Immigration policy isn’t just contentious among human societies: Different species of mongooses also have radically different ways of dealing with newcomers. For instance, some mongoose societies gladly take in immigrants based on merit—while others will deport even their own relatives to protect their offspring.
Even for relatively welcoming groups, however, immigrants may face a long road before they find acceptance. "Things might take a bit of time. There’s a bit of an integration or adjustment period needed for both parties—residents and immigrants,” says Julie Kern, a post-doctoral biology research associate at the University of Bristol and the lead author on a study published today in the journal Current Biology.
Dwarf mongooses are Africa’s tiniest carnivores, measuring in at under a foot long. They are typically found throughout East Africa, from Somalia and Ethiopia all the way down to South Africa, Zambia, Angola and Mozambique.
Inquisitive and glossy-furred, these highly social animals live in cooperative groups of up to 32 individuals, where they share danger-watching duties. Some will play sentinel, standing up on their hand legs and keeping their eye on the horizon, much like their meerkat cousins; meanwhile, others forage for insects, scorpions, snakes and other gourmet dishes in the underbrush.
During sentinel duty, those on the lookout must continually ease the minds of their hungry counterparts with small chirps that let them know they are keeping watch. "These are just small 'brrrp' things,” says Kern, imitating the low-key trill sound the animals make. This helps their companions focus in on the task at hand: "You can't forage and be vigilant at the same time.”
Dwarf mongoose societies are fairly egalitarian, but there are still a dominant male and female who do most of the breeding and a larger share of the sentinel work. Other adults wait in line until they're large and established enough to become head honcho. "Within both sexes you have this linear dominant hierarchy coming down in terms of who's queuing for the next breeding position,” Kern says.
But not all dwarf mongooses are willing to play the waiting game. Some will set off to a new group in order to become a bigger fish in a smaller pond. "If you are a particularly heavy individual, so you're quite competitive, you could sometimes go and join a group and skip straight to the top," says Kern.
However, mongooses that immigrate to new groups often have a rough time settling in. First off, life on the run is difficult for mongooses since they don’t enjoy the benefit of having another play lookout while they forage around for food. Kern and her coauthors found that when new immigrants joined a group, they were often undernourished and weren’t much use to the group.
“When individuals first join a new group, they don't contribute much—they don't do much sentinel duty,” Kern says.
And when they do feel up to contributing, their adopted family isn't always trusting. The new group was more than willing to pay heed to warning sounds when predators like raptors, small cats like servals or caracals, jackals, and snakes were spotted. "If you get it wrong and don't respond when you should have done, you risk being captured by a predator or eaten or seriously injured,” Kern says.
But the locals didn’t always trust the everyday work ethic of newcomers.
Kern and the others conducted a series of experiments in 2014 and 2015 on eight populations of dwarf mongooses in the Sorabi Rock Lodge in South Africa. These mongooses had become habituated to humans, thanks to the teams spending time sitting closer and closer to the groups until they were used to their presence (a process that took “a lot of patience” according to Kern).
By recording sentinel sounds made by different mongooses and then playing back the recorded sounds of a new immigrant as well as an established group member, they found that the others in the group didn’t pay much attention to the newcomers’ efforts to contribute. When immigrants chirped, foragers spent less time on finding a meal and stopped more to stand up and look around for potential predators.
After five months, the researchers conducted the same experiment and noted a big difference in trust. Foragers now trusted the calls of the newcomers—which had become louder, lower frequency and more dominant—about the same as they did other long-term residents. "Often the individuals we follow as immigrants have become dominant in five or six months,” says Kern.
Emma Vitikainen, a biology researcher at the University of Helsinki, calls Kern's study "really cool." "It's a fantastic use of the long-term data set they have on dwarf mongooses," she says. Vitikainen has coauthored a number of studies on banded mongooses, another related mongoose species found widely in sub-Saharan African. Banded mongooses, like meerkats, are a social species but are relatively close-minded when it comes to newcomers.
In a study published November in Animal Behaviour, Vitikainen found that banded mongoose females in Uganda will sometimes deport even their close relatives from a group in an effort to give their own offspring an upper hand. "They never voluntarily leave their group, but sometimes when the group size gets too large, older females will kick out the younger females," Vitikainen says. This generally occurs when resources get too sparse, she says.
Under normal circumstances, banded mongoose females are famous for their cooperation: females all give birth on the same day, and can't even discern between their children and those of other females in the group. According to other work by Vitikainen, the females in a given group will nurse each other's pups and raise them equally. In fact, the pups will try to conceal their identities as the female mongooses sometimes practice infanticide. Showing they aren't too closely related to a particular female may increase their chances at survival.
Unlike dwarf mongooses, banded mongooses don't have much chance to immigrate to other groups after they are kicked out of a given group. "A lone mongoose would be a dead mongoose in the banded's case quite often," Vitikainen says. However, if a female is kicked out, younger males in the group looking for a breeding opportunity might follow them out the door, thereby creating a fledgling new group.
Between infanticide and forced evictions among banded mongooses, Kern says that on the whole, dwarf mongooses enjoy "a lot more of a peaceful society" than their cousins.