Introduction
Neovison vison (the American mink) and Chinchilla lanigera (Chinchilla) are both small mammals from the wild. Currently, there are two existing species of mink and chinchilla. The North American sea mink was hunted to extinction[1], and Chinchilla chinchilla is currently facing extinction[2].
The American mink is native to North America and because of past and present human involvement, its habitat has largely expanded to Europe and South America. Especially in American history, minks were hunted for their desirable fur. Only later was domestication practiced and minks used for household purposes as well.
The chinchilla is native to a much smaller habitat in South America in what are now Chile, Argentina, Peru and Bolivia. Like the mink, chinchillas were hunted for their fur, which caused their populations to dwindle drastically. Unlike its carnivorous counterpart, however, hunting of chinchillas continued toward the end of the 20th century, threatening its preservation status[2].
In order to compare the two species, which seemed relatively similar at first glance, skeleton preparations were carried out under the auspices of the University Of Michigan Museum Of Zoology. Skeleton preparations have a lengthy history and are intimately tied to the beginnings of the modern museum. In the modern era, they are used by zoologists, for example, in identifying fossil bones and tracking evolution.
From the skeletons and dentition, evolutionarily adaptive functions were extrapolated and used to explain current niches in their respective habitats. With additional literature, the current biological and evolutionary historical context of the mink and chinchilla were investigated and then compared. Of primary concern was understanding evolutionary strategy in relation to functional anatomy.
Materials & Methods
A preserved mink and chinchilla were graciously provided the University Of Michigan Museum Of Zoology. The chinchilla received was domesticated, and it is believed that the breed originated from Chinchilla lanigera[3]. Both animals were skinned and stripped of their fur coats before removing all internal organs with the exception of the brain and eyes. Fur and organs were then discarded. Carcasses were then set in a dermestarium, where carrion-eating dermestid beetles (Dermestes maculatus) stripped the bones of remaining soft tissue[4]. Depending on the condition of the preserved animal, cleaning the bones may have lasted from a few days to a week for satisfactory results. After this period, the bones were thoroughly examined and assembled using glue and wire.
Noted difficulties with skinning the mink included removal of tail fur and the anal glands. The potent chemicals stored in the anal glands, if nicked, could cause severe olfactory discomfort not only for the preparer but also for those in his/her immediate and distant vicinities.
The given chinchilla had been preserved in the freezer for many years and was thus heavily freezer-burned. Particular hardships were noted with skinning the tail and limbs and removing the fur entirely from the skull. Miniscule amounts of fur therefore remained on the paws, tip of the tail, and nose. Sawdust had to be used in order to soak up fluids leaking from the deteriorated viscera while the fur continuously shed itself. After skinning, the carcass had to be soaked in water for almost an entire week before it could be considered degradable in the dermestarium[4].
Discussion
Biological Context of Neovison vison
The American mink is found throughout the United States and is also present in most areas of Canada with the exception of the Arctic coast and some islands. The British Isles are also a range for the American mink, where they pose a potentially serious problem for native wildlife because they are non-native predators. The most optimal habitat for the mink is an area with access to brushy or rocky cover in close proximity to streams, ponds, or lakes[5].
The coveted fur of a mink is primarily a darker brown, and often includes white patches on the chin, chest, and throat areas. The thick fur coat is waterproofed by oily guard hairs, and their toes are partially webbed; both of these showing the mink’s ability to live a partially aquatic lifestyle (natatorial). Their long and slender bodies usually range from 46 to 70 centimeters, approximately half of this coming from tail length. Usually, the females are noticeably smaller than males, ranging between 0.7 to 1.1 kilograms, as compared to the males which range between 0.9 and 1.6 kilograms as full-grown adults[5].
During the winter months, the estrous cycle readies the female mink’s fertility and mating begins; the mink becomes sexually mature at around ten months. The gestation period for the American mink is usually between 40 to 75 days, resulting in live birth around the months of April or May. The litter size typically ranges between one to eight newborns, each weighing in at around 8 to 10 grams. The mink is born with a thin coat of white fur covering its underdeveloped body. The young are able to open their eyes at just over three weeks, and are finally weaned at week six. It is not until the fall till the mink leaves its mother to go about establishing territory of its own. The typical lifespan of mink in the wild is ten years[5].
The mink usually lives alone (solitary), as the males do not get along, and the boundaries of their homes are marked using secretions of their anal glands. The American mink is both nocturnal and natatorial; they are proficient swimmers and can dive to depths of five meters to hunt for prey. The mink lives in either self-made burrows in riverbanks, lakes, and streams or may also utilize the dens of other mammals. Interestingly, their burrows are often lined with dried grass and leaves on the inside, along with a proud display of the fur from their prey. Mink communicate heavily through chemical signaling for territorial issues and mating, but may also rely on visual and auditory signaling; they have a keen sense of vision, gustation, and hearing[5].
As a member of the order Carnivora, Neovison vison has a fleshy diet varying by season. During the warmer months, the diet includes crayfish and smaller frogs, as well as small mammals like the shrew, rabbit, mouse, and muskrat. Alternatively, the mink also preys on fish, duck, and other water fowl, and primarily small mammals in the winter months. They play an important role in helping to maintain a balance in the ecosystem because they are such active predators. On the other hand, the American mink does not have many predators to avoid. Humans continue to be deadliest for the mink, but they are hunted by coyotes, bobcats, and other carnivores occasionally (including snakes and birds of prey). Like most species in the mustelid family, the mink is aggressive and fearless, not hesitating to defend itself against larger animals. It is no question that these animals are secretive in nature, and their cryptic nature (well-camouflaged) helps confirm their overall avoidance of most predators[5].
Humans continuously prove to be most dangerous towards the American mink, and because of this humans have devised tactics to keep the population densities constant. Trapping seasons are limited and quotas on catch size are in effect. As with all animals, human destruction of habitat always remains a concern, and contaminants such as mercury can accumulate in the mink’s tissues, causing further complications[5].
The nature of the mink’s fur allows the animal to thrive in such temperate weather throughout North America. The insulation provided by the fur of the mink should allow it to withstand the bitter temperatures of winter in the north. In addition to the thickness of the coat, the oils used to waterproof the animal make the animal more adapt for harsh winters and survival in general. Because water is a much better conductor than air, one would expect the mink to lose far too much body heat in the bitter months of winter if its coat was not waterproofed. In addition to helping N. vison maintain body temperature, the waterproofed coat would reinforce the animal’s agile avoidance of predation because it would prevent the additional water-weight from becoming a burden to carry.
The mink has also adapted to its predatory nature though its interaction with others. Because other minks pose a threat from a competitive standpoint, the solitary behavior of the mink is well-purposed. In addition to competing for prey, finding a mate also gives reason for such independence. The time for a mink to leave its mother also seems quick, and this is yet another support for the species’ need for competition-based independence.
As an altricial species, bearing many underdeveloped young per birth, the newborns are expected to survive because minks are not prey to many. Not being able to open their eyes till over three weeks into life, it is clear that the way the mink has adapted has to do with its nature as a predator.
Biological Context of Chinchilla lanigera
The desire for chinchilla fur has reduced the geographic range of the chinchilla to the mountains of northern Chile. The climate is best described as dry, arid and sandy[3]. In the high elevation, they live in dens such as crevices or holes. Said fur is dense and soft, with single follicles producing up to 60-80 hairs alone[6]. Color will vary from animal to animal. The head is typically broad and the external ears are large. Other characteristic traits include large, black eyes and vestigial cheek pouches (functional ones are found in other rodents such as the golden hamster). All feet have four phalanges and weak claws[6].
It is a sexually dimorphic species with males weighing around 500 g and the more massive females at around 800 g. Typical head-to-tail length is around 22.5-38 cm with the tail taking up the majority of the length, 7.5-15 cm[6]. Females are socially dominant and during estrous appear very aggressive towards both sexes. Despite this, serious scuffles are uncommon. It’s also a mostly monogamous species, breeding between May and November. The gestation period lasts for 111 days, somewhat longer than most rodents. Two litters with two to three young a litter are expected each year. The young are well-developed (precocial) at birth and reach sexual maturity after 8 months, allowing them to breed within the year. Life expectancy in the wild is about 10 years, with domestic life expectancy reaching over 20[6].
C. lanigera is quite social and exists usually in herds of over 100 individuals. This allows for mutual grooming and cleaning. They are mainly nocturnal but have been observed being highly active in the daylight[6]. Its diet consists mainly of grass and seeds and vegetation as filler. On occasion, they feed on insects and bird eggs but only when imminent threat is low. Besides humans, predators include foxes, birds of prey, felines, and snakes[7].
Human involvement beginning since the Inca Empire has drastically reduced the population of the chinchilla and its range. Despite attempts to stimulate reproduction, the population remains low and is on the decline. The species is considered critically endangered and there are no strong theories as to why the population continues to decline[2].
The extremely dry climate has shaped the appearance and size of the chinchilla. Living in the semi-desert of the Andes, chinchillas are small and extremely furred. Their size allows them to dissipate heat efficiently while their unique fur allows them to retain it during colder times. Their size and coating reduce water demand and loss, respectively. Divergence from other rodent lines has reduced the size and use of their cheek pouches, possibly indicating a climate-influenced shift in diet back in its evolutionary past.
With many natural predators, the chinchilla has developed many methods of evasion. Living in a herd allows for additional protection. If a threat is encountered, defense mechanisms in order of severity are as follows: standing up on hind legs to appear larger and to confuse, vocalizing while showing off incisors, grunting and using its paws to shove away the threat, airborne urination coupled with releasing odors from the anal glands, biting, and running away. If the predator bites the hide, the chinchilla will simply shed its hair and run or jump away[8]. Not only that, but the physical maturity of the chinchilla at birth is comparable to that of the adult, giving them less vulnerability early on.
Biological Comparisons: N. vison & C. lanigera
The chinchilla belongs to the mammalian order of Rodentia, a mostly herbivorous group known for their gnawing habits. Out of all mammals, rodents are most diverse and comprise the largest portion, at least 40%[9]. Characteristically, they are relatively small in size. The upper and lower pairs of incisors grow all throughout their lives, preventing long-term wear from the diet. Behind these is a large diastema or gap. There are no canines and in their stead are premolars or molars[9]. Next down in the taxonomy is family Chinchillidae and genus Chinchilla.
The mink belongs to the order Carnivora, which mostly includes carnivores. Animals in this order have evolved traits which allow them to hunt effectively. Members often bear teeth specialized for cutting soft tissue. Heightened senses and larger brains are also common features which are vital in hunting[10]. Next down in the taxonomy is Mustelidae, also known as the weasel family, and after that, the genus Neovison.
Upon searching the literature, it was revealed that the chinchilla also has anal glands. While the mink uses the glands to mark territory[5], the chinchilla uses them for defense[8]. This is not a result of divergent evolution because many mammals have such an organ located near their anus. However, usage has been adapted among certain species for specific purposes. For example, dogs will use it to mark territory as well as identify other dogs in the vicinity[11], whereas the skunk famously uses its anal glands to defend itself against predators.
Further comparisons in height and weight reveal that the mink is approximately twice the length and body mass of the chinchilla in both sexes. The limited number of chinchillas has decreased the genetic variation among the herds and perhaps has restricted range of weight. Chinchillas have denser fur, with two to three times the hairs per follicle when compared to the mink, which has 9-24 through one opening in the epidermis but each has its own follicle[12]. The density of fur in the chinchilla is related to its defense mechanism, as it can willingly shed its fur if bitten to escape a predator’s grasp[8]. It is also vital in preventing moisture loss in its arid environment. The mink, in contrast, is semi-aquatic and therefore, the fur is adapted to be water resistant and does not require a mechanism against drying out.
Upon comparing osteologies, the functional and phylogenetic differences become much more apparent. As typical of carnivorans, the mink skull has a sagittal crest, though less pronounced when compared to other species, as well as defined temporal attachments. The orbits are oriented forward, positioned for binocular vision, and the dense skull has a larger cranial capacity. Finally, upon examining the mandible, the coronoid process is noted to be very prominent, dominating the upper portion of the ramus, and the mandibular condyle is low-set. The sagittal crest, forward-oriented orbits, larger cranial size and jaw shape are all typical of this order[13]. They all aid in hunting prey. The sagittal crest and jaw shape indicate strong biting force and large vertical mobility while the orbit position helps with depth perception. The brain size is related to their intelligence and cunning when it comes to hunting[10]. For the chinchilla, the skull is much lighter and less dense, with bones forming cavities of air. In contrast to the mink, it appears more gracile and has no sagittal crest whatsoever because of its simple herbivorous diet. Were its diet to include more fibrous foods or plants, a sagittal crest would also be present. Position of the orbits in this skull is more lateral. The cranial capacity is smaller, which could indicate a less intelligent animal. This, however, is not reflected in its social nature. The mandible is vastly different in function and form. While the mink has a highly defined coronoid process and a low-set mandibular condyle, the chinchilla mandible has a thin ramus with both features present. The motion is restricted more so in the vertical plane but freer in the horizontal plane. This allows the animal to process the fibrous foods in its diet more. The mink has a typical 3.1.3.1 (incisor, canine, premolar, molar) maxillary dental and 3.1.3.2 mandibular dental pattern[14]. The canine here is the most pronounced of the teeth, sitting right next to three small incisors. They are useful in grasping flesh while the incisors chop. The third maxillary premolar and first mandibular molar are morphologically complementary and the largest teeth after the canines. They are known as the carnassials because of their blade-like structure and shearing capabilities[15]. Because the mink has little to no horizontal mastication, the teeth tend to be narrow and sharp. The reduction of post-carnassial molars, which are used for crushing and grinding, indicates the adaptation of the mink to a carnivorous lifestyle. In stark contrast, the chinchilla has an identical maxillary and mandibular dental pattern of 1.0.1.3[9]. The incisors here are incredibly pronounced, as is with most rodents. Behind these incisors, there is a large, non-functional diastema and no canines. Throughout carnivoran evolution, diastemata served as gaps in the gumline so that opposing canines could fit, allowing the mouth to close comfortably. However, diastemata are also common where teeth have been left out of development, such the chinchilla and other rodents lacking canines entirely[9]. The premolar and molars are aligned in even rows with minimal grooves and serve to grind rather than tear. Chinchillas eat grasses and seeds for the most part and do not require vertical motion but rather a more flexible horizontal motion for processing tougher foods. One particular feature unique to rodents is the lifelong growth of incisor enamel. The foods encountered in the diet are surprisingly not the only cause of this adaptation. The mutual grinding of the top and bottom pairs actually sharpens one another, effectively creating better gnawing tools[16]. Both the food and grinding wear down the enamel quickly, and it must be replenished throughout its life.
From the mink skeleton, a total of 49 vertebrae are present – 27 cervical to lumbar, 3 sacral, and 19 tail. The chinchilla has a total of 49 vertebrae as well – 26 cervical to lumbar, 3 sacral and 20 tail. The thoracic vertebrae are where the two species differ, as the mink has 15 pairs of ribs and the chinchilla has 13. In the mink, they are attached to a proportionally longer sternum composed of a manubrium, 8 sternebrae and one xiphoid process. The chinchilla ribs are attached to only 5 sternebrae along with the manubrium and xiphoid process. The difference in number could be due to functional differences. The mink is adapted for swimming and diving and requires higher oxygen intake for its physical demands. The lungs need to be larger, which in turn requires a larger thoracic cavity. The vertebrae of the mink in general are much thicker, especially the spinous processes, which indicate stronger muscle attachments.
The bones of the mink arm are particularly robust when compared to that of the chinchilla. The humerus, radius and ulna are not only thicker, but they have pronounced muscle attachments, indicating heavy use. The shoulder and elbow joints also have higher degrees of freedom in movement. Both fall along the line of their use in swimming as well as hunting. Chinchillas mainly gather food and do not require heavy use of the arms. The humerus is shorter in contrast to the radius and ulna, which are practically fused. These bones are much thinner in comparison and are fragile. The chinchilla tends to move only in the sagittal plane of motion whereas the mink needs movement in at least two planes to hunt, swim and dive. Finally, being carnivorous, the mink has claws to help capture prey, sometimes nearly as large as itself. The chinchilla has small forepaws to deal with much smaller items, such as seeds and blades of grass.
Both sacra are attached to relatively flat pelves. The chinchilla pelvis, however, is much longer and has much longer iliac alae. The mink pelvis, like the rest of the skeleton, is dense and compact. It is speculated that the mink pelvis stays short in proportion to its body length in order to maintain its mobility. The mink faces both terrestrial and aquatic obstacles, and a smaller pelvis increases lateral movement. Chinchillas tend to move linearly and their pelves tend to be longer and larger in proportion to body size. It is speculated that the length assists in this unidirectional movement.
The leg bones of the mink are approximately equal in length and robusticity. As is expected with the forelimbs, they are pronounced from heavy use. The leg bones of the chinchilla, however, differ drastically in length and use. The femur is shorter than the tibia and the fibula is practically non-existent, which indicates lack of weight-bearing responsibility. The change of proportion in body length is associated with functionality, as the chinchilla leaps away in confrontations. The longer tibia allows for a better jumping motion.
The hind feet of the mink are adapted for swimming and are approximately the length of its tibia. This again aids in its partially natatorial lifestyle, as the feet are typically webbed. The low force to area ratio is related to its heavier weight. The hind feet of the chinchilla are narrow and adapted for terrestrial life. The opposite effect is seen here: the hind feet have a high force to area ratio because the animal is lighter. This also aids in the jumping escape that is so common with chinchillas.
Concluding Remarks
By looking at the biology of an animal, one can draw conclusions about its functional structure. These may not all be correct, but they are made intelligently based on observations. Throughout the history of science, this exact ‘observe and speculate’ process has been used to further the workings of the natural world. Preparing a mink and chinchilla skeleton and doing an extensive literature search on the biology of the two species almost created a puzzle, and it was with inference and speculation that conclusions were drawn to fill in the gaps. With more extensive knowledge in the respective fields, more accurate conclusions could be made about the two species; however, the main intent of this investigation was to improve critical thinking, especially within the disciplines of osteology and mammalian biology, by going through the exercise of skeleton preparation and analysis. These skills can then be applied to other scientific fields, and it allows early work in determining function by looking at structure and scientific discovery in general to be more appreciated.
Special thanks to Dr. Phil Myers and Dr. Stephen Hinshaw for their support, giving us access to their facilities and allowing us to perform a skeleton preparation. Without it, this project would not have been possible. Dr. Vanessa Agnew we thank especially because without her, this project would not have existed in the first place. She has critically altered our perspectives on the history of science and science itself.
References
1. Wund, Matthew, and Philip Myers. University of Michigan Museum of Zoology, "Class Mammalia." Accessed December 17, 2011. http://animaldiversity.ummz.umich.edu/site/accounts/information/Mammalia.html.
2. D'elia, G. & Teta, P. 2008. Chinchilla lanigera. In: IUCN 2011. IUCN Red List of Threatened Species. Version 2011.2.
3. Animal World, "Chinchillas." Accessed December 17, 2011. http://animal-world.com/encyclo/critters/chinchilla/chinchilla.php
4. Hinshaw, Stephen. University of Michigan Museum of Zoology, "Dermestarium." Accessed December 17, 2011. http://www.ummz.umich.edu/mammal/dermestarium.html.
5. Schlimme, Kurt. University of Michigan Museum of Zoology, "Neovison vison." Accessed December 17, 2011. http://animaldiversity.ummz.umich.edu/site/accounts/information/Neovison_vison.html.
6. Hendricks, Colette. University of Michigan Museum of Zoology, "Chinchilla lanigera." Accessed December 20, 2011.
7. Chinnie Chin Bin, "The History of Chinchillas." Accessed December 17, 2011. http://www.chinniechinbin.chinchillas.org/about.html.
8. Deane, Amy. Field Trip Earth, "Searching for Wild Chinchillas." Last modified January 23, 2008. Accessed December 17, 2011. http://www.fieldtripearth.org/article.xml?id=1290&ordinal=4.
9. Lewis-Clark State College, "Rodentia." Accessed December 17, 2011. http://www.lcsc.edu/mjbrady/mammalogy lectures/Mammals of the world/rodentia.htm.
10. ZonotriKia, "Carnivora." Accessed December 17, 2011. http://www.zonotrikia.com.ar/.
11. Ruben, Dawn. PetPlace, "Structure and Function of the Anal Glands in Dogs." Accessed December 17, 2011. http://www.petplace.com/dogs/structure-and-function-of-the-anal-glands-in-dogs/page1.aspx.
(Ruben)
12. Kaszowski, Sylwester, Charles C. Rust, and Richard M. Shackelford. "Determination of Hair Density in the Mink." Journal of Mammalogy . 51. (1970): 27-34. http://www.jstor.org/pss/1378528 (accessed December 17, 2011).
13. Bureau of Information and Education, Pennsylvania Game Commission, "Examining Predator and Prey - Carnivore, Herbivore and Omnivore." Accessed December 17, 2011. http://www.envirothonpa.org/documents/skulldrawings.pdf.
14. Aulerich, Richard J., and Daris R. Swindler. "The Dentition of the Mink (Mustela vison)." Journal of Mammalogy. 49. (1968): 488-494 . http://www.jstor.org/pss/1378207 (accessed December 17, 2011).
15. Bels, Vincent L. . Feeding in domestic vertebrates: from structure to behaviour . Cambridge, MA: CABI Publishing, 2006. http://books.google.com/books?hl=en&lr=&id=2pjkMUEhScAC&oi=fnd&pg=PA61&dq=dentition of minks&ots=6TLdKJzP5B&sig=aHl- (accessed December 17, 2011).
16. Hurst, J.L., (1999). Introduction to rodents. In: The UFAW Handbook on the Care and Management of Laboratory Animals, Vol. 1, Terrestrial Vertebrates, 7th edn. Ed. Poole, T., pp. 262–273. Blackwell Publishing, Oxford
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