Humans like to think they’re special, but at the end of the day, they’re just another primate. For proof, look no further than our DNA: we’re up to four percent Neanderthal, 99 percent chimpanzee, and 50 percent banana (the most monkey of all fruit, and you know it.)
But despite knowing all this, you still might not be too happy to be informed if you have “gorilla ribs” – which is a shame, because there’s a non-zero chance that you really do.
What are gorilla ribs?
Besides the obvious – the ribs which are located inside a gorilla – you may not have heard of the anatomical variant sometimes known as “gorilla ribs”. That’s not surprising: they’re rare, potentially only occurring in around one in every hundred people, and even that small group generally won’t know they’ve got them unless something goes wrong.
They “are usually detected as incidental findings during routine radiographic imaging of the lumbar region,” notes one 2017 case report of a man whose gorilla ribs were only discovered at autopsy. “The majority of cases […] have been asymptomatic.”
So, what precisely are we dealing with, here? Well, gorilla ribs are more properly known as “supernumerary lumbar ribs” – a name which is both completely and accurately descriptive, and at the same time entirely unhelpful to anybody outside of the medical field. Translated into plain English, though, it’s a lot more straightforward: they’re an extra pair of ribs, found underneath the normal ones.
“Lumbar ribs most commonly arise from first lumbar vertebra, although it has been reported to arise from the second and third vertebrae as well,” explains a 2013 case report of a man whose extra rib was discovered after a routine examination for back pain.
“Lumbar ribs are a common finding in standard developmental toxicology bioassays that are performed globally,” the report adds. They “are associated with longer ribs, increased numbers of vertebrosternal ribs, and the presence of extra presacral vertebrae.”
Okay, that’s a lot of words – it might be time for a little anatomy lesson.
Why are they sometimes called gorilla ribs?
Outside of anomalies, the human spine has 33 vertebrae – 24 in adulthood after ones at the bottom fuse. The first seven from the top are the cervical vertebrae – the vertebrae of the neck, which are found immediately below the skull. In fact (fun side trivia here) all mammals have the same number – yes, even giraffes – with only three exceptions: the manatee, which has six; the two-toed sloth, which has five to seven; and the three-toed sloth, which has eight or nine.
Below the cervical vertebrae you should find 12 thoracic vertebrae – easy to spot, because they’re the ones that have ribs attached. However, not all ribs are created equal: the top seven pairs, are known as fixed or vertebrosternal ribs, are called “true”, meaning they are connected directly to the sternum, or breastbone; the next three pairs, conversely, are “false”, because while they do meet the sternum, it’s via the cartilage of the ribs above, rather than a direct connection.
Finally, there are the two “floating” ribs, so called because they – well, they kind of float there. They’re attached to your spine, obviously, but not to anything else; they just sit there, protecting your kidneys and diaphragm and occasionally sliding out of place and making you think you’re dying.
So much for the ribs and thoracic vertebrae; carry on downwards, and we next come to the lumbar vertebrae. You should have five of these, making up the spinal column of your lower back. Below that is the sacrum, which is the middle bit of your pelvis (the outer wing-like bits being the ilia), and underneath that is the coccyx – the sad vestigial reminder of our once-mighty tails. These latter two parts technically make up the last nine of your vertebrae, bringing the grand total to 33, but they’re all fused together inside your butt, so we may as well ignore them.
Now, supernumerary lumbar (or “gorilla”) ribs are an extra 13th pair that sprout from – you guessed it – the lumbar vertebrae. That might mean you just have fewer non-ribbed vertebrae in your spinal column, or it may – as the 2013 case report noted – involve having one more vertebra than most people. Either way, it can be quite confusing for the poor radiologist who first notices them, since vertebrae are usually either counted down from the last rib or up from the sacrum – with an extra rib and/or vertebra in the mix, those two methods no longer agree.
But here’s the thing: you may have noticed that we specifically said the human spine. Despite our species’ closeness on the evolutionary tree, there are quite a few differences between humans and our primate cousins – and one that you probably haven’t thought about too much is sitting just inside our trunk.
“The majority of mammals possess 19 or 20 thoracic and lumbar vertebrae, for a total of 26 or 27 [cervical, thoracic, and lumbar] vertebrae,” explained researchers from New York University, following a 2019 study into mammalian spinal variations.
“There is little variation in these numbers, either within species or across different species – or even different species separated by over 160 million years of evolution,” they pointed out. “Humans, with 24 [vertebrae], are one of the exceptions.”
But you know who’s not an exception? Almost all the other great apes. Chimps, bonobos, and, yes, gorillas all have 13 pairs of ribs, with only four lumbar vertebrae underneath (unless you’re an eastern gorilla, in which case you have just three). The overall effect is one of a human ribcage – albeit a conical and flared-out kind of one – with an extra couple of ribs underneath the normal set.
So, when a human turns out to have an extra pair of ribs below the others? Well, we could have called them “chimpanzee ribs” or “bonobo ribs”, or even “gibbon ribs” if we wanted – but for whatever reason, “gorilla ribs” was the name that stuck.
When gorilla ribs go wild
Supernumerary lumbar ribs may be rare, but they’re far from the only way in which the human body can give you a little extra bone in your breastplate. For a few people – estimates vary wildly depending on what populations you’re measuring, why you’re measuring them, and who is doing the measuring – their extra rib isn’t at the bottom, but the top of their ribcage.
Known as cervical ribs for obvious reasons – viz, they sprout from the cervical vertebrae – these are basically, well, ribs in your neck. Like gorilla ribs, they’re rare, and usually go unnoticed even when they’re there, since they tend not to cause many problems – although they can occasionally compress the nerves in your shoulder, leading to a condition called thoracic outlet syndrome.
But where things get really hinky is in the few – very, very few – people for whom one set of extra ribs just doesn’t cut it.
“Lumbar ribs most commonly arise from [the] first lumbar vertebra,” says the 2013 case report, but “it has been reported to arise from the second and third vertebrae as well […] Rudimentary ribs have also been reported at the sacral site and coccyx.”
And if all else fails, you can always just shove in some extra vertebrae to cope with your gorilla-esque anatomy – like one 35-year-old who, in 1969, was found to “[have] 15 pairs of thoracic ribs arising from 15 thoracic vertebrae.”
“In our search of the literature we have not found another case of a patient with this many pairs of thoracic ribs,” the authors of that particular case study noted.
You know what? Forget gorillas – that guy was clearly a snake.
All “explainer” articles are confirmed by fact checkers to be correct at time of publishing. Text, images, and links may be edited, removed, or added to at a later date to keep information current.
The content of this article is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of qualified health providers with questions you may have regarding medical conditions.
