The term coloniality refers to group living inasmuch as conspecifics (members of the same species) are significantly and measurably interacting and living close together. While physical distance between members, group density, population size, and other factors come into play, behavior and social context is also key to defining a colony. Some species exhibit coloniality during particular life stages, such as breeding and nesting, while others, like the prairie dog, are permanently and actively colonial. In the animal kingdom, coloniality is seen across many taxa to varying degrees of population density, seasonality, and conspecific cooperation.
All species of prairie dog live in colonies (sometimes called towns). Typically, colonies are subdivided by landscape features into subcolonies (or wards), where adjacent populations are aware of each other but do not tend to interact. The five species do differ in levels of population, and in ward size and density, in addition to many behavioral differences among them. Black-tailed prairie dogs (BTPDs) for example, can be found in colonies in the thousands, with many wards and coteries (family groups dominated by one or two adult males) within wards. In other species, the equivalent of coteries are called clans. The clan (or coterie) is a contiguous family group, and the home territory is where the majority of a prairie dog's time is spent. Gunnison’s prairie dogs (GPDs) live in smaller wards as well as smaller colonies overall; however, dependent on other environmental factors (climate, predators, etc.), BTPDs can be found at dramatically lower densities, or GPDs could be found at higher densities. The same can be said for all species. Many factors come into play for colony populations, including landscape features, predator populations, anthropogenic (human) presence, and history.
Though coloniality is more complex than a simple cost/benefit analysis, drafting a list of advantages and disadvantages offers a good idea of the whys and what-fors of colonial living across different taxa (mammalian, avian, reptilian, etc.):
Advantages (not all of which are necessarily found in all colonial animals) include:
- Shared vigilance of surroundings and increased predator awareness
- Lower chance of being targeted by an incoming predator, as there are more of you to choose from
- A chance that another member will allocate energy toward caring for your offspring in some way
- Closer access to sexually receptive individuals during the mating season
- The easier spread of disease in close groups
- The risk of inbreeding and the result of genetic drift (in that genetic variation is compromised)
- Higher potential mortality in numbers from single events such as natural disasters, weather events, or anthropogenic events
- Competition for resources (i.e. food, space, mates) and resulting aggressive interactions
- The possibility of allocating energy to the care of offspring that are not blood related
While there are plenty of disadvantages that may seem to work against the evolution of coloniality, the dominating advantage of predator protection has been attributed as the strongest incentive for colonial living. This is described in part by the selfish herd effect.
The Selfish Herd
Biologists, ecologists, and even the casual naturalist may be familiar with the term “selfish herd,” a dynamic studied across all taxa. The selfish herd phenomenon is defined by the Oxford Dictionary of Zoology as:
A theory proposed in 1971 by W.D. Hamilton (1936-2000) according to which the risk to an individual of predation is reduced if that individual places another individual between itself and the predator. When many individuals behave in this way an aggregation is the inevitable result and, because the risk is least near the center and greatest at the edge, individuals of high social status will tend to occupy the center and subordinate individuals will be pushed to the edge.
In simpler terms, as the benefit of eluding predation outweighs the costs of sharing space and resources, conspecifics may aggregate into groups. We call this safety in numbers, or as John puts it, getting "lost in a crowd of conspecifics." As described in the zoological definition, the level of protection for the individual increases closer to the center of the group. We will sometimes see yearling (and more rarely adult) prairie dogs living on the edge of a colony ward seemingly alone, interacting little with other prairie dogs except for the occasional meeting. It is often difficult to devise why these individuals have been pushed to the edge, but they certainly do not exhibit dominant behavior. Dominance belongs to the one or two dominant breeding males in a family group. These stronger males will often push weaker (sometimes younger, sometimes older) males to the edges of a subcolony, where they are more vulnerable to predation.
The benefits of the selfish herd are more evident within wards (subcolonies), as across the greater colony distance negates the effects of audio and visual cues that benefit the group, such as alert postures and alarm calls.
PRAIRIE DOG COLONY CHARACTERISTICS
Prairie dogs are extremely social animals at every life stage. For the majority, these creatures are peaceful and cooperative, but hostilities do occur and competition is a necessary part of colony life. While some behaviors - such as grooming each other (allogrooming) - are specific to a species (the black-tailed prairie dog), most behaviors can be seen across all species. These include friendly and hostile kisses, territorial disputes and territorial calls, alarm calls, contact fighting, play (especially among juveniles and yearlings), chases, and foraging together or apart. Certain species are considered more social than others: the Gunnison's prairie dog, for example, is among the least social species and exhibits a lot of solitary behavior; while the black-tailed prairie dog is considered the most social species and exhibits a high degree of togetherness throughout the day. Even within a single species, you will see denser groups in more patchy vegetation landscapes, and looser groups where vegetation is more spread out. Other factors include space, human presence, and other landscape features like bodies of water.
Philopatry & Dispersal
Female prairie dogs exhibit philopatry, defined as fidelity to one area, and do not commonly disperse from their natal (birth) territories. When or before they become sexually mature, male prairie dogs disperse away from their birth ward, thus avoiding any chance of inbreeding with their relatives. Female prairie dogs in a ward, however, will generally remain in their birth area for their entire lives, living with their mothers, sisters, aunts, cousins, and nieces. This localization strategy is called matrilocal (matri- from the Latin for mother), and most female prairie dogs are matrilocal for their entire lives.
Like all things wildlife, there are exceptions to matrilocality in prairie dogs. In a 2013 paper in Science Magazine, John describes female natal dispersal, uncommon in all species - but it was found that in the absence of nearby female relatives, young females are more likely to disperse from their natal wards. In other words, the likelihood that a female would disperse from her natal ward increases as the number of female kin in her territory decreases.
Allogrooming is the act of grooming each other, and has been observed in the black-tailed and Utah prairie dog species. By grooming each other, blood-related prairie dogs reinforce social bonds while simultaneously controlling exoparasite (flea) infestations. Among the other species of prairie dog, grooming is a solitary behavior (called autogrooming) and so has no social component but does serve the same function for flea control as well as keeping the fur clean of dust and dirt.
Life in the grasslands is rich with drama for colonial prairie dogs, and especially during the mating season there is much ado between territorial males. We code territorial disputes as TDs in our datasheets, and TDs can last from a few seconds to several minutes. When the dispute escalates into a contact fight, the two prairie dogs engage in a flurry of blows that can sometimes leave the defeated with visible injuries. More importantly, losing a contact fight can have serious implications for the loser's standing in the clan, and consequences for passing along his genetics during the mating season. As mentioned above, each clan has one or two dominant males who hold the territory and burrow sites; losing one's territory can be a worst-case scenario for a dominant male.
Most territorial disputes, however, do not intensify to the level of contact fighting. Often a few-seconds-long hostile kiss followed by a chase or a runaway will satisfy the aggressor for at least the time being, until the two males cross paths again from minutes to hours afterward. If the researcher is close enough, she will often hear the disputing males chattering their teeth in anger. Sometimes a loud territorial call will be emitted, especially if the aggressor has managed to herd the challenging dog into a burrow, sometimes followed by interment behavior (that is, burying the chastised prairie dog with dirt kicked into the burrow). Interments are most often seen when an adult male has chased and herded a yearling male of the ward into his burrow as a way of exerting dominance. Though yearling males do not typically mate, and will disperse in their second year, they are often the sufferers of harrassment from adult males (and rarely adult females) in their clan.
It is not uncommon to see aggression among females as well. In the early spring before they have mated, most females behave amicably toward each other and other members of their ward. After becoming pregnant, however, and thereafter until their offspring emerge from their burrows, the females become aggressive and take to defending their nursery burrows combatively. While territorial disputes between females may be seen at any time, it is especially during the months of pregnancy and subsequent lactation that we will see disputes involving females, not uncommonly escalating into contact fights.
A series of burst photos below captures a fight involving approximately three seconds of contact. A territorial dispute between males R21 and 49 begins as a hostile kiss. After breaking the kiss, 49 pounces on R21, they roll in a ball as they bite and kick to injure each other, then break apart and out into a chase as the smaller R21 tries to get away. Though these fights are quick, they can sometimes result in minor or even devastating injuries. Rarely, an especially brutal series of fights could kill a losing male. In most cases, however, the injured will eventually relent and run off to lick his wounds.
As much as friendly interactions, hostile interactions are equally important for enforcing social parameters within the colony.
MATING AND REPRODUCTION
Prairie dogs are polygynous (in which a single male mates with multiple females) but they are also often polyandrous (in which a single female mates with multiple males). During the mating season, the dominant male of a territory will attempt to mate with as many females as possible. But research has found that there are genetic benefits to the population when females mate with more than one male as well. Though it may not prove true for every single mother, statistically, mothers who mate with more males tend to birth larger litters. The often exhausting effort to mate with as many males as possible, however, does not come without its costs in fitness. You can read more about polyandry under THE MATING SYSTEM.
The prairie dog gestation period is about 26-30 days long depending on the species. Offspring are fed mother’s milk underground in their nursery burrow for 5-6 weeks, emerging aboveground before they’ve been fully weaned but are ready to forage on grass and foliage. Species-specific nuances can be seen at this stage as well: for example, in black-tailed and white-tailed prairie dogs, you will sometimes see juveniles nursing aboveground; in Gunnison’s prairie dogs, on the other hand, it is rare to see juveniles nursing aboveground. It is sometimes difficult to discern the reasons for these differences, and questions like these are what continue to drive continued research. You can read more about juveniles under CHILDCARE.
Infanticide & CANNIBALISM
Over the years, John has determined that among black-tailed prairie dogs, over a third of juveniles fall victim to infanticide. While infanticide can be committed by invading unrelated males as well as local males (more rare), most killers of offspring are lactating females in the same clan/coterie and who are closely related to the mother of the victims. These lactating female killers have their own litters to feed and protect nearby. Because infanticide is almost always followed by the cannibalizing of the carcass, it is clear that the sustenance and nutrients offered by the body are beneficial to the killer. The ultimate drivers behind high incidences of infanticide are not completely understood, and may sometimes defy our understanding of cooperative living situations, especially in species like the black-tailed prairie dogs where so many juveniles are lost to this behavior every year. You can read more about John’s discoveries on infanticide, including differences among species, under INFANTICIDE.