INFANTICIDE
Describing Infanticide
The killing of juveniles in their first year is known as infanticide, and in prairie dogs infanticide occurs during a baby's first summer, most often before the baby has emerged from its nursery burrow for the first time, but occasionally after emergence as well. This type of killing has been observed to varying degrees in three species: the black-tailed (BTPD), Utah (UPD), and Gunnison's prairie dogs (GPD). Typically, the carcass of a killed juvenile is immediately (or shortly afterward) consumed by the killer and/or opportunistic prairie dogs in the area. This activity - eating of one's own species - is called cannibalism. Cannibalism is common across all taxa in the animal kingdom, the benefits of which, especially the nutritional value of consuming the carcass (waste not), have been well-studied.
Opportunity and Occurrence
The most common occurrences of infanticide will be found in BTPD populations. John’s many years of research on BTPDs revealed that over a third of juveniles will typically fall prey to infanticide. The killer is most often a lactating female who is in the same area and kin to the victimized mother. Unlike BTPDs, the most common infanticidal killers among UPDs and GPDs are marauding males; that is, males invading from adjacent or nearby territories.
The opportunity to commit infanticide can vary depending on the behaviors typical of a certain clan or colony, and of course individual mothers. If a mother is away from her nursery burrow for too long, likely foraging (or in some cases even committing infanticide herself), she leaves her offspring defenseless in their nursery burrow. Among BTPDs, this is when usually a female relative (who is taking care of a litter herself) will enter and kill the undefended offspring. Among UPDs, the offender will typically be a marauding male. Among GPDs, a marauding male or sometimes even a member of the same clan. Questions abound over this behavior.
As for white-tailed prairie dogs (WTPDs) and Mexican prairie dogs (MPDs), there is little data available. In all his years of research, John did not witness a single infanticide among WTPDs (which he studied for a combined 10 years at two different sites). As for MPDs, further research would shed light on the occurrence in this species, but the scarce data so far appears to indicate that infanticide is rare in the species.
UNCOVERING WHY
Among BTPDs, the benefits of infanticide and subsequent cannibalism have been well-studied, and written on by John in his 1995 book on the species (available through Amazon). The killer is most often a female who is genetically related to, and lives in the same territory as, the victim mother. By killing the unprotected litter, the offender gains assistance from the aggrieved mother, who moves on from her loss within a few days. The offender also relieves some risk, as the victim mother loses her incentive for committing infanticide herself. That incentive for the infanticidal killer includes the bonus nutrients and nourishment from consuming the carcass of your victim, which is an obvious advantage when weaning young of your own.
In The Black-tailed Prairie Dog, John lists five potential benefits to the killer specifically from infanticide and subsequent cannibalism (some of which we've already touched on):
1. Removal of future competitors (especially in a philopatric community)
2. Increased sustenance (proteins and nutrients from cannibalizing the carcass)
3. Increased foraging area (as a result of decreased territoriality by the victimized mother)
4. Victimized mothers become better helpers (shifting their focus from offspring back to community)
5. Victimized mothers are less likely to kill (non-lactating mothers do not typically commit infanticide)
Many of the above benefits would also apply to male killers among UPDs and GPDs (and the rare male killer among BTPDs). In addition, if the male killer is an invader from another area or a non-breeding yearling, he has killed potential later competitors for estrous females. Among UPDs and GPDs, the killer is most often an invading male. Rarely, the killer is a male in the same clan, who may possibly even have sired the victim offspring - in these cases, the benefits which might outweigh killing one's own genetic offspring remain unclear.
Documenting Infanticide Events
While carrying a killed juvenile aboveground offers clear evidence of infanticide and cannibalism, many of these events take place entirely underground. To verify if an infanticide is occurring or has occurred, we keep 20-minute checks on males and keep close track of all members (males, females, yearlings) closely during the lactating season. If a marauder descends into a nursery burrow for longer than 5 minutes (sometimes up to half an hour or longer), we are on alert for ABOVEGROUND CUES upon emergence, including a bloody face, licking of the front paws, and rubbing their face or body in the dirt. All the indications combined will lead us to conclude that an infanticide has likely occurred, and when the time comes for the litter to emerge and they never appear, the data from the day of the possible infanticide is verified.
Over the years, John has documented many infanticide events. Aside from species-specific differences, several other factors may play into the prevalence of infanticide year-to-year; these factors require further study, and have not been within the scope of John's behavioral research, including: vegetation abundance, weather and climate, the presence of predators, or even human disturbance.