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= Gnatocerus cornutus =

The broadhorned flour beetle (Gnatocerus cornutus) is a common species of beetle in the tenebrionidae family. It is predominately found in mills as a storage pest and has a wide distribution.

Physical Appearance
Broadhorned flour beetles show sexual dimorphism, with males having large mandibles and a small pair of horns at the apex of their head while females lack these characteristics.

Diet and Development
G. cornutus mainly feed on various types of grain, yeasts, and flours.

In the larval stages of broadhorned beetles, the quality of their diet affects their development rate but not their survival. Larvae with poor quality diets can survive albeit having a longer developmental period compared to larvae that has access to a higher food quality.

The quality of food has no effect on the body size and longevity of the beetles. However, it does affect the size of male mandibles, with larvae that have a poor quality diet having smaller mandibles and vice versa. The expression of sexually selected traits depend on larval diet quality.

Increased larval density (number of larvae present) can slow the rate of development of broadhorned flour beetles, affecting their body size. It can also lead to an increased mortality rate and cannibalism between other broadhorned beetle larvae.

Juvenile Hormones
Juvenile hormones (JH) are involved in the regulation of moulting and metamorphosis of insects and is also associated with the exaggeration of sexually selected traits.

It was found that the head size, gena, horn, and prothorax increased when exogenous JH was added, indicating that perhaps these body parts are associated with their mandibles, a sexually selected trait.

Excess JH reduced the size of the male beetle’s hindwing area and elytra length, along with reduced testes, illustrating the trade off between having enlarged weapons for competition and a reduction in reproductive organs and flight.

Male-male competition
In male-male competition, the males show aggressive behavior such as interlocking horns, pushing, lifting their opponents, and biting. In general, the weapon size (mandibles) typically is larger with a larger body size. Males with larger weapons are typically better fighters and have enhanced fighting endurance

Behaviour
The experience of losing fights can affect male behaviour. With each losing experience, the males reduced their attempts at attacking, often dispersing from the fight more quickly, thus decreasing the duration. However, after four days the effect of losing disappears, ending quite abruptly instead of a gradual decay. The males are able to fight normally on the fifth day without retreating or reducing their attempted attacks.

Genetics and Heredity
The length of male mandibles is heritable but can be evolved based on the environmental pressures.

There appears to be genetic correlations between the size of the mandibles and the size of the head, prothorax, and legs, which were all significantly larger if the beetle had large mandibles. In contrast, there seems to be no genetic correlation between the size of the mandibles and the size of the antenna, eye area, elytra length, and head horn. (although having the head horn may aid the beetle in pushing away the opponent when their mandibles interlock)

Female mate choice
Females often prefer males that court (attractive males) more under non-competitive mating situations. However, under competitive mating events, females tend to mate with competitive males (males with larger weapons and superior fighting capabilities) although there is no current evidence that either attractive males or competitive males affect the females longevity or reproductive success of females.

When females mate with competitive males, the large mandible size phenotype of the males is transferred to the daughter. While the daughter will not inherit large mandibles, the competitive phenotype can masculinize the typical female phenotype, causing a reduction of egg space in their abdomen. This reduces the daughter's overall fitness. Therefore mating with an attractive male, which often have smaller mandibles, reduces the masculinization of the daughters.

Mating with either attractive or competitive males often results in their sons inheriting the traits of attractive or competitive male, which may increase their fitness indirectly. However, there are no benefits in the daughters.

Polyandry
Polyandry in female broadhorned flour beetles has shown to improved their fecundity and reproductive success, as females that mated multiple times often lay more eggs than those that have only mated once.

Sperm Competition
Despite competitive males having high reproductive success due to forced copulation with females and defeating other males, they often transfer fewer sperm and have smaller testes. Thus, they are at a disadvantage in sperm competition. There is a smaller chance for competitive males to have offspring with the female as less sperm equates to a lower probability of fertilizing the females eggs. In contrast, attractive males which allocate their energy in courtship rather than weapon size, often can transfer more sperm, having an advantage against competitive males.

These two male strategies most likely depends on the population densities. In high density populations, having a competitive strategy and phenotype is advantageous as intrasexual competition is commonplace. So having a large weapon size often indicates greater reproductive success as competitive males defeat their opponents and then proceed to copulate with the female. In lower densities, having an attractive strategy and phenotype may attract females easily, as females often prefer attractive males. There is also less competition against males with enhanced weapons