males to help care for the young.

Data collected at Jane Godall’s research center at Gombe National Park in Tanzania indicate that female chimpanzees mate several hundred times. It is possible that these sexual rendezvous protect against inbreeding and may provide insurance against infanticide. 

Polyandry and polygony sometimes occur in the same species. After mating, the female giant water bug attaches a clutch of eggs to the back of her mate, and she can go on copulating with another male. If the male has space for more eggs he can mate with another female. 

Among some species, a male guards his partner after copulation to ensure that she will not copulate with another male. For example, dominant male African elephants guard a female only during the phase of oestrus when she is most likely to have an fertile egg. One cost of mate guarding is the loss of opportunity for a dominant male to mate with other females. 

Males among some species are monogamous, they mate with a single partner during a breeding season. Monogamy is rare among mammals, but common in birds. This mating system also occurs when males are needed to protect and feed the young, for example in wolves and many other carnivores. 

A pair bond is a stable relationship between two animals of the opposite sex that may ensure co - operative behaviour in mating and the rearing of the young. 

 

 

Many organisms care for their young

 

Care of the young is an important part of successful reproduction in many species. The benefit of investment in parental care is the increased probability that each individual will survive

The costs include a reduction in the number of offspring, and the risk taken when protecting them from predators.

Females of many vertebrate animals have more invested in gamete formation than males, they produce relatively few large eggs. Because of energy and time spent producing eggs and carrying the developing embryo, the female has more to lose than the male. Parental care is especially for the female in mammals because female mammals, provide milk to nourish their young. 

Effort in care of the young is usually less advantageous to a male, for time spent in parenting is time lost from inseminating other females.

In some situations however, a male can benefit by helping to rear his own young or even those of a genetic relative. Receptive females may be scarce, breeding territories may be difficult to establish or guard, and gathering sufficient food may require more effort than one parent can provide. In some habitats, the young may need protection against predators or cannibalistic males in the same species.  

Among many species of fishes, the male cares for the young. The ultimate cause for this behaviour appears to that the parenting costs to the males are less than they would be for the females. A male fish must have a territory in order to attract a female. While guarding his territory, the male guards the fertilised eggs as well. Having eggs also appears to make the male more attractive to potential mates. 

As a conclusion you can say that when a specie takes advantage of reproductivity, they will be more able to fit into the surroundings – “survival of the fittest “ 

Resource: Biology-fifth edition, Solomon, Berg and Martin 1999