Sinaloan pocket mouse
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The Sinaloan pocket mouse (''Chaetodipus pernix'') is one of 17
species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...
of
pocket mice Perognathinae is a subfamily of rodents consisting of two genera of pocket mice. Most species live in complex burrows within the deserts and grasslands of western North America, They feed mostly on seeds and other plant parts, which they carry i ...
in the
genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...
''Chaetodipus.'' Two
subspecies In biological classification, subspecies is a rank below species, used for populations that live in different areas and vary in size, shape, or other physical characteristics (morphology), but that can successfully interbreed. Not all species ...
of ''C. pernix'' are recognized, ''C. p. pernix'' and ''C. p. rostratus,'' all are
endemic Endemism is the state of a species being found in a single defined geographic location, such as an island, state, nation, country or other defined zone; organisms that are indigenous to a place are not endemic to it if they are also found elsew ...
to
Mexico Mexico (Spanish: México), officially the United Mexican States, is a country in the southern portion of North America. It is bordered to the north by the United States; to the south and west by the Pacific Ocean; to the southeast by Guatema ...
. This small mouse lives among shrubs and has a wide range of coat colors.


Range and habitat

''C. pernix'' mostly lives in the coastal plain of
Sinaloa Sinaloa (), officially the Estado Libre y Soberano de Sinaloa ( en, Free and Sovereign State of Sinaloa), is one of the 31 states which, along with Mexico City, comprise the Administrative divisions of Mexico, Federal Entities of Mexico. It is d ...
state within Mexico, which extends from southern
Sonora Sonora (), officially Estado Libre y Soberano de Sonora ( en, Free and Sovereign State of Sonora), is one of the 31 states which, along with Mexico City, comprise the Administrative divisions of Mexico, Federal Entities of Mexico. The state is d ...
to northern
Nayarit Nayarit (), officially the Free and Sovereign State of Nayarit ( es, Estado Libre y Soberano de Nayarit), is one of the 31 states that, along with Mexico City, comprise the Federal Entities of Mexico. It is divided in 20 municipalities and its ...
. Their habitat consists mostly of small trees, shrubs, and cacti. Sinaloan pocket mice are found on sandy soil, living under low bushes or dense networks of vines and grasses. Their habitat does not contain any rock material, unlike in its sister species, the
rock pocket mouse The rock pocket mouse (''Chaetodipus intermedius'') is one of 19 species of pocket mice in the genus ''Chaetodipus''. It is sometimes grouped in the genus ''Perognathus''. Description Found mainly in rocky outcrops in the deserts of the southwes ...
.


Description

The Sinaloan pocket mouse is one of the smallest in the genus ''Chaetodipus'' with a total length of less than 200mm. Their skull is narrow with an elongated nose and medium-sized ears. ''C. pernix'' has a thinly haired tail that is longer than the body length. Their coat color is variable (usually yellowish-brown), but always darker on the back and lighter on the sides and belly. Their diet consists of grains and seeds. The average litter size is seven. Color polymorphism Coat color is a
genetic polymorphism A gene is said to be polymorphic if more than one allele occupies that gene's locus within a population. In addition to having more than one allele at a specific locus, each allele must also occur in the population at a rate of at least 1% to gen ...
. Many different coat colors are seen between species of pocket mice and even within the same population because of differences in their
genotype The genotype of an organism is its complete set of genetic material. Genotype can also be used to refer to the alleles or variants an individual carries in a particular gene or genetic location. The number of alleles an individual can have in a ...
. Multiple aspects of the environment put selective pressures on pocket mice to adapt their coat colors. Variation in coat color between pocket mice can be a selective advantage for two main reasons: concealment and regulation of physiological processes. Concealment offers a large selective advantage for pocket mice that are able to successfully hide from potential predators, also known as
Crypsis In ecology, crypsis is the ability of an animal or a plant to avoid observation or detection by other animals. It may be a predation strategy or an antipredator adaptation. Methods include camouflage, nocturnality, subterranean lifestyle and ...
. Pocket mice generally show two forms of crypsis:
camouflage Camouflage is the use of any combination of materials, coloration, or illumination for concealment, either by making animals or objects hard to see, or by disguising them as something else. Examples include the leopard's spotted coat, the ...
and
countershading Countershading, or Thayer's law, is a method of camouflage in which an animal's coloration is darker on the top or upper side and lighter on the underside of the body. This pattern is found in many species of mammals, reptiles, birds, fish, and ...
. Camouflage is seen when an organism's coloring helps it blend into its background or environment. Evidence of camouflage is seen between different populations of pocket mice that live in two different environments. The Sinaloan pocket mouse usually has a yellow-brown coat with black hairs because they live in a sandy environment under shrubs, while the
Rock pocket mouse The rock pocket mouse (''Chaetodipus intermedius'') is one of 19 species of pocket mice in the genus ''Chaetodipus''. It is sometimes grouped in the genus ''Perognathus''. Description Found mainly in rocky outcrops in the deserts of the southwes ...
usually has a grey or black coat color and lives among rocks. Mice who show a very different coat color than their environment are more susceptible to predation because they can be seen easier than mice that blend in with their environment. Countershading is another type of camouflage and is seen when an organism's pigmentation is darker on the upper side of the body and lighter on the underside of the body. Sinaloan pocket mice almost always have a lighter belly and sides compared to the darker color of their back. The selective advantage of this color patterning is again, concealment from predators, mostly owls in this case. Countershading helps the pocket mouse to reduce the appearance of their shadow in sunny environments. Living in the sunny desert, this color pattern has a large selective advantage. Mice whose shadows were easily seen by owls flying overhead would more likely become prey, than mice whose shadows weren't as obvious. Coat color in pocket mice can also be a selective advantage because it can help regulate physiological processes of the body. Light coat colors have a selective advantage in hot, sunny environments because they can help regulate body temperature. A dark coat in a sunny environment would attract sunlight and cause the mouse to overheat more easily, requiring more energy from the mouse to try and lower their body temperature. A light coat color requires less energy input to regulate body temperature and therefore has a selective advantage over dark coat color which requires more energy. The selective advantage of coat color for concealment and regulation of body temperature likely caused geographic variation of coat color as well. For both concealment and body temperature regulation, the mice better adapted to their environment will likely survive better and produce more offspring. This means that the mouse that blends in with its environment the best, in a way that uses the least amount of energy will have a selective advantage. Going along with this, mice in different environments require different coat colors to blend in most effectively. Geographic variation between environments is another selective pressure that helped color polymorphism evolve between pocket mice. Pocket mice that live in sunny environments with an abundance of sand will have a selective advantage with light colored yellow or brown coats. In contrast, pocket mice that live on rocks in environments with less sun are more likely to have a selective advantage with darker colored grey or black coats. Genetic basis The genetic difference for varying coat colors is controlled largely by two
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...
s in pocket mice:
eumelanin Melanin (; from el, μέλας, melas, black, dark) is a broad term for a group of natural pigments found in most organisms. Eumelanin is produced through a multistage chemical process known as melanogenesis, where the oxidation of the amin ...
and
pheomelanin Melanin (; from el, μέλας, melas, black, dark) is a broad term for a group of natural pigments found in most organisms. Eumelanin is produced through a multistage chemical process known as melanogenesis, where the oxidation of the amino ...
. Production of eumelanin gives hair a black or brown color, while pheomelanin gives hair a red color.
Melanocyte Melanocytes are melanin-producing neural crest-derived cells located in the bottom layer (the stratum basale) of the skin's epidermis, the middle layer of the eye (the uvea), the inner ear, vaginal epithelium, meninges, bones, and heart. ...
s are pigment producing cells, which are controlled by
gene In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...
s. Differences in
gene expression Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product that enables it to produce end products, protein or non-coding RNA, and ultimately affect a phenotype, as the final effect. The ...
cause melanocytes to produce varying levels and types of pigment. Eumelanin is produced when the
G protein-coupled receptor G protein-coupled receptors (GPCRs), also known as seven-(pass)-transmembrane domain receptors, 7TM receptors, heptahelical receptors, serpentine receptors, and G protein-linked receptors (GPLR), form a large group of evolutionarily-related p ...
called melanocortin-1-receptor (MC1R) is activated. Pheomelanin production is controlled by the
agouti-signaling protein Agouti-signaling protein is a protein that in humans is encoded by the ASIP gene. It is responsible for the distribution of melanin pigment in mammals. Agouti interacts with the melanocortin 1 receptor to determine whether the melanocyte (pigment ...
, which is an
inverse agonist In pharmacology, an inverse agonist is a drug that binds to the same receptor as an agonist but induces a pharmacological response opposite to that of the agonist. A neutral antagonist has no activity in the absence of an agonist or inverse agon ...
of MC1R. Therefore, Agouti expression causes decreased levels of eumelanin production, which allows increased levels of pheomelanin expression. Variation in the environment can cause proteins to be expressed differently in the
genome In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding ge ...
, producing variation in coat color between two different environments.


References

*Patton, J. L. 2005. Family Heteromyidae. pp. 844–858 ''in'' Mammal Species of the World a Taxonomic and Geographic Reference. D. E. Wilson and D. M. Reeder eds. Johns Hopkins University Press, Baltimore. {{Taxonbar, from=Q172553 Chaetodipus Endemic mammals of Mexico Sinaloan dry forests Mammals described in 1898 Taxa named by Joel Asaph Allen Taxonomy articles created by Polbot Sonoran–Sinaloan transition subtropical dry forest