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Members of the dog family are usually socially monogamous, so that males and females either literally live together in a shared den, or at least share overlapping territories. It can be difficult to tease apart to what extent this is facultative or obligatory, since either could be the case. For instance, social monogamy could have originally evolved in the group because individuals tend to live far apart, but once it had, it makes sense for both parents to help in raising their young - it might not be essential, but it's probably going to help, so the animals that do it gain an edge.
This might be the case, for example, with the kit fox (Vulpes macrotis). Kit foxes are small foxes native to the American west, being found from southern Oregon to central Mexico. They prefer arid or semi-arid terrain where food is inevitably rather scarce, so it's unsurprising that they do tend to live far apart. They live in dens, sheltering in them from the daytime heat, and coming out at night to forage for food.
Although they forage individually, rather than as pairs, the lifestyle of kit foxes is socially monogamous, with a single male and female cooperating to raise their young. Estimates of how much the hunting ranges of the members of a mated pair overlap vary from 70% to 79%, which is significant even at the low end.
The way that this works is that the female begins to scout out suitable locations to rear her pups in September and October. Typically, she will pick one of her existing dens, although only after checking out all of them and clearing them of any debris before making her final choice. Then she adds a few extra entrances/escape routes to it in preparation. She only comes into heat once a year, mating in the winter, and giving birth in February or March after a pregnancy lasting between 49 and 55 days.
From then on, she devotes most of her time to suckling and caring for her young, with her partner being the one who heads out at night to forage. He doesn't necessarily live in her den while he does so, but the fact that he keeps bringing her food while she stays at home indicates that there is obviously a strong bond between the two. The pups are weaned at eight weeks, after which the mother is able to forage for herself again, although her partner continues to assist her for another one to two months until the pups are old enough to join in. The family breaks up around October, making way for the next year's litter.
All of which sounds very sociable, and it has been reported that, even if they drift apart in late summer and autumn, mated pairs keep returning to one another every year, maintaining a lifelong pair bond. However, even if that's true of their social behaviour, that doesn't mean that there isn't more going on than is immediately apparent. Humans, after all, are also socially monogamous (at least in most societies), but that doesn't mean that no human has ever had an extramarital affair.
Taken as a whole, kit foxes are reasonably common animals, at least for something that lives in marginal habitats where food is scarce and there is no significant risk to their continued existence. That's less true of the San Joaquin subspecies of central California, which is on the US list of federally endangered species. How many other subspecies there might be is a matter of debate, but however many it is, their populations are thought to be at least declining as housing and industry expand into their habitat - including, in recent years, the growth of solar power farms in Arizona and Nevada.
Those farms are probably better than the alternative of increasing climate change in areas that are already hot and dry, but that doesn't mean that we needn't think about future conservation of desert-dwelling animals such as kit foxes. Understanding issues such as their genetic variation, which is affected by how genuine their monogamy might be, could well help inform such efforts.
To this end, a recent study took a look at the genetics and paternity of the kit foxes of the Colorado Desert in southern California, sometimes placed in the "desert" subspecies (C. m. arsipus) that also inhabits Arizona, southern Nevada, and northern Sonora. The researchers took samples from 102 kit foxes, including adults, juveniles that had left home but were not yet living in a pair, and weaned pups. Using a mixture of radiotagging and cameras placed near birthing den sites, they were then able to compare the genetic ancestry of the pups with the identity of the parents raising them.
Perhaps of most relevance to conservation efforts, the results of the genetic analysis allowed the researchers to evaluate how diverse the community was. The results were encouraging, with the sample, collected from the kit foxes living in a 100-square-mile (260 km²) area of desert, showing as much genetic variety as that collected in similarly broad studies of other fox species, including Arctic, red, grey, and bat-eared. In fact, there was higher variation than seen in most other studies, notably including some on the San Joaquin subspecies of kit fox.
This shows that the population here, at least for the time being, as healthy as we could reasonably expect. Since we know the population is declining, it's good to see that it isn't declining enough to be a problem yet.
Examining the patterns of relatedness confirmed that, as was already known, and is the standard for most mammalian species, young males travel further on leaving home than females do. This makes it less likely that they will end up living alongside their own sisters, which has obvious benefits for genetic diversity in any population.
By comparing the genetics of the young pups with that of the parents raising them, however, the study showed that three of the mothers had cheated on their partners, accounting for 17% of the pups examined. Some previous studies have shown younger males helping out mated pairs, perhaps because one of them is their mother and they just haven't left home yet. This wasn't the case in this one, however, so that potential confounding factor can be ignored - we know exactly which animals were raising which pups.
(I should note at this point that litters aren't all or nothing with dogs. In each of the three litters showing infidelity, at least some of the pups were the biological children of the father raising them, just not all of them. It's perfectly possible for a single litter of fox pups to have two different fathers.)
In one case, the paternity tests showed that a female had been cheating on her partner with his half-brother from a different litter. The biological father had shown no interest in raising his cubs at all and, for all we know, was looking for a longer-term partner outside the study area. While there was no camera evidence he had ever visited the mother's den, since the foxes forage alone at night, he wouldn't have needed to. A chance meeting at the right time of year is all it would have taken.
In the other two cases, the biological father was unrelated to the one raising the young, but had been caught on camera sharing the birthing den with the pups' mother. In one instance, the biological father, who had been living with a different partner a couple of miles (3.3 km) away, was killed by coyotes the following year, and the mother went on to raise a second litter with her actual partner. In the other, there was no such misfortune, but the mother found herself a different partner the following year anyway, abandoning both of her former mates in the process.
While she apparently stayed faithful to that third partner, at least until the study ended, this does show that pair bonding in kit foxes is not necessarily for life.
Even if you're a kit fox, some rules, it seems, are made to be broken.
[Photo by Clint Wirick, in the public domain.]
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