Gnatocerus cornutus

The broadhorned flour beetle is about 3.5 to 4.5 mm long and is red-brown in color. The males have a pair of broad, stout horns on their mandibles. These beetles show sexual dimorphism, with males exhibiting larger body sizes and distinctive characteristics such as enlarged mandibles, widened genae, and two small horns on the vertex. These physical traits are not observed in females. == Habitat ==

The broadhorned flour beetle is a stored product insect found all around the world, more commonly in warm, tropical areas. In warmer regions, this beetle is often found in cassava root and cotton, whereas in the colder regions such as the temperate climate of the UK, it is often found in flour and provender mills. This is due to their increased susceptibility to colder climates and innate desire to shelter from the cold. == Diet and development ==

Gnatocerus cornutus mainly feed on various types of grain, yeasts, and flours. In the larval stages of broad-horned flour beetles, the quality of their diet influences their development rate without affecting their survival significantly. Larvae with poor quality diets can survive, although they experience a prolonged developmental period compared to those with access to higher food quality. The development rate of broad-horned flour beetles is also affected by larval density. As larval density increases, time for development from mature larvae to adults extends, affecting body size and secondary sexual traits. This delayed development is believed to be in response to heightened tactile stimulation between neighboring larvae. Increased larval density also increased mortality rate and cannibalism where larvae were seen eating pupae and other larvae. Moreover, individuals consuming higher-calorie diets exhibit enhanced survival and faster development, growing larger much more quickly. These beetles are in better condition, enabling the allocation of resources to both naturally and sexually selected traits. The ratio of ingested nutrients proves critical for trait expression, with an optimal balance observed at a specific protein-to-carbohydrate ratio of 1:2. A carbohydrate rich diet is likely to enhance male fitness, improving offspring survival, development rate, and morphological traits whereas a protein rich diet is essential for the expression of sexually selective traits. Although the genitalia of G. cornutus are subject to sexual selection, it is clear that the size of the aedeagus is not particularly sensitive to nutrition and is an unlikely signal of a male's underlying genetic quality. Males infected with parasites may have decreased horn size as resources are allocated towards building immune response. This reduction in horn size may limit mating opportunities for infected males. == Genetics and heritability ==

One specific type of Insulin-Like Peptide (ILP), GcorILP2, has been shown to have an impact on nutritional signaling and promoting mandible growth. GcorILP2 positively correlates with nutrional condition and promotes the growth and development of mandibles along with increasing the size of the head and prothorax. GcorILP2 is primarily synthesized during the larval stage, specifically at the postfeeding metamorphic stage and is retained fat, retaining information on the beetle's nutritional state and directly coupling this nutritional condition with the growth of exaggerated traits in adult G. cornutus. 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). Ornamentation Sexually selected exaggerated traits such as mandible length, size of the gena, and horns on the vertex are all ornaments which confer mating advantages to males. However, these genes are costly if expressed in females which is why they are expressed at different levels amongst the sexes and regulated differently. 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 to male broadhorned flour beetles, 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. == Reproduction ==

Male-male competition In male-male competition, the males show aggressive behavior such as interlocking horns, pushing, lifting their opponents, and biting. Males with larger weapons are typically better fighters and have enhanced fighting endurance Because of this, fighting is common and males will often fight for mates at least twice in their life. Males that win these fights obtain mating success whereas mates that lose rarely find success. The experience of losing fights can affect male behavior. == Parental care ==

Oviposition The larvae of G. cornutus are approximately 2-6 mg in weight, measured 30-60 days from egg oviposition. These males were seen as poor reproductive competitors in relation to males with larger size and mandibles. However, these mothers not only produced smaller children, but they produced a greater quantity of daughters. In contrast, mothers in good nutritional conditional will even out the sex ratio. Under crowded conditions, a similar trend is noticed where females produce larger broods with sex ratios biased towards females, thereby maximizing the number of potential, future colonizers. == References ==