On the nature of pain

Last week, a manatee was found in Florida waters, with the word ‘Trump’ scraped on its back. Although this kind of ruthless mutilation is horrific in itself, I started wondering if the animal felt any kind of pain.

I must admit, up until I came across the news about what happened to the manatee in Florida, I knew very little about manatees, in general. And my first thought was whether, during the scraping proccess, this manatee suffered at all. To my dismay, there were not many scientific papers dealing with the somatosensory system in manatees. However, I did find something that eased my soul a little bit: one of the articles reporting on the dreadful event states that the scratch was done in the algae growing on the animal’s back. Still, in the same article it is said that manatees have sensory hairs and nerves in their skin, which means that, if the cuts had touched the skin, they could have caused pain; not to mention the infection that the skin was at risk of, due to the open wounds.

The video below shows the above-mentioned manatee swimming, with the human-made scars on its back.

After reading all these news articles, I was left with some questions that kept occupying my mind: What are manatees?; To what extent can they feel pain?; And can we talk about ‘pain’ at all in manatees, or just nociception? Lastly, how did pain evolve throughout the animal kingdom?

What are mantees?

Also known as ‘sea cows’, manatees (Trichechus manatus latirostris) are herbivorous acquatic mammals of the Order Sirenia. As the name of their order suggests, manatees are believed to be the animals behind the myths of mermaids. For those interested in how manatees inspired mermaid legends, please check out the video below.

Manatees are the largest vegetarian animal to inhabit the sea, and they communicate with each other through high-pitched sounds. They are also very gentle and lack defense mechanisms, given that they do not have any natural enemies. However, they have become and endangered species, due to human activity, which is the manatee’s greatest threat. According to the Florida Fish and Wildlife Conservation Commission, the year 2020 was a hard one for manetees as well: 637 of them died, 90 of which were victims of boat collisions, and another 15 were killed by other interactions with humans.

Although manatees do not possess a highly acute visual system, they compensate for that by the presence of tactile hairs, or vibrissae, spread all over their body, especially on the face. This distribution of vibrissae is something unique among mammals, and to manatees it is highly useful in allowing them to navigate in the water.

Since mantees rely tremendously on tactile inputs, it comes as no surprise that their brains are organised to support somatosensation. The primary somatosensory cortex of manatees occupies 25% of their neocortex. Moreover, the sixth layer of their cortex contains clusters of neurons, known as Rindenkerne, which are believed to process information related to the manatee’s facial and bodily vibrissae. Although the Rindenkerne cells of manatees are somewhat similar to other cortical representations of vibrissae, termed ‘barrels’, in rodents, shrews, opposums and hedgehogs, Rindenkerne are unique to sirenia. These neuronal aggregates become active when manatees engage in tactile exploration and object recognition.

At the subcortical level, manatees possess three types of somatosenroy nuclei in their brainstems, namely the Birchoff’s nucleus, which receives information from flukes, the cuneate-gracile nucleus, which processes inputs from flippers and body trunk, and the trigeminal nucleus, which receives sensory inputs from facial vibrissae. Figure 1. below shows the somatosentory representations of the manatee’s body parts, in a coronal section of the brainstem. The thalamus also has specialised somatosensory nuclei, which differ in size, depending on their functional relevance to somatic sensation.

Figure 1. Left diagram based on image by Isuru Pryiaranga. Right image from Sarko et al. (2007), showing functional divisions withing the brainstem, corresponding to the manetee’s body parts. 

Given that somatosensation is so developed in manatees, one burning question is whether they feel pain.

What is pain?

Pain is different from nociception. However, pain from injury cannot occur without nociception. The latter reffers to the process of detecting injury by the activation of a special class of receptors found in the skin, as well as deep tissues and organs, known as nociceptors. The detection of potentially or actually damaging stimuli is followed by a reflex withdrawal reaction, or nociceptive behaviour, mediated by nerves in the spinal cord. The nerve fibres that detect noxious stimuli are Aδ fibres and C fibres, which have their cell bodies in the dorsal root ganglion (DRG) of the spinal cord, as shown in Figure 2.

Figure 2. Illustration taken from a student presentation at Heidelberg University, Germany.

Aδ fibres are mechano-nociceptors, meaning that they are activated by high mechanical pressures. C fibres are polymodal, which means that they respond to a variety of noxious stimulations, such as noxious chemicals (e.g., acids), extreme temperatures and high mechanical pressures. They not only encode the stimulus modality (type), but also their intensity and duration, which are relayed to reflex centres in the central nervous sytem, mediating withdrawal reactions.

The nociceptive information travels from the DRG to different parts of the brain via spinothalamic tracts (from the spinal cord to the thalamus) and sensory fibres of the trigeminal tract (from the face to the thalamus). And it is within the brain that pain happens.

Pain is a complex feeling. Many brain areas are involved in not just generating pain, but also in ameliorating it. Structures from the limbic system, such as the amygdala, receive and integrate nociceptive and affect-related information. The amygdala can lead to increased nocifensive and affective pain behavior, while, under certain circumstances, it can also contribute to endogenous pain inhibition. Pain is also processed in the hypothalamus, the basal ganglia, the insula and the somatosensory cortices. Because these areas play a role in metabolism, as well as fear, pleasure and homeostasis, the nociceptive information is integrated and modulated according to the current state of the individual. In some situations, pain becomes pathological, as it is the case in neuropathic pain, where either previously innocuous stimuli become painful (aka, allodynia), or previously painful stimuli become even more painful (aka, hyperalgesia).

There are two brainstem structures, which are highly involved in controlling pain and generating analgesia. One of them is the periaqueductal grey (PAG) and the other is the rostral ventromedial medulla. These regions exert control over pain to prioritise competing stimuli, and to maintain homeostasis and survival. You might have noticed that, in highly stressful situations you do not feel pain. This is known as stress-induced analgesia, a phenomenon whereby the brain responds to stress by the production of endogenous opioids that act as natural analgesics in the nervous system. The opioid receptors found in the brain are the same ones which analgesic drugs, such as synthetic opioids and morphine, act on to relieve pain.

The evolution of pain

Many animal taxa have nociceptors. A schematic of the evolutionary development of nociceptors and the types of noxious stimuli they respond to is presented in Figure 3. In order to process nociceptive inputs, animals need a central nervous system (spinal cord and brain). It might come as a surprise that such a system, though at different levels of complexity, is found in all kinds of animals, including insects (like Drosophila melanogaster, the fruit fly), C. elegans (a type of worm highly studied in neurosciences), fish, amphibians, reptiles, birds and, of course, mammals.

Figure 3. The different types of nociceptors across animal taxa, from an evolutionary perspective.
Taken from Sneddon (2017)

Life-history shapes pain perception. A very interesting example is the African naked mole rat, which lives in underground burrows that are poorly ventilated, hence contain high carbon dioxide levels. As a result, the C fibres of the naked mole rat are unresponsive to acid, which means that, while other mammals find acidic environments nociceptive, the African naked mole rat does not.

When it comes to acquatic animals, such as manatees, they are expected to have differences in their sensory system compared to terrestrial ones, due to distinct ecological and evolutionary pressures. In water, any chemicals become dilluted, shifts in temperatures are less common, and there is no mechanical damage due to falling. Thus, acquatic animals are possibly at a lower risk of damage than terrestrial animals, which has implications on their nociceptive system.

As far as manatees go, it is still unclear to what extent they feel pain. The fact that they are an endangered species makes is difficult to study them. But given that they posses a very well-developed somatosensory system, which is even more advanced than in other mammals, it is expected that manatees are familiar with pain. Moreover, we still do not know enough about their stress, fear, memory and pleasure systems, which all play a role in pain processing.

It would be great if we managed to achieve a better understanding of these amazing marine animals. But, until then, let us enjoy their existance peacefully, without interfering violently with their lifestyles and without exposing them to any potential pains.

For a more in-depth view on pain, as well as more information about manatees, I highly encourage you to read the papers and articles listed in References.

Special thanks to Isuru Priyaranga for creating the cover image. He is a fellow blogger and YouTuber, and I highly recommend visiting his blog and YouTube Channel.


Track the turtles

Those of you who love Neuroscience and Neuroscience only, I must warn you: This is not a Neuroscience-related article! This article is about turtles. And as much as I would like to get an insight into these creatures’ minds, knowing how invasive the current techniques are and that there is not yet a vast literature on this subject, the today’s article will not consider the brain.

Why turtles then? Are they interesting enough for us to mind them? The answer is YES! I am an animal lover, and I believe that all beings of any sort are fascinating and that they deserve to live a happy life. That is not how I became so amazed by the turtles, though.

This summer I volunteered in Kefalonia (one of the Ionian Sea’s Greek islands) for two weeks to work in the only sea turtle conservation there. I spent a wonderful time and I came back with lots of memories and things to share. But the most important thing I gained through my experience there was what I have learnt about sea turtles. The organisation that runs the conservation volunteering projects is called Wildlife Sense.  You can have a look and if you are turtle lovers and enjoy adventures, you should definitely sign up for volunteering work, which I can assure you, is very rewarding.

As volunteers in the egg-laying season, our task was to take many types of measurements, observe and analyse turtle tracks, the sand, the temperature on the beach, the amount of light and estimate light pollution, find turtle nests, relocate (if necessary) these eggs, tag and microchip the adult turtles etc. It might not sound so exciting, but I felt like both a researcher and an animal protector and I acquired a lot of skills needed for future experiences of this sort and much more.

Know your sea turtle

It is important to know the difference between a turtle and a tortoise. The latter are the ones who walk on the ground and can retract all four limbs and head. Turtles have fins (or flippers) and these are not completely retractable. There are 7 species of sea turtles: leatherbacks (the biggest), loggerheads, greens, kemp’s Ridley, olive Ridley, hawkbill and flat-back. On Kefalonia one can only find loggerheads (Caretta caretta) and some say, greens (but we’ve never seen those).

A few anatomy things

Loggerheads are big turtles, with 80 to 110 cm in carapace length and contrary to what some of you might think, they are very strong and can be very aggressive: they can bite and scratch with their claws (one of the volunteers in my team got her trousers ripped off by a turtle). Marine turtles, just like tortoises, terrapins and other reptiles, are covered in scutes and scales. The scales are on the carapace, while the scutes cover the skin and the ones on the head profile are like human fingerprints – they are unique to each turtle. There are some differences between male and female turtles, including size (females tend to be a bit bigger), but to be sure about the gender, look at their tails: males have a tail coming out from underneath the carapace. Nevertheless, some males hide their tails from time to time, giving researchers a hard time about gender identification.

Laying eggs

When a turtle needs to lay her eggs, some very interesting things happen: she usually returns to the same beach she was born on, even though they travel long distances and get in the ocean throughout their lifetime. For that, she uses something called geomagnetism which involves Jacobson’s organ for olfaction and the geomagnetic orientation trigeminal system. When she comes out of the sea, a turtle pays a lot of attention to the environment: she wants to make sure there are no predators and that the sand is good enough, so she is very focused on anything that is moving, as well as noises and lights.

Loggerheads alternate their tracks when they move on sand and the tracks are a good indicator for the directions to and from the sea. When they start digging the nest, marine turtles use their front flippers to remove the sand around them, while their bodies form a distinctive (and very relevant for researchers) shape in the sand – a body pit (or an extended body pit). They lay around 100-120 eggs, but due to external factors only few of them (1 in 1000) can reach maturity and lay eggs. Temperature determines the embryo’s sex – eggs kept warmer become females – but it can also affect the embryo’s survival. Therefore, the depth of the whole is very important (16-34 cm from the top five eggs), because it significantly influences the temperature of the egg chamber.

It is said that turtles cry after they lay their eggs. There is a grain of truth here, the turtle does indeed drop a few tears, but this is not because she is sad to leave her babies out in the nowhere (even though, that does not mean she has no feelings of this sort, we do not yet know). It all comes down to maintaining the salt balance and those tears actually help the turtles excrete the excess of salt in their eyes.

After she lays her eggs, the turtle returns to the sea and she might never see her babies again. The hatchlings come out of the eggs after one month and a half-two months and orient themselves towards the sea using light (the sea is the brightest thing on the beach if there’s moon light to reflect in it). They show a tropotactic behaviour (they compare intensities in both eyes and move accordingly). Light pollution from artificial lights on the beach is fatal for many hatchlings (amongst other factors like predators), because the poor babies often get confused and don’t know where to go. Sea turtles do not perceive red light, so volunteers were advised to use red lights when looking for turtles at night, in order not to disturb the egg-laying process. The wavelength of light perceived by marine turtles usually ranges between 360 and 600 nm; green turtles see yellow light and do not mind it, while loggerheads are xanthophobic (averse to yellow-orange light). Once in the sea, baby turtles can encounter many other dangers, but if they survive, they swim to other seas and oceans and they can live up to 60 years.

A sea turtle story’ link – A MUST WATCH

As for genetics…

It was discovered that male turtles do not have an SRY gene on their Y chromosome, although the presence of another gene, the SOX9 gene, influences the formation of testies.  Steroidogenic genes are also thought to be involved in sex determination, along with the DAX1 nuclear receptor protein (encoded by an ‘antitestis’ gene) and the anti-müllerian hormone (for testis differentiation).

Marine turtles, and turtles in general, are still a mystery for biologists, but what has been discovered so far about them did nothing but prove how marvellous these animals are. If what you have read in this article aroused your curiosity, I can only hope you will allow the turtles to amaze you in the future as well.

Below, I have inserted a link to a very interesting paper that raises awareness about relocating eggs and explains it from a different point of view.

For further information:

Lutz et al., 2003. The biology of Sea Turtle, Vol. 2. CRC Press


Wildlife Sense link