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Darwin's Imps of Darkness - The Galapagos Marine Iguana | Part 2

187 years ago, Charles Darwin, an unmotivated and failing medical scholar, departed England alongside Captain Robert Fitzroy on a five-year voyage onboard the HMS Beagle. He boarded the boat as a travel companion and budding naturalist, with little ambition for ground-breaking scientific research. Despite this, in society today, perhaps our first association with the word "Galapagos" is the name "Darwin."

During his journey on the HMS Beagle, he recorded the stunningly unique creatures along the South American coast. The ship stopped over in three Galapagos Islands, where he began to notice variation in species from island to island. This visit had a resounding impact on the formation of his Theory of Natural Selection, which drastically and controversially altered the scientific view on the biological origins of life.

Not all the island's inhabitants were depicted as marvels of the natural world. In fact, some species, received a rather poor rap. Darwin once wrote in the stained pages of his journal -

"The black Lava rocks on the beach are frequented by large, disgusting clumsy Lizards. They are as black as the porous rocks over which they crawl & seek their prey from the Sea. Somebody calls them ‘imps of darkness’".

Darwin was aptly referring to the Marine Iguana (Amblyrhynchus cristatus), which is now arguably ranked as one of the most remarkable and distinctive organisms of the archipelago, being the only lizard with morphological and behavioural adaptations to the marine environment (they feed exclusively on intertidal and subtidal products, such as algae). Despite the species having captured the curiosity of the world through documentaries and film, there is still little known about the species behaviour, performance, evolution and ecomorphology.

Marine Iguana located at Playa Loberia (prominent salt crown on head)

This endemic species has suffered from population decline since human settlement, with modern threats including invasive species, anthropogenic disturbance, foreign pathogens and the effects of climate change. Studies have documented that the most severe threats are environmental variables, such as oceanic changes due to climate change and increased El Nino event severity. Marine Iguana area occupancy and fragmented populations have made the population reproductively unstable and at an elevated risk of being endangered. As a result, the ancient species has been listed as Vulnerable by the IUCN. Additionally, there has been a decline in global concern for the well-being of the Galapagos Islands, despite an increase in tourism, a higher frequency of environmental contamination and climate change. It is therefore of utmost importance to investigate the unique ecomorphology of the Marine Iguana to contribute to their conservation efforts.


Last year, Professor Christofer Clemente and I journeyed to San Cristobal, the oldest island in the Galapagos archipelago to conduct a research project on Darwin's 'Imps of Darkness' - the Marine Iguana. We decided to investigate the locomotory mechanics of the Marine Iguana, in hopes of it providing evolutionary insight into their performance trade-offs and morphological modifications to the marine and terrestrial environment. Below is a brief background summary on the relationship between movement and habitat -

Iguana being measured for research - ML Parker Media

Variation in a species physiology and morphology is thought to determine variation in ecologically relevant traits, thus determining the individual’s fitness in relation to habitat. As a result, natural selection acts on intermediate traits, such as locomotory performance, which is recognised as being the foundation of all behaviour undertaken in their ecological niche. Locomotion is used to accomplish a multitude of fitness related tasks, such as home range, defence, finding mates or food, escaping predators and dispersal. The interaction between locomotory performance and habitat is complex, with habitats often being three dimensional. As a result, individuals must negotiate substrates of various diameter, incline, height and texture, which in turn can influence the behavioural task they are engaged in. Individuals may adjust their locomotion to address the effects of habitat and behavioural task, thus reiterating the complexity of this relationship. As locomotory performance of an individual influences the overall fitness, this intermediate trait can impact geneflow over multiple generations. Consequently, the congruence between morphology and habitat use is generally considered one of the most illustrative outcomes of adaptive evolution. Therefore, examination of locomotory performance gives an understanding of both the difficulties an individual may encounter in their habitat and the morphology that aids in overcoming these difficulties (this can help with species management and conservation!).

Marine Iguana clings to a rock underwater

As previously mentioned, species must overcome numerous challenges associated with habitat structure, consequently resulting in evolutionary trade-offs and variation in design. Although the first reptiles were apparently adapted to live on land, multiple lineages of reptiles have reinvaded freshwater and marine environments, such as the Marine Iguana. The adaptive success of reptiles in terrestrial environments has led to fewer than 8% of the 6000 species being primarily aquatic. Despite only few species being primarily aquatic, reptilian design has many physiological traits that would allow for a simple transition towards aquatic behaviour and locomotion. We hypothesised that these variations in physiology and design may result in evolutionary trade-offs with locomotory performance in terrestrial environments. Therefore, through studying Marine Iguana locomotion, we can understand the evolutionary transition from a terrestrial to marine environment.


Now, how does one actually study Marine Iguana locomotion? It involves scrambling over lava rocks to catch agile iguanas, running them through racetracks and playing with high-speed cameras - what's there not to love about that!

As I explained in my previous post, whilst in the Galapagos we were introduced to a team of Veterinarians from the University of North Carolina. The team has been conducting population health assessments on Marine Iguanas for several years now, having even collated data during the most recent El Nino (where the population suffered losses of up to 90%). Leading up to our expedition with this team, we scoured the small town of Puerto Baquerizo Moreno for parts which we could use to construct an Iguana-proof racetrack. We soon began to realise that finding parts in the Galapagos was an island-wide scavenger hunt (let's just say I'd hate to try to find all the ingredients to bake a cake with). As a result, we ended up finding wooden planks from a construction site and disassembling my tripod to make our racetrack.

The team sets out to Isla Lobos to find some Iguanas

With our trusty racetrack in our satchels, we set out in the early hours of the morning to Isla Lobos. Isla Lobos is a small flat island, located 10km northeast of Puerto Baquerizo Moreno and is only accessible by boat. It is most definitely one of San Cristobal's hidden gems, with a sheltered channel of crystalline water to the east and a sandy beach with rolling waves to the west. In the chilling waters, Sea Lions frolic in the surf and Blue-footed Boobies dart between schools of fish. Green Sea Turtles glide above the eroding lava rock and Marine Iguanas dive for submerged algae. On the land, windswept trees line the shore and the shrills of Frigate birds fills the air. A small sandy beach runs along the western side of the island, which is where we set up the field laboratory. In the far distance, we could see Kicker Rock protruding from the ocean (which makes for a spectacular dive site!).

Sea Lion Basking on the rocks (Kicker rocks in the distance)
The channel that isolates Isla Lobos from the mainland is teeming with life

Marine Iguanas were collected by hand, with no nets or lassos to help us. Luckily, there was an abundance of iguanas on the island, so missing a few here and there wasn't a great loss. With majority of individuals basking on the lava rocks (having returned from foraging in the sea), it proved a treacherous task to retrieve them. The Marine Iguanas certainly had the home turf advantage, with the rocks being difficult for us to manoeuvre across. In addition, all it took was for the iguana to dart into the rumble or the sea for it to be lost. To speed up the process, we had five guides and vets out searching for specimens. It wasn't long after that we were starting to get a queue of iguanas at the field lab, all of which were to be weighed, sampled and then released down our racetrack.

After being handled for several minutes, the individuals were eager to escape back into the wilderness. As a result, we had no problem getting them to run in front of our cameras (well, aside from the few iguanas that went sideways into the cameras instead of towards the sea!). Below is an image from the high speed camera showing a specimen locomoting down the racetrack. We had two cameras angled to

obtain a dorsal and a lateral view of the iguanas as they ran. They were perfectly synced to allow footage to be digitised using Argus and MatLab.

This allowed for a thorough analysis of the spatial, kinematic and temporal variables. Digitalised points were examined within the stride, which began when the ankle came into contact with the substrate. The stride ended at the next footfall of the same limb. The measured variables and corresponding terminology were similar to those in previous studies. Each stride was described in two phases, being: stance phase and swing phase. The stance phase is defined as the proportion of the stride that the hindfoot is in contact with the substrate, whilst the swing phase is the proportion of the stride in which the foot is not in contact with the substrate. Stride frequency was taken as the distance moved across the x-y plane by the lumbar vertebra, divided by stride duration. Stride length was calculated as the distance travelled by the lumbar marked along the x-y plane between successive footfalls. Speed was also calculated using the distance between footfalls of a cycle and the duration a cycle.

Additionally, several angular kinematic variables were calculated in relation to an individual’s stride. Three angles were calculated to describe the movement of the femur in relation to the hip, being: femur rotation, femur retraction and femur abduction. Angles of the knee joint, the ankle joint and the metatarsal joint were calculated to describe the movement of the hindlimb segments relative to each other. The absolute angle value for each of these variables and the total change in each angle was calculated. Midstance was selected as the ground reaction forces are generally highest during this phase of the stride. The total change in each angle represents the compliance of the limb during the stance phase. Therefore, there were a total of 12 angular kinematic variables assessed to examine Marine Iguana movement. With the assistance of R Studio and powerful statistical analyses, we were able to gain the first insight into the performance of Marine Iguanas and how they modulate their speed.

Our analyses revealed that the species locomotory performance varied from that of other taxa (such as varanids, agamids and crocodillians), which is likely to be a result of adaptation to the marine environment. We identified that the Marine Iguana modulates speed predominately with stride frequency, as opposed to stride length. Additionally, it was found that alteration in stride length requires changes in the angular kinematics of the hindlimb. In comparison, stride frequency was found to be associated only with distal joints. When compared to other taxa, it was identified that the kinematic associations with the species speed modulation strategy were often a result of adaptation to habitat.

However, it would be recommended that further research is conducted to improve accuracy of findings and determine the species’ performance in relation to habitat. Additionally, the effect of mass and other morphological variations on Marine Iguanas requires further investigation, as it is hypothesised that the high genetic diversity and incipient speciation will have caused intraspecific variation in performance. Further, comparative studies can be conducted to document the evolutionary transition from terrestrial to marine and identify the trade-offs on morphology, physiology and performance. It is our hopes to return to the Galapagos to address these gaps in literature and ensure that effective conservation management strategies are employed to preserve this species.

Charles Darwin may have doubted their importance, describing them as “hideous-looking” and “most disgusting, clumsy lizards.” However, the ancient and endemic Marine Iguana provides a remarkable testimony to the process of evolution and is beautifully adapted to best exploit its environment. For this reason, we must continue to investigate and preserve Darwin's 'Imps of Darkness'.


That concludes the stories of our research that we conducted on some of the most iconic species of the Galapagos. Thank you for reading and in turn spreading awareness.

Let me know in the comments below whether you would love to see some more posts about travelling to the islands and what to see and do when you are there (yes, even researchers still get the chance to be tourists!).



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