Sunday, 10 September 2023

Skunks of the World: Spotted Skunks

Eastern spotted skunk
At least outside of the Americas, the most familiar species of skunk is the striped sort and, as I mentioned last time, even scientifically speaking, this is the animal that defines the skunk family. It's perhaps surprising then, to note that, despite it also being very widespread and highly visible, it wasn't the first species of skunk to be named.

That honour goes to the spotted skunk, which appeared in the earliest recognised list of scientific animal names in 1758. This isn't to say that nobody knew at the time what a striped skunk was, merely that the naturalists of the day had yet to identify them as something distinct from the spotted sort, and it was the latter that happened to be described first - the striped skunk followed less than twenty years later, in 1776. Before they were given their own genus, both species were originally placed in Viverra, which comes from the Latin word for "ferret" but seems to have been used for any small, slender mammalian carnivore that didn't fit elsewhere (not including, ironically, the ferrets). 

In the two centuries since, it has become clear that the spotted skunks do not represent a single species. Four species are currently recognised, but it's worth noting that a paper last year used broad DNA analysis to identify seven. It seems quite likely that that will stand but, lacking a crystal ball, I am going to stick with the four we have today, with the proviso that this may not last long. 

When Linnaeus named the spotted skunk, he said that it lived in "South Carolina", so it's the species that lives there that gets to keep the original name. This is now known as the eastern spotted skunk (Spilogale putorius) and it's found through much of the eastern half of the US. It has three subspecies, one living between the eastern foothills of the Appalachians and the Mississippi from southern Pennsylvania in the north to the Gulf of Mexico in the south, and another restricted to Florida. The third subspecies - which the new paper recommends splitting off - inhabits the Great Plains on the other side of the Mississippi, crossing the Canadian border just north of Minnesota and the Mexican one near the Texan coast. 

Spotted skunks are much smaller than striped skunks and, being more thoroughly nocturnal and hiding in long grass or vegetation when they can, are probably not as commonly seen as their larger cousins. While a male striped skunk can weigh as much as 4 kg (9 lbs - the size of a small cat), spotted skunks are rarely more than 800g (1 lb 13 oz.) and usually less. They are black with white stripes and a bushy tail that makes it obvious that they are, indeed, skunks. However, they have a total of six stripes in a complex pattern, and these are often broken into shorter dashes, which is what gives the "spotted" appearance. There is some evidence that this spotted pattern makes them harder to discern at a distance, imitating dappled sunlight in the way that a spotted deer might, even if they are very visible close up in order to serve as a deterrent once hiding is no longer an option.

They are not as effective at digging as striped skunks are, and so are more likely to shelter in ready-made dens, typically those constructed by other animals, but hollow logs, natural rock crevices and the like are also used. Where possible, they prefer dens concealed by undergrowth or beneath a thick tree canopy, which may help both with hiding from predators and with staying cool in the summer. This fits with their broader habitat preferences of woodland and vegetated terrain; they can also be found in rocky slopes with plenty of hiding places, but not in open terrain or flooded wetlands. They are generally solitary, and males in particular can become violent if they encounter one another.

They eat small mammals, insects, and the like, and are said to be effective at keeping down the population of mice on farms, making them more beneficial than their smelly reputation may suggest. An unusual feature of their behaviour is that they sometimes crack eggs open by picking them up with their front paws and throwing them back between their hind legs, giving them a kick on the way in the hope that it hits something solid and breaks.

Of course, it is the spray for which spotted skunks are most famous. This is similar to that of striped skunks, with most of the same chemicals, although there are some differences. These differences mean that it lacks the extra component in striped skunk spray that allows the stench to linger for a long time, even though the primary blast is just as potent. More obviously, however, it is delivered differently. Spotted skunks are generally more agile than their larger kin, and are, for example, better at climbing trees. But they really demonstrate this agility when they are warning off a potential threat, doing a handstand and waving their tails in the air to make themselves look larger and more impressive. While they can spray while performing the handstand, it's intended as a deterrent, and if it doesn't work, they'll usually drop to the ground, bend into a U-shape so they can see where they're aiming, and let rip.

Spotted skunks were once common across their range, and, at least in Precolombian times, may have once lived further afield, into the Great Lakes region. However, they were widely trapped for their fur well into the 20th century and suffered a catastrophic population collapse from around 1940 onwards. Across the whole of their range, the population is thought to have declined by 90% between the late '30s and early '40s, and had dropped to one-hundredth of its former level by the '80s. While it has almost certainly disappeared altogether from some areas over that time span, they are good at hiding, and small populations have been uncovered in recent decades in South Carolina and Tennessee, where their survival was in doubt. 

The decline is sufficient that they were listed as a vulnerable species in 2016, although they remain sufficiently widespread to avoid the higher classification of "endangered". For whatever reason, however, they remain locally very common in Florida, with population densities of up to 23 skunks per km² (60 per square mile) having been reported from wilder parts of the state.

Western spotted skunk

The western spotted skunk (Spilogale gracilis) is essentially indistinguishable from its eastern counterpart. They are, on average, slightly smaller and slimmer, with a longer tail, and the skull tends to be a little flatter - but the overlap is sufficiently great that this is of little use when simply looking at the animal For most of the 20th century, therefore, they were considered to be a single species. While the suggestion had been made earlier, it was only in the 1990s that sufficient genetic evidence built up to distinguish them as more than subspecies. 

The 2022 paper I mentioned above recommends splitting them further, with a 'new' species inhabiting Arizona, New Mexico, western Texas and much of northern Mexico and the 'original' one west and north of that, from Baja California in the south to southern Montana and British Colombia in the north. The dividing line between the latter and the eastern species is roughly the western edge of the Great Plains where they but up against the mountains. Even where the species is not split further, these are thought to represent at least subspecies, perhaps having sheltered in Texas and California respectively during the Last Ice Age. Among the other currently recognised subspecies, one is unique to the California Channel Islands, having arrived there around 10,000 BC.

Eastern and western spotted skunks also differ little in their behaviour. While they have few preferences in terms of their overall habitat, in Texas they spend most of their time in cover provided by mesquite trees, while in Wyoming, they favour rocky outcrops - both of which mirror the terrain types that the eastern species uses. They also give the same handstand warning that the eastern species does and, in one instance, an individual was reported to have successfully scared a cougar away from its kill - an animal a hundred times larger than it is.

It's often the case that after two apparently similar species are separated on purely genetic grounds, we take a second look at them and discover differences we hadn't really noticed before. So it is in this case. It turns out that the main breeding season for eastern spotted skunks is in March and April, while the western species does almost all of its breeding in September and October. This doesn't absolutely make breeding between the two impossible, since sometimes a female can come into heat again at a different time of year... but it does make it a lot harder.

This was less obvious than it might have been because both species give birth in the spring, roughly between April and June. They can do this because the fertilised egg does not immediately implant in the wall of the womb and instead enters a period of what's effectively suspended animation before suddenly starting to divide and grow into an embryo. It so happens that the western spotted skunk does this for a much longer time - several months instead of just a couple of weeks - than does the eastern. This had, in fairness, been spotted in 1968 but nobody seems to have paid it much attention at the time.

Southern spotted skunk

Much less is known about the remaining two species. The southern spotted skunk (Spilogale angustifrons) is the most recent to be split off from the others, following the discovery that it had a different number of chromosomes from its more northerly relatives - making fertile crossbreeding between them difficult. This was first noticed in 1996, although it took a decade or so for it to filter through to textbooks and become widely recognised. Once again, the 2022 study proposing new species recommends splitting this into two, with the subspecies native to the Yucatan peninsula being the one that gets promoted.

As the common name suggests, this species lives further south than the other two. It is found throughout much of southern Mexico and through Central America as far as Costa Rica. One can reasonably infer that it prefers warmer weather than its relatives, but that's about all we can say at present. We don't, for instance, yet know when its breeding season is, assuming it has a specific one.

Both the western and the southern spotted skunks are common and widespread, never experiencing the population crash that affected the eastern species. The remaining species is not quite so lucky, although it, too, avoids the formal 'endangered species' designation. With at least slightly more originality than the other three, this one is known as the pygmy spotted skunk (Spilogale pygmaea). It lives solely along the west coast of Mexico, in patchy dry forests, sometimes inhabiting hilly terrain, but mostly in the coastal plain below 350 metres (1,100 feet). 

This is the smallest of all skunks, with a typical adult weighing about 250 g (9 oz.) although it otherwise looks much the same as other spotted skunks. Unlike those, small rodents form only a minor part of its diet, and it primarily feeds on invertebrates, preferring insect larvae and centipedes when it can get them but also eating beetles, grasshoppers, and spiders, along with some fruit. We actually do know a little about its reproduction; breeding is between April and August and the kits are born with fine white hair, rather than hairless as those of its relatives are. It does, however, use the same handstand threat behaviour that they do.

While the species is sometimes hunted, and remaining populations are declining rapidly, at least as much of the threat to its survival comes simply from development. It was never very common, living as it does in relatively isolated patches of forest, but the growth of tourist resorts along the sunny west coast of Mexico (Acapulco is in its range, although less popular as a destination now than it used to be) has meant a corresponding increase in recreational and transport infrastructure that only increase its rarity.

In addition to the striped, spotted, and hooded skunks, there is also a fifth species of skunk that lives in the US. It is to that, and its various Latin American relatives, that I will turn next time...

[Photos by the US National Park Service, in the public domain, and Brian Kentosh and Heidi Donat, from Wikimedia Commons. Cladogram adapted from Caro et al. 2012 and McDonough et al 2022.]

1 comment:

  1. I don't recall if I've mentioned here that I was a seasonal Ranger Naturalist at Channel Islands National Park in 1987. While I learned about the endemic deer mice, island foxes, dwarf mammoths, plants (I even gave talks about the latter two), and land snails while I worked there, I never knew that the two largest island in the park had their own endemic subspecies of spotted skunk. Thanks to you, I now know. It's always a good day when I learn something new.