Beginning in late June, the prevailing winds in the American Southwest change directions. Warm, moist tropical air from the Gulf of California suddenly finds itself being pulled into the dry Arizona landscape. With this influx of warm, moist air, the North American Monsoon begins. Storms pop up all over Arizona and the rest of the Southwest, but this rain doesn't come in the form of widespread and gentle storm fronts as it often does in Ohio. Instead, isolated, but intense, thunderstorms with torrential rainfall form around the region, dotting the landscape. On one typical July day during the summer field season, the dorms at the Appleton-Whittell Research Ranch in Southeast Arizona were surrounded by these monsoon thunderstorms.
Okay, it’s time to delve into some science. What is a
species? Although it seems like a simple question, it turns out it’s quite
difficult to define what a “species” is, and it’s even more difficult to have
the majority of biologists agree with a single definition. Most people have
been taught that a species is a group of animals which can freely reproduce
with each other. This is not really correct however, as many different species
can hybridize with each other. The concept of a species is more complex than
that. So then, what is a species? Well, there’s a lot of different concepts. In
fact, there’s more than two dozen species
concepts, and a list of the 26 most common ones throughout recent history
can be seen here. Nowadays, most biologists define a species using the Unified Species Concept. This concept
defines a species as a separately evolving metapopulation lineage. This
definition isn’t as confusing as it may sound, and if you want to read all
about it, please see Kevin de Queiroz’s 2007 paper at this link. I won't get into the details of the Unified Species Concept, but just know that our changing definition of a species has resulted in some problems when it comes to classifying animals.
Historically, animals were classified into species according
to differences in morphological (or physical) characteristics. Aphonopelma tarantulas were one such
group of animals. They proved quite a challenge though. As it turns out, many Aphonopelma tarantulas look very, very
similar. Classification based on morphological characteristics became
difficult, and the genus descended into a taxonomic chaos. Then everything
changed when it became possible to look at the genetics of an organism. Being
able to see how closely related or diverged one organism’s DNA is to another
revolutionized the field of systematics (the field of classifying animals).
With this new approach, scientists began classifying species based on their
DNA. This approach uncovered an interesting secret. Scientists were finding
that some animals which looked exactly
the same (and therefore were thought to be the same species) actually had very different DNA. Thus the concept of a “cryptic
species” arose. A cryptic species is one which looks exactly like another
species, but in reality is a completely different one. A non-tarantula example
of this would be the Northern and Southern Ravine Salamanders, which were
previously thought to be a single species as they look exactly the same, but
DNA analysis in 1999 revealed two separate species.
Of course, scientists decided to re-evaluate the Aphonopelma genus using a genetic
approach. Maybe this would reveal
some truth in the current taxonomic mess that was Aphonopelma. I want to stress that doing something such as revising
an entire wide-ranging genus is a tremendous
undertaking. First, genetic work is expensive. Second, you would need DNA samples from hundreds and hundreds of individuals in order to get a representative view of the evolutionary relationships, as tarantulas are so wide-ranging (they span throughout the south from the Mississippi River west to the Pacific Ocean). Getting enough samples is not only expensive due to the traveling necessary, but is also incredibly time intensive.
And this is exactly the undertaking that Chris Hamilton, Brent Hendrixson, and Jason Bond took on. This team of scientists from Auburn University and Millsaps College just published (Feb. 2016) a massive revision of the Aphonopelma genus. In this study, they combined the relatively new genetic methods of classification with the classical morphological and ecological methods in order to “delimit” the species within the genus. This integrative approach is much more effective and thorough than simply using any one technique alone. Before this study, there were 55 Aphonopelma tarantula species described in the US. This study found that there were actually only 29 true species. Only 15 of the 55 originally described species were supported by their findings. In addition, they described 14 new cryptic species. The rest of the originally described species were found to be either unsupported or a case of a single species getting named twice or more by separate scientists.
However, the authors pointed out that this is only the
start. Although they sampled 1000+ tarantula individuals from a wide range of
localities, there are probably many more species out there that they simply
didn’t come across. Between incredibly remote and rough terrain, and the
difficulty of finding individuals in the wild, it is probably nearly impossible
to completely sample every species in the US. There are surely more species
out there, but many of those are likely to be highly localized or in hard to access
localities. If you want to learn more, I highly recommend reading the first several pages of this giant
study (which comes in at a staggering 340 pages). The paper can be
found at this link.
I think it's safe to say that a lot of people probably fear tarantulas. They're big, they're fuzzy, and, gasp, they're spiders. Now, I love spiders, but I can understand the fear. Though, as with many animals, much of that fear is rooted in misunderstanding. Tarantulas in the United States aren't dangerous. None of the American species have dangerous venom. A bite from one is equivalent to a bee sting. It's not going to feel pleasant, but it's nothing to freak out about either. American tarantulas rarely bite though. A more common line of defense in the Aphonopelma tarantulas is their use of urticating hairs. These tarantulas have species bristles on their abdomen that, when the tarantula feels threatened, can be "thrown" off with their legs. The tarantula will throw these bristles toward the threat, where they will embed themselves in the skin. These bristles then cause irritation to a varying degree, depending on the species. This irritation is essentially just an uncomfortable burning sensation. It's similar to the kind of reaction you would get from touching Stinging Nettle. Even so, American tarantulas are pretty docile. It really takes some harassing for them to defend themselves, and that level of harassment should never even happen. If you leave a tarantula alone, they pose literally no threat to you at all.
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This post ended up being a lot longer than what I had planned, but my intent on identifying this tarantula to species took me down the systematic rabbit hole. This post is a bit more science-heavy than usual, but hopefully I explained the concepts well! If you have any questions, or want a more in-depth explanation of something discussed, feel free to drop me a comment below! Thanks for reading! I would also like to thank Dr. Chris Hamilton for identifying this individual to species.
Kyle. You've convinced me. I may put "holding" a tarantula on my bucket list.
ReplyDeleteIf you see a wandering male in the summer, it's super easy to get them in your hand! You just put your hand in front of them and they walk right on.
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