Polygyny in Prairie Dogs
Prairie dogs are found to exhibit polygynous behavior (Hoogland, 1995). The coterie is composed of one male with multiple females, usually two to three adult females (Hoogland,1995). There are many reasons that polygyny may be the mating system of choice for the prairie dog. First and foremost, because the females stay in their natal territory in a group of kin, it is easy for one male to claim this female group as his own. According to Hoogland, the development of a coterie begins with the formation of female kin groups as protection against predators. Soon this group evolves into group defense of the food supply. Then, the claiming of the group of females that live on the territory by a breeding male occurs (Hoogland, 1995). Thus, the type of polygyny that exists within prairie dogs is resource defense because the male secures control over the females within the coterie by defending the resources that they need (Case, 2004). Because of the behaviors of prairie dogs, such as the scattering out of males from their natal territory once they reach sexual maturity and the strong force to stay in kin groups for females, little inbreeding exists (Dobson, Chesser, Hoogland, Sugg, & Foltz, 1997).
Photograph courtesy of Michael Bates
Because offspring grow up fast, it can be assumed that is probably little need for male care of the offspring. While the mother cares for the young by nursing them, defending the home nursery burrow and collecting grass for the nest, the male has less parental responsibilities. These responsibilities include territorial defense and maintenance of burrows (Hoogland, 1995). Young prairie dogs spend the first six weeks of their lives below the ground, where they are nursed by their mothers (Chance, 1976). After these first six weeks, however, the young pups are weaned and finally come up to the surface from the burrow. Within another five months or so, these same pups are almost adult size (Chance, 1976).
Another reason why polygyny is the mating system of choice for the prairie dog is based on a model by Con Slobodchikoff (Travis & Slobodchickoff, 1993). Slobodchikoff’s model, known as the habitat- variability mating system model says that as food resources are abundant and patchily distributed as well as scarce and patchily distributed coteries will be composed of several females per male. However, when food resources are scarce and uniformly distributed as well as abundant and uniformly distributed, there will be harems of single females. The reasoning that Slobodchikoff has as to why resources that are patchily and abundantly distributed would lead to harems with more than one female is that these patchily resources are spread out over a larger area, and thus more difficult for one female and male to defend. Because the food patches that Slobodchikoff talks about are small, a male and female pair would not be able to guard enough space to ensure enough food for themselves, and thus must have the group defense that comes along with a polygynous mating system with multiple females. The other resource classification of scarce and patchy would also lead to polygynous mating systems, as long as the competition is not so great as to make group defense worthless. Therefore, there must exist a greater amount of food resources than the minimum threshold. Any amount below this threshold would cause monogamy to be the existing mating system (Travis & Slobodchickoff, 1993).Along the same lines of reasoning, in the resource distributions of scarce and uniformly distributed, there will be monogamous pairs because there are not enough food resources to guarantee food for more than the pair. Under abundant and uniform resources there would also be monogamous pairs because the competition that would exist if there was more than one female would outweigh the need for cooperative defense (Travis & Slobodchikoff, 1993).
In a competing hypothesis to the habitat- variability model is the predator hypothesis proposed by Hoogland. This hypothesis states that as the incidence of predation increases, so will the group size and thus the incidence of polygyny because there will be more individuals around to give the alarm call (Travis & Slobodchickoff, 1993).
Multiple Paternity and Paternity Uncertainty
Because of the inability of prairie dog fathers to recognize their own offspring, paternity uncertainty is a problem for them (Hoogland, 1995). However, the difference in the cost of rearing another male’s offspring varies between prairie dog species. Foltz and Hoogland found that for the majority of the time for black- tailed prairie dogs, males within the home coterie territory fathered the offspring within their territory (1981). Thus, multiple paternity seemed to exist only occasionally. However, because the act of copulation among prairie dogs is very rarely seen above ground, genetic evidence as well as observations of males present on the home territory were used to establish paternity. Because there were very little observations of non- resident males on the home coterie territory, it is concluded that not many copulations occurred between non- resident males and females (Foltz & Hoogland, 1981).
Photograph courtesy of Roberta Stacy
In contrast, a similar experiment was conducted by Haynie, Van Den Bussche, Hoogland and Gilbert to determine the percentage of multiple paternity in Utah and Gunnison’s prairie dogs. The percentage of multiple paternity for Gunnison’s prairie dogs was much higher than the 5-10% multiple paternity seen in Black- tailed prairie dogs, but the percentage was still lower than the 89% multiple paternity seen in California ground squirrels (Haynie, Van Den Bussche, Hoogland, & Gilbert, 2002.). The question of why Gunnison’s prairie dog females mate with more than one male seems puzzling when one realizes the amount of costs associated with mating outside the territory (Hoogland, 1996). These costs probably include a greater exposure to parasites and disease such as the bubonic plague, greater exposure to predation, and a decrease in parental care from the primary male (Hoogland, 1996). However, there are also benefits that may stem from copulating outside the home territory. These benefits include an increased supply of food due to courtship feeding, as well as less antagonistic behavior from males due to increased number of males copulated with (Hoogland, 1996). If the male provides parental care for offspring of all females copulated with, a copulation outside the home territory could result in increased parental care. In addition, a larger litter and more fit offspring may result because of the increased genetic diversity within litters due to increased competition between sperm (Hoogland, 1996). Another reason for this multiple paternity could be to ensure insemination (Haynie, Van Den Bussche, Hoogland, & Gilbert, 2002.; Hoogland, 1996).
Extra- pair copulations for female Gunnison’s prairie dogs seem to be a way of coping with the conflict between male and female reproductive strategies (Hoogland, 1996). Female Gunnison’s prairie dogs have two facts working against them. These aspects include the fact that male Gunnison’s prairie dogs copulate with more than one mate, decreasing the amount of sperm available and that these females only come into estrus once a year (Hoogland,1996). The length of this estrus only lasts for one day in Gunnison’s prairie dogs (Hoogland, 1998). It is therefore important for these females to become pregnant during this time period (Hoogland, 1996). There is some evidence, however, that if female Gunnison’s prairie dogs do not become inseminated during this first time period, they may go into estrus again during that season (Hoogland, 1998). Females try to gain greater reproductive success by copulating with more than one mate, while males try to prevent them (Hoogland, 1996). Males may insert copulatory plugs into the female to prevent her from mating right after he mated with her (Hoogland, 1998).
Sexual Behaviors
The breeding season for Black- tailed prairie dogs usually occurs in the later part of January. Gestation lasts for about a month, and the females birth the young in March or April (DesertUSA, 1996).The act of copulation between prairie dogs takes place underground within the burrow (Hoogland, 1998). This is because the risk of predation while copulating is lower in the burrow, as is the risk of another male competing to copulate with the estrus female. Because the act of copulation occurs within the burrow, it is increasingly difficult to tell whether a prairie dog is in estrus (Hoogland, 1998). However, according to Hoogland, there are five distinct behaviors that one can use to identify a prairie dog in estrus. These behaviors include underground consortship, excessive female attention to an estrus female, self- licking of genitals, dust bathing, and late entrance into a burrow at night.
Photograph courtesy of Roberta Stacy
When not in the breeding season, female prairie dogs return to the burrow within five to ten minutes of the other females of the coterie (Hoogland, 1998). However, when in estrus, the female will return to the burrow an hour to an hour and a half after all the other females who are not in estrus (Hoogland ,1998). It is believed that during this time period, females in estrus look for males who they have not yet mated with (Hoogland, 1998).
The acts of self- licking of genitals as well as dust bathing may have a sanitary purpose behind them. Self- licking of genitals may lessen the instance of sexually transmitted diseases and genital infections (Hoogland, 1998). Dust bathing serves the purpose to remove fleas and other parasites after copulation.
Other behaviors that may signal that copulation has already occurred include the change in behavior of both male and female prairie dogs (Hoogland, 1998). After copulation, a female prairie dog will not allow a male to sniff her genitals any more. The male prairie dog is less interested in the female sexually, although he does seem to guard her against any other males that are interested in her now (Hoogland, 1998). This seems to exist for anti- cukoldry purposes (Hoogland, 1998).
Another, less understood mating behavior is that of the mating call (Hoogland, 1998). This call can be given by the male either before or after copulation. According to Hoogland, in most instances this mating call occurred before the sexual encounter right after the male sniffed the females genitals. This mating call may sound similar to the antipredator call, but consists of a set of 2-25 barks with a 3-15 second pause between each one (Hoogland, 1998).