The claw or the formation of making one with your hands is an amazing tool to help get a grip of an object or prey, or even to break something to get the materials inside. Humans even use this technique by using their index finger and thumb at a very young age to show affection toward another human, but that tends to hurt them. They are also used in adulthood for a similar affection and to enact sexual activity with their partner. In nature though, animals use claws in more of survivability, resource-gathering, and efficient manner. Some of these manners include communication, attracting mates, excavating burrows, gathering food, climbing, and scratching.
The most common usage of the claw is for gripping which is when an animal uses their claw to hold down their prey . Another type would be by attracting males by showing off their claw and color pattern on them, such as the fiddler crab that will be discussed later. The claws can be used to dig a burrow in the ground to protect them from hostile environment, such as the ghost crab. The claw can be used for climbing up steep walls, trees, mountains, etc as like the lizard species and how they evolved. Claws can be used to scratch and mark territory for other animals to see such as the black bear. Lastly I will talk about the earliest known species, the Teilhardina brandti that had both claws and nails and how they might have been the one that started the evolution of separating them into two different categories.
One type of species that I will talk about is the crab. Crab are a class of Malacostraca, order of Decapoda, and suborder of Pleocyemata. They are part of the Arthoropoda phylum which means they are invertebrate (they have an exoskeleton, paired jointed appendage, and a segmented body. Since they are also crustaceans, they form with other insects, myriapods, and arachnids, to make Euarthropoda. The crab’s exoskeleton is mostly made with chitins and they can be found anywhere in the ocean, and on land. I will talk more on how powerful the grip of a coconut crab can be, how fiddler crabs use their claws to attract mates, and how ghost crabs use their claws to dig deep burrow underground.
Another type of species is the lizard which uses their tiny but efficient claw to grip onto any types of surface. I will be talking about them in a broad sense so that you will know the advantage of tiny claws and large toes might give and vice versa. American Black bear, Ursus americanus, have many uses of their claws but I will show a study on how they mark their territory with them. Lastly we will talk about the Teilhardina brandti and that they might have evolved from claws to nails and started a new path in the Phylogenetic tree. This paper will show published work featuring the evolution of claws across studies of different animals and the many different usages of the claws. This also might help answer the question of how the claw on animal and nail on human came to be and why they have split in the evolution timeline.Now that I have introduce you to the many species that we will talk about, let’s start with the three species of crabs claw I have here.
Crabs are known for their huge claws (pinchers) since that is their main features. In this study Oka, Shin-Ichiro, et al. (2016), explore the crushing grip of the Coconut crab, Birgus latro, one of the largest terrestrial crustacean that can lift up to around 30 kg. Oka, Shin-Ichiro, et al. (2016), wanted to determine the pinching force of the coconut crab and how these animals acquired their mighty claws, by the usage of metric and ecological data (Oka, Shin-Ichiro, et al. 2016). Since there was no previous study on the actual pinching force Oka, Shin-Ichiro, et al. (2016) wanted to see for themselves. Oka, Shin-Ichiro, et al. (2016) also mention that the Coconut crab shared a common ancestor with the terrestrial hermit crab, Coenobita spp., but the coconut crab has lost dependency on the shells for protection during the adult stage and instead develops a calcified body.
29 wild coconut crab, Birgus latro, were collected from the northern part of Okinawa island to perform the pinching force measurement. Oka, Shin-Ichiro, et al. (2016) measured their thoracic length and body weight, which ranged from 16.2 to 64.5 mm and 33.0 to 2120 g. Each crab, now down to 23 coconut crab, was measured using their left claw to get the claw length, height, and width. The crab pinching force was measured using the device called SKYSCIENCE, SK-MBF-01F) Oka, Shin-Ichiro, et al. (2016). This device can measure the force exerted by the crab by having the crab pinch the stainless steel stick-shaped sensor. These sensors have load cells embedded in them which can accurately measure the pinch force to the nearest 0.1 kg. Once the measurement was obtained, it was then converted into newton and the crab was then released back to their capture point. Oka, Shin-Ichiro, et al. (2016) got a maximum pinch force range from 29.4 to 1765.2 N.
These values that they got were significantly greater than the pinching force of the proportional to muscle cross-sectional area they have predicted. Instead, Oka, Shin-Ichiro, et al. (2016), showed that coconut crab pinch force was greater than the force of any animal above 0.14 kg body weight. Oka and his research then track the coconut crab back to their similar ancestor, the hermit crab and see what evolved the coconut to have such large claws and no shell like the hermit crab. This is most likely due to the switch from shell protection to claw protection that most likely happens during the Pliocene era. Since the coconut crab has lost its shell due to the thorax and pleon hardening, this allowed them to grow exponentially bigger and calcification of the body to protect from predators. As they get bigger, they lose more predators and are able to freely consume a variety of food due to their large claw, which in terms turns them to predators themselves and have the ability to eat smaller crab and crack open the smaller crab shell which allows them to maintain their large body size.
When humans found them, they saw them cracking open coconut and gave them the name of coconut crab. Overall, these results suggest that coconut crab has evolved into using their claws for the sole purpose of crushing and protection. Fiddler crab is known for its massive claw when compared to its body ratio. These massive claws would seem like they would be used for crushing tiny nuts since fiddler crabs are quite small but were later disproven as fiddler crabs use their smaller claw to dig and find food in the sand. In this peer review, Mowles, Sophie L., et al. (2017) shows us how the male fiddler crab, Uca sp., uses their massive claws for mating display. Not only do the fiddler crab use their claw to attract mate, Mowles, Sophie L., et al. (2017) also shows us that in some species of the fiddler crab, they also produce an additional sound/signal by rapidly rubbing its claw together and creating a “drumming” noise toward the potential female mate.
In Mowles, Sophie L., et al. (2017) study they are trying to answer whether the additional “drumming” is just a reinforce information communication toward the female that the male fiddler crab want to mate (the redundant signal hypothesis) or does the “drumming” show different aspect of the male’s quality/fitness to the female (the multiple message hypothesis)? Female fiddler crabs prefer males with larger claws and that is waved at higher rates, female also goes of the mating system of comparative evaluation of males. They tested to see the different in number of wave the male fiddler crab would do when they had a female fiddler crab tied to a stick and was held at 10, 5, 2.5, and 0 cm from the male burrow. The results that Mowles, Sophie L., et al. (2017) got was that the further the female fiddler crab were, the more the male fiddler crab used their waving claw, as the female fiddler crab got closer, the male tended to switch from waving to drumming bouts significantly. This showed that there was a significant positive relationship between the total number of waving and drumming. Overall in this case studies, Mowles, Sophie L., et al. (2017) showed us that the fiddler crab used their large claws to attract potential mating partners and how they show off to the female.
Ghost crab, Ocypode cordimanus, are an unique type of crab in which they live in very hostile environments that have extreme weather changes which include going from extreme heat to extreme cold in a couple of days. Ghost crab anatomy for their claws allows them to scoop up the sand with one or both claws and hold it in a cradle shaped form like if a human was holding a baby. In this study, Watson, Gregory S., et al. (2018) have done their research in Warana beach, which is located in the Sunshine Coast region of Australia. They wanted to test the effectiveness of the ghost crab burrow and how it helped the crabs to survive. They randomly selected Ghost Crab burrows that were around 15m of the vegetation zone, which is where the highest density of ghost crab were. Watson, Gregory S., et al. (2018), then observed the crab burrow temperature around morning before sunrise (6-8am) and around noon to afternoon (12-4pm). They also wanted to test the crab’s thermo regulation too, and so they captured at random 35 crabs and took measure of them. After a day of putting the crab in a controlled environment, Watson, Gregory S., et al. (2018) picked 18 at random and were experimented on.
The result that Watson, Gregory S., et al. (2018) got were that the burrow during the morning when it was cold, were at a higher temperature and vice versa during late afternoon. When testing the crabs they have captured, they have found that the temperature of the crabs were very similar to the burrow rather than the temperature outside. Watson, Gregory S., et al. (2018) also found that the crab temperature that they experimented on changed rapidly when exposed to controlled temperature which show that ghost crabs can heat up or cool down extremely fast. Overall this study shows the efficiency of how well the Ghost crab can build their burrows.
Lizards are known for their ability to climb up walls and even defy gravity and stick to the ceiling! There are many types of lizards out there with many types of designed feet and toes. In this study, Zani. (2000) tested the relationship between toe and claw morphology and clinging performance to see if there is a correlation with one another. Zani. (2000) did this by examining the evolution trends across 85 lizards from 13 families. After many tests and measurement of the body size and what family the lizard came from, Zani. (2000) was able to make a comparison of both the claw and toe of a lizard do have correlation with the clinging performance. Zani. (2000) was able to come to a conclusion that lizards that have large toe width, claw curvature, and increase in adhesive lamellae number are able to perform better on clinging onto smooth surfaces. Also that lizards with longer claws and shorter toe are able to perform better on rough surfaces. Overall, this study shows the relationship of how lizards use their claws to climb surfaces and how lizard’s claws evolve over time to their surrounding environment.
American black bear, Ursus americanus, are the smallest but widely distributed bear species, they are omnivores which can vary depending on the seasons and location. In this study, Taylor, A. Preston, et al. (2015) will be examining the bears marking behaviors during their breeding season. They set up remote video cameras to capture the activity of the bears and see how they mark their territory and how they do it. They wanted to know the gender, age, and during what time of the day they did it. Taylor, A. Preston, et al. (2015) had to first find the right location and tree of where the bears mark their scent on most frequently during mating season. They decided to do their study in the northwestern California in Redwood National Park which has some of the highest density of black bears.
Taylor, A. Preston, et al. (2015) method is to set up six cameras to record marking events between April 25 to August 9 of 2013, these dates are chosen because that is when breeding season starts and finishes. The result that they were able to gather from the months of examination, were the many behaviors of how black bears mark their territory. The bears were examined to bite, back/belly rubbing, smelling the ground/tree, pede marking (twisting both back and front leg on the ground to leave a track, and scratching. Taylor, A. Preston, et al. (2015) were then assigned the bears into categories of gender and age. They found that out of the 31 bears they captured marking the tree, only 16 were able to be identified, in which they were all male.
They were also able to tell that 29 out of 31 bears were adult bears. Most of the marking were during the days and the most behavior marking was by bipedal marking and the least common were scratching and biting. Overall, this study showed that even though the american black bears did rarely use their claws to mark an area, it still shows how they use their claws to sometimes mark a tree other than just for climbing up trees. Teilhardina brandti is not a known species in today’s world but they were part of Omomyidae which is a family of early primates that lived during the Eocene era that happened between 55 to 34 million years ago.
