ethology Ethology is a branch of zoology that studies the behavior, behaviour of non-human animals. It has its scientific roots in the work of Charles Darwin and of American and German ornithology, ornithologists of the late 19th and early 20th cen ...

, male-male intrasexual competition occurs when two

male Male (Planet symbols, symbol: ♂) is the sex of an organism that produces the gamete (sex cell) known as sperm, which fuses with the larger female gamete, or Egg cell, ovum, in the process of fertilisation. A male organism cannot sexual repro ...

s of the same

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

compete for the opportunity to

mate Mate may refer to: Science * Mate, one of a pair of animals involved in: ** Mate choice, intersexual selection *** Mate choice in humans ** Mating * Multi-antimicrobial extrusion protein, or MATE, an efflux transporter family of proteins Pers ...

with a

female An organism's sex is female ( symbol: ♀) if it produces the ovum (egg cell), the type of gamete (sex cell) that fuses with the male gamete (sperm cell) during sexual reproduction. A female has larger gametes than a male. Females and ...

Sexually dimorphic Sexual dimorphism is the condition where sexes of the same species exhibit different Morphology (biology), morphological characteristics, including characteristics not directly involved in reproduction. The condition occurs in most dioecy, di ...

traits, size,

sex ratio A sex ratio is the ratio of males to females in a population. As explained by Fisher's principle, for evolutionary reasons this is typically about 1:1 in species which reproduce sexually. However, many species deviate from an even sex ratio, ei ...

, and the social situation may all play a role in the effects male-male competition has on the

reproductive success Reproductive success is an individual's production of offspring per breeding event or lifetime. This is not limited by the number of offspring produced by one individual, but also the reproductive success of these offspring themselves. Reproduct ...

of a male and the

mate choice Mate choice is one of the primary mechanisms under which evolution can occur. It is characterized by a "selective response by animals to particular stimuli" which can be observed as behavior.Bateson, Paul Patrick Gordon. "Mate Choice." Mate Choi ...

of a female. Larger males tend to win male-male conflicts due to their sheer strength and ability to ward off other males from taking over their females. For instance, in the fly '' Dryomyza anilis'', size shows the strongest correlation to the outcome of male-male conflicts over

resources ''Resource'' refers to all the materials available in our environment which are Technology, technologically accessible, Economics, economically feasible and Culture, culturally Sustainability, sustainable and help us to satisfy our needs and want ...

like territory and females.

Influencing factors

Sex ratio

There are multiple types of male-male competition that may occur in a population at different times depending on the conditions. Competition variation occurs based on the frequency of various mating behaviours present in the population. One factor that can influence the type of competition observed is the population density of males. When there is a high density of males present in the population, competition tends to be less aggressive and therefore sneak tactics and disruptions techniques are more often employed. These techniques often indicate a type of competition referred to as scramble competition. In Japanese medaka, ''

Oryzias latipes The Japanese rice fish (''Oryzias latipes''), also known as the medaka, is a member of genus ''Oryzias'' ( ricefish), the only genus in the subfamily Oryziinae. This small (up to about ) native of Japan is a denizen of rice paddies, marshes, po ...

'', sneaking behaviours refer to when a male interrupts a mating pair during copulation by grasping on to either the male or the female and releasing their own sperm in the hopes of being the one to fertilize the female. Disruption is a technique which involves one male bumping the male that is copulating with the female away just before his sperm is released and the eggs are fertilized. However, all techniques are not equally successful when in competition for reproductive success. Disruption results in a shorter copulation period and can therefore disrupt the fertilization of the eggs by the sperm, which frequently results in lower rates of fertilization and smaller clutch size.

Resource value and social ranking

Another factor that can influence male-male competition is the value of the resource to competitors. Male-male competition can pose many risks to a male's fitness, such as high energy expenditure, physical injury, lower sperm quality and lost paternity. The risk of competition must therefore be worth the value of the resource. A male is more likely to engage in competition for a resource that improves their reproductive success if the resource value is higher. While male-male competition can occur in the presence or absence of a female, competition occurs more frequently in the presence of a female. The presence of a female directly increases the resource value of a territory or shelter and so the males are more likely to accept the risk of competition when a female is present. The smaller males of a species are also more likely to engage in competition with larger males in the presence of a female. Due to the higher level of risk for subordinate males, they tend to engage in competition less frequently than larger, more dominant males and therefore breed less frequently than dominant males. This is seen in many species, such as the Omei treefrog, '' Rhacophorus omeimontis,'' where larger males obtain more mating opportunities and mate with larger females.

Winner–loser effects

A third factor that can impact the success of a male in competition is winner-loser effects. Burrowing crickets, ''Velarifictorus aspersus'', compete for burrows to attract females using their large mandibles for fighting. Female burrowing crickets are more likely to choose winner of a competition in the 2 hours after the fight. The presence of a winning male suppresses mating behaviours of the losing males because the winning male tends to produce more frequent and enhanced mating calls in this period of time.

Effect on female fitness

Male-male competition can both positively and negatively affect female fitness. When there is a high density of males in a population and a large number of males attempting to mate with the female, she is more likely to resist mating attempts, resulting in lower fertilization rates. High levels of male-male competition can also result in a reduction in female investment in mating. Many forms of competition can also cause significant distress for the female negatively impacting her ability to reproduce. An increase in male-male competition can affect a female's ability to select the best mates, and therefore decrease the likelihood of successful reproduction. However, group mating in Japanese

medaka The Japanese rice fish (''Oryzias latipes''), also known as the medaka, is a member of genus ''Oryzias'' ( ricefish), the only genus in the subfamily Oryziinae. This small (up to about ) native of Japan is a denizen of rice paddies, marshes, po ...

increases the fitness of females due to an increase in genetic variation, a higher likelihood of paternal care, and a higher likelihood of successful fertilization. Exposure to environmental estrogens, such as some herbicides, can confuse female choice of males.McCallum, M.L., M. Matlock, J. Treas, B. Safi, W. Sanson, J.L. McCallum. (2013). Endocrine disruption of sexual selection by an estrogenic herbicide in Tenebrio molitor. Ecotoxicology 22:1461-1466.

References