Sexual selection in social insects

Social insects take over many different habitats and possess a complex relationship with their societies. William Morton Wheeler called the social insect colony a superorganism because of the extent to which individuals appear to operate as an entity solely dedicated to the continuance and reproduction of the colony as a whole ^[1] Among social insects, males have evolved exaggerated adornments and mechanisms for self-defense. These traits play a role in increasing male reproductive expectations by triggering male-male completion or influencing the female mate choice.^[2] Essentially, sexual selection in social insects can be thought of as functioning on three different levels: individuals, colonies, and populations within an area.^[3]

Anatomy of social insects

There are four sexual organs involved in the mating among social insects. These are the testes, accessory testes, accessory glands, and the external genitalia, which are often flexible body parts. The production of sperm in the testes of males culminates after males reach their prime and achieve sexual maturity. The sperm number in many social insects is fixed, except in termites.^[3]

Male mating tactics

Male mating tactics are essentially regulated by two factors. These factors are space and time. They involve the female ready to mate and the competing male. The amount of space accessible and the time both male and female usually determines the site of copulation. For example, many hymenopteran males constantly fly around specific sites where approachable females are found. These sites are usually the tops of tall trees, summits, or along hedges. These locations can also be resources visited by females for the purpose of feeding. An important and common tactic seen in males is similar to a scramble contest for males. This is seen in honey bee queens. Honey bee queens join the mating area alone and are then pursued by a dynamic comet of males. The males are located in the front of the line are the ones that usually achieve mating with the female. This is seen as a swarming behavior. A queen usually mates with twelve males on an average of two mating flights. She stores sperm throughout her entire lifetime. Colonies sustain thousands of males but only a few queens, so only very few males are able to successfully mate with the queen.^[4]

Male providing

Sexual selection is stronger in males than in females. Males can be choosier than females. As an offering, males are capable of providing nuptial gifts to the females, parental care, and foraging and/or nesting sites. For example, in some species, male nuptial gifts secretions can be more expensive than female reproductive investment.^[2] In species where males provide the parental care, the reproductive costs in males are higher than in females because they invest large quantities of resources and time after egg fertilization . Moreover, these factors decrease their subsequent male reproductive opportunities, and therefore reduce the availability of males. Although, there are few examples of male parental care, it can be found in belostomatid water bugs. The male allows the female to glue her eggs onto his back. However, the male may not accept the eggs unless he has fertilized them. He broods the eggs until the nymphs hatch 2–4 weeks later. The eggs are large and reduce the ability of the male to fertilize many females. This makes the male less efficient in catching prey, and increases its predation risk.^[5]

Common roles of the females

Females are usually in conflict over the acquisition of resources, thus generating a sex role reversal. Females also tend to compete for male provided resources. They also displace rivals and have claspers that appear to be specialized in keeping their hold on males and in avoiding takeover attempts by other females. Females compete to monopolize parental care.^[6]

Male characteristics

In general, because the access to females limits the reproductive success of males, the males compete among themselves for access to females. They also compete with other males for resources that the females require for themselves and their offspring. Thus, intrasexual selection or male-male competition can explain the morphological traits and behaviors that increase the possibilities of the males gaining access to the females. Large body size, weapons, territoriality, and early sexual maturation have evolved by intrasexual selection.^[7] In many social insects, larger males are more competitive and tend to monopolize females or the resources that they need. For example, in bees, females may select males, directly by mating with males who are faster, more agile, or more persistent fliers. Sequestering of females and forced copulation may also favor large male size. Males with more developed weapons are also more competitive. Males with longer horns and mandibles can use them to fight, or for take over attempts when rivals are copulating. In beetle species, the male horns can be as long as the male’s body. Early sexually mature males can start their reproductive life before the rest of the male population. Under scramble competition to inseminate females, the early matured males are smaller than the rest of the population and can attain the highest mating success. In addition, small males can be favored in some cases where courtship occurs on the ground. Thus, in the fly Drosophila suboscura, smaller males are better than large males at tracking females during the courtship dance.^[8]

Sexual pheromones

Sexual pheromones are defined as odors and are one of the prevalent ways in which social male insects find females to mate with. They can be produced in either males or females. The central roles of pheromones are to trigger behavioral interactions between the two sexes.^[9] Ultimately, this brings the sexes together for mating purposes. In addition, females discriminate against mates by the judgment of their sex pheromones.^[10] Courtship pheromones are issued in many cases by males. In the queen butterfly Danaus gilippus and the Arctic moth Utetheisa ornatrix, the chemical stimulation of the female is necessary for male mating success.^[2]

Acoustic and visual signals

In species such as the Coleopteran family Lampyridae, the males fly in the darkness and emit a species-species specific pattern of light flashes, which are answered by perching receptive females. The color and temporal variation of the flash contribute to the success in attracting females.^[11] Acoustic signals are produced by many groups including several orthopeteran species, neuropterida, cicadas, flies, cave planthroppers, leaf hoppers, and tree hoppers. In crickets and other Orthopeterans, loudness and/or uninterrupted songs often attract more females. In other insects, females may choose a mate in relation to aspects of his songs. In any case, it is possible that females compare the songs of several males before making a choice.^[12]

Benefits

The effect of mating on female fitness may vary depending upon the kind of benefit that the females receive from males as a result of their choices. The benefits can either be direct or indirect. Direct benefits include nutritional resources to be used by females. Donation of foods to mates is also common. Males can offer prey to the female. This is seen in scorpion flies and dance flies. Males can donate secretions or nutritional substances to the females which are transferred in the ejaculate or produced by male glands. They can contribute to increase female fecundity or to increase in her lifespan. These types of compounds are produced by several orthopteran species, butterflies, flies, and beetles. For example, in the polyandrous butterfly Pieris napi, a virgin male can transfer an ejaculate containing 14% nitrogen by dry mass. Females utilize the nutrients transferred from the male in order to increase their nutrients. These nutrients essentially contribute to ensure successful egg production. A positive relationship between the amount of ejaculate material received and lifetime reproductive output, as male-transferred material increases female longevity. Male-transferred material also increases female longevity. In some insect species, males can donate some body parts to females such as the leg spurs or the fleshy hindwings of jumped-winged crickets, or they may be completely cannibalized, as occurs in mantis, some dipterous, scorpions and some spider species.^[3]

Female calling syndrome

The female calling syndrome is when female social insects emit a typical call to the males. They do this in close proximity to their natal nests. Males that are attracted by the callings fly extensively in pursuit of potential mates. Their gasters raise and stings start to protrude. The venom glands are a basis of a chemical male attractant. For example, in the species Xenomyrmex floridanus, the queen’s venom glands and gasters both attracted males and prompted copulation.^[10]

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