In some bear species, males kill unrelated cubs to create mating opportunities with victimized mothers. To protect their cubs, mother bears pick places where humans can act as their metaphoric shields, according to findings published in Proceedings of the Royal Society B this week.
Finding the right habitat in a risky landscape is critical. In many predator-and-prey systems, the prey species will use what’s known as “protective associates” as a shield against their main predator. That protection can often take the form of an apex predator. Top predators often instill fear in so-called mesopredators since they can be killed as competitors and also exploited as prey. In many ecosystems, humans take on the ecological role of the local top predator. Humans and our footprints have acted as shields for moose against brown bears in Yellowstone and mountain nyala against spotted hyenas in Bale Mountains National Park in Ethiopia.
A mechanism like that should also evolve in species where conflict between the sexes affects survival of the offspring. In a population of Scandinavian brown bears (Ursus arctos), predation doesn’t impact offspring survival as much as sexually-selected infanticide – in which unrelated adult males kill dependent cubs. Because this is so costly to females, several counterstrategies have evolved against it. They’ve been known to alter their habitat choice to avoid infanticidal males, and they might end up in areas with lower food quality. Whether this avoidance strategy pays off remains unknown.
A team led by Sam Steyaert from the Norwegian University of Life Sciences wanted to see if the use of protective associates – in this case, human shields – affects offspring survival. After all, brown bears have lost its apex status to human hunters in most of their geographic range. The team put GPS-collars on 26 mother bears in an intensively managed boreal forest in south-central Sweden during mating season (May to July from 2005 to 2012). The study area includes a dense network of roads and a scattering of villages and isolated houses. Bears are hunted annually from August to October, but family groups are protected.
Adult males and solitary females strongly avoid human footprints throughout the year. That includes roads, human habitations, and clearcut forests. During mating season, successful mothers with litters that survived were more likely to use human shields. Unsuccessful mothers, who suffer complete litter loss, avoided humans. Successful mothers ventured hundreds of meters closer to human habitation.
After the mating season – when there’s no risk for infanticide – females with cubs avoid humans again and shift to areas with protein-rich, low-fiber food. The findings suggest that predator-prey principles can be extended into the context of sexual conflict.