Dung Beetles, Although Not One of the Favorite Insects of Many People, May Perform Essential Services to the Environment in Many Ecosystems
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Dung Beetles, their Mating Habits, and their Effect on the Environment Emily Chaskey [email protected] BI507 Introduction Dung beetles, although not one of the favorite insects of many people, may perform essential services to the environment in many ecosystems. Their gathering, transportation, and burying of the dung of various omnivores and herbivores helps to remove accumulations of manure, fertilize the surrounding areas, aerate the ground, and to disperse various seed types. The term “dung beetles” typically refers to beetles belonging to the order Coleoptera, Families Scarabaeidae and Geotrupidae, and subfamilies Scarabaeinae and Aphodiinae. There are three main types of dung beetles: rollers, tunnellers, and dwellers. As the names suggest, rollers roll dung into balls, roll it away, bury it, and use it for food or egg- laying; tunnellers bury the dung near to where it originally lies for the same uses as rollers; and dwellers simply live in the dung pats. The front legs of most dung beetles have serrated edges for use in digging in dung and for digging holes in the soil. Female beetles typically have shorter back legs, while the males have long back legs for use in rolling dung balls along the ground. Also, the age of the beetles can be estimated by the serrations on the front legs, since they are worn down through a lifetime of digging and rolling. Adult beetles are drawn to manure by the odor, and some species will fly up to ten miles in search of the right patch of dung. Adult dung beetles use only the liquid contents of the manure for their nutrients; they do not actually “eat” the dung. Dung beetle larvae eat the dung brood balls that they are laid inside. After hatching, the larva eats 40 to 50% of the 1 dung contained in the ball, and then pupates inside the brood ball as well. After the young adult beetle emerges, it eats its way out of the brood ball, tunnels to the surface, and starts the cycle over again. One benefit from the relocation and burying of herbivore and omnivore dung is a re-introduction of nutrients to the soil. This raises the general quality of the soil, and can lead to a significant increase in the growth and health of pastureland plant species. The aeration of the soil by burrowing beetles also increases the soil’s ability to absorb and hold water. These improvements make the land more efficient for cattle grazing. Another benefit is simply the removal of the feces from the pastures. Cattle benefit from the reduction in fly populations and recycled gut parasites. Removal also helps with contamination of the grazing area, allowing more cattle to feed. Variations in Species Different species of dung beetles have varied feeding patterns and preferences (fresh vs. old dung, nocturnal vs. diurnal, etc). For example: Aphodius granarius and Aphodius erraticus are present in North Carolina from late winter through spring, but are not common during summer and fall; Phaenus vindex are found spring to fall; many Onthophagus species, including O. hecate and O. taurus, are present starting in late March and are most active through October. Dung beetles also have predilections for distinct climates, from arid desert-like conditions to humid forests (Thomas, 2001). In addition to variations in seasonal abundance and location, there are distinctions even between the times of day these beetles feed, with many being nocturnal and relying on the polarization of moonlight to navigate (Dacke, et al., 361-365). Many experts are now advocating the introduction of foreign species to cattle ranges. This diversification would increase the dung beetle population 2 overall, and, through careful planning, could also give us a continual cycle of dung removal. A continual cycle of dung removal could be the key to ensuring that our pastures are more desirable for the cattle grazing in them, and could increase the usability of these pastures and the production levels of our livestock. Mating Behavior Many dung beetle species have dimorphic males. These males are divided into major and minor morphs, where major males have large horns, and minor males have very short horns, or no horns at all. Oddly enough, the differences in horn length, while not due directly to parental genetics, are due to the parents of the beetles, as will be explained. Major and minor dung beetle males have significantly different behaviors in courtship, mating, and postcopulatory behavior (mostly parental provisioning). In Onthophagus males, all males show distinctive “upward jerks of the head and pronotum” when initiating courtship with a female dung beetle. During this behavior, major males are more likely to be in direct contact with the female beetle than minor males (82% versus 56%), and were also more likely to contact the female head on (78% versus 39%). This has a negative impact on the number of times the major males get to breed, as it appears to visibly shake the females, to the point of pushing them backward. In a study by Cook (1990), 97% of hornless males initiated courtship behavior, compared to 82% of horned males. However, while 61% of the hornless males were successful in inseminating the female, only 13% of horned males successfully mated. Although this would seem to be a notch in favor of the hornless males and their ability to produce offspring, this may not be the case. Horned males are much more likely to cooperate with the females they mate with after copulation. In O. bindis, this amounted to 3 ten occasions of cooperation between a female and a large male, while hornless males were never seen cooperating with their mates (Cook, 1990). Horned males are involved in moving the dung to the burrow entrance and then passing it down into the burrow, as well as sometimes gathering the dung. This involvement is actually positively correlated (Hunt and Simmons, 2002). Males and females do not act alone. Rather, they cooperate with one another, filling in the gaps in parental care that the other may have left. This cooperation is also vital to the weight of the brood masses, as a male and female together are able to compile a larger amount of dung before the eggs are laid. Cook (1990) also hypothesized that the horned males mate less often on purpose. “By lifting and shaking the female during courtship, the horned male may acquire information on the female’s size or body weight. Males might choose females on the basis of size since size is positively correlated with fecundity in female O. bindis as in the dung beetle Copris diversus.” This discrimination toward potential mates would also help account for the larger size of the brood masses and potential offspring. In contrast, minor males have actually been known to dig holes along side of the holes being guarded by horned males in an attempt to mate with the female that is already preparing to lay eggs in a brood ball. This behavior alone shows that hornless males are indiscriminate in their choice of a mate. O. bindis females produced more offspring when paired with horned major males than when they pair with minor, hornless males (Cook, 1990). In other studies, such as those by Kotiaho (2002), it was shown that species such as O. taurus produce larger offspring when they mate with the larger horned males than when they mate with hornless males. This is probably a result of the involvement of the major males in the production of brood balls and the guarding of the nests, giving their offspring more nutrients and protection from 4 competing dung beetles and other predators. This phenomenon, where females mated with large males provide more resources to their offspring, has actually been termed the “mother- in-law effect” by Hunt and Brooks (2002). The lighter brood masses (less offspring) produced in O. taurus by mating with large males leads to the production, therefore, of larger offspring from the same amount of initial investment of dung. These larger offspring are, in turn, able to better provide for their offspring, continuing the cycle. Dimorphism in male horn lengths is a result of parental care rather than the genetics of the parents of the larva. The amount of nutrients provided for a larva in the initial brood ball is an accurate indicator of whether or not the resulting male offspring will have horns or not (Moczek 1998, Moczek 1999, Hunt and Simmons 2000, Hunt and Simmons 1998). Once male O. taurus larvae reach a body size of 5 mm protonum width, they change from investing in body size to investing to the production of horns (Hunt and Simmons, 2000). If there are enough nutrients in the brood ball to support both growth to that size, and development of a horn, the larvae will develop into a major male, and will generally be large as well. Therefore, the involvement of the male in production and protection of the larva provides significant pushes toward the development of a large major male. Even though the horned phenotype is not genetically passed down, it is still more likely that the progeny of a horned male will have horns than the progeny of a minor male. The use of major males in breeding and addition of dung beetles to stock land would be advantageous. Females that mate with major males can produce either larger offspring or more offspring. Both results would lead to a greater use of dung piles, as larger offspring and larger brood sizes would require a larger initial stock of food, and would require more dung throughout their lifetimes. This advantage would be passed down from generation to generation, as the large offspring had large offspring, and so on.