Red flour beetle

Multiple mating events can ensure that females obtain a greater net amount of sperm, resulting in an increased likelihood of successful fertilization.[5] In nature, repeated matings could result in males that have a low sperm count.[5] Due to the males' low sperm count, a female may need to mate with several males before being successfully inseminated.[5]

Although multiple mating events may result in an increased likelihood for finding genetically compatible sperm, genetic compatibility cannot always be considered a major fitness advantage for polyandrous behavior.[5] The increased viability of embryos—due to increased genetic compatibility—did not significantly increase the number of adult beetles over time, and therefore, did not play a significant role in the fitness of the overall population.[5] However, increased genetic compatibility could increase the genetic diversity of the population, which maybe useful in various different environments.[6] High genetic diversity within a population can lead to high phenotypic variation, which can subsequently enable some variants to better survive and reproduce given a sudden environmental change.[6]

The availability of resources and population size can greatly affect how many matings each individual participates in. Increased population size within a given area with fixed resources can limit how many offspring can survive.[6] Therefore, males must often compete with other males to be the last male that mates with the female, to increase his fertilization rate.[7] By being the last male to mate with a female, it is likely that his ejaculate removed previous ejaculate from previous males, increasing the chances that his sperm fertilizes the female.[7] In fact, in areas with limited resources, higher rates of cannibalism among competitor males can result in an overall decrease in fitness of the population since there is a net decrease in offspring production and survival.[6]

Polyandrous behavior may not always result in the propagation of adaptive genes. In the red flour beetles, the ability of a male to attract females—through pheromones—is genetically based. Males vary in the ability to attract females.[8] However, offspring fitness is not related to the ability of the males to attract females.[8] In other words, just because a male reproduced more often due to increased ability to attract females, does not necessarily mean the offspring have inherited the traits that result in increased fitness.[8]

Females of different geographic regions—and subsequently, different genetic backgrounds—often show great variation in mating behavior.[6] Certain strains of females avoid multiple mating events while other strains of female engage in higher degrees of polyandry.[6] This variation suggests that polyandry can be advantageous in some populations but not in others.[6]

Female beetles vary in which males they choose to copulate with. Moreover, female beetles can specifically choose which male's sperm is utilized for fertilization through cryptic choice.[9] Females that have multiple sperm receptacles can store sperm from different males and can later choose which sperm is used for fertilization.[9]

Male beetles can also vary in the females they choose to mate with. Males are extremely selective in their mate choice. They prefer to mate with mature, virgin females.[7] If a male mates with a virgin female, his sperm has an extremely high chance to fertilize the female if another male does not mate with her.[7] Males are able to differentiate between virgin females and non-virgin females through scent; the wax-like secretions of competitor males could be found on the reproductive glands of non-virgin females, but not on virgin females.[7] Males that possess an increase in the number of odor receptors are better able to choose which females to reproduce with and subsequently, increase their fitness.[7] Some males possess better suited characteristics to detect the maturity and reproductive status of the female, and as such, will preferentially breed with only those females that will have the highest production of offspring.[7] Likewise, males that deposit stronger scents will have an indirect fitness advantage due to their[6] odor deterring other potential mates from an already inseminated female.[7]

In red flour beetles, females that engage in polygamous behavior produce more offspring than those that are less polygamous. Polygamy is mostly seen in populations that lack genetic diversity. Polygamy in less genetically diverse populations is a means of avoiding fertilization between beetles that are closely related since they may be genetically incompatible.[10] The more partners that a male or female has, the higher the chances that at least one of the matings is with an unrelated partner and the greater the genetic diversity in the offspring. In this way, genetic incompatibility is reduced and diversity is increased in a population. For this reason, females copulate with more males when genetic diversity is low in order to attain fertilization success and also increase fitness in their subsequent offspring.

In some studies, however, it has been noted that fertilization can still occur when related beetles mate. Nonetheless, it is worth noting that there is a significantly lower number of offspring produced when inbred beetles mate than when the matings are between out-bred partners. Successful fertilization observed in a small portion of research in related beetles has led some biologists to claim that there may be no inbreeding depression in red flour beetles.[11] Even though there is successful fertilization, it is observed that a lower number of total offspring is produced, which can be argued to be a type of inbreeding depression since it lowers reproduction fitness.

During mating, red flour beetles are known to engage in polygamous behavior. Male flour beetles have been known to recognize their relatives while the females do not have this capability. Lack of the ability to recognize their relatives has led females to mate with any male within the population.[11] Female red flour beetles are also known to store sperm after mating. More sperm is stored by the first mating, which leads to less sperm stored in subsequent matings. However, amount of stored sperm does not stop the last male mate from fertilizing the egg.[12] This is due to the fact that with each mating, males can remove previously stored sperm thus giving their own sperm an advantage to fertilize the egg.