Table 2Specimens ofMustela frenata(north of the range ofM. f. frenata) arranged by sex and under each sex by age
MaleFemaletotal number of ♂ and ♀total number of adults, ♂ and ♀adult ♂♂ ad., % of total adultssubadult ♂young ♂juvenal ♂total number of ♂♂, % of totaladult ♀♀ ad., % of total adultssubadult ♀young ♀juvenal ♀total number of ♀♀, % of totalMay295541475459244519337419153June425314408975938474252694116680July5970185521305925305582904122084August40772355..113741223225..392615252September157925121517542149..17256819October1158467..4366842131..22346519November4170481..887318301221332712159December5969431..10873263115.....412714985January8069322112672363114.....5028176116February4566195..8273233443..302711268March38722.....5770152881..24308153April306724339671533..2219335845Totals489672811962298868244338213510471321,459733
I suppose that in nature there are approximately equal numbers of male and female weasels and further suppose that the selective factors which cause more males than females to be caught are the greater distances traveled by the males and their greater weight.
At a late stage in the preparation of this manuscript a total of 5,457 specimens had been examined. For the most part these were conventional study-specimens; that is to say, they were stuffed skins with the skulls separate and each was accompanied by the customary data as to locality of capture, date of capture, name of collector, external measurements and sex recorded on the labels by the collectors. Skulls unaccompanied by skins, nevertheless, comprised a large share of the total and a small proportion was made up of skins unaccompanied by skulls, mounted specimens, skeletons, and entire animals preserved in liquid.
It was the recognition of this need for specimens from extensive areas from which no specimens previously had been collected that influenced me, approximately a year after the study was begun, to allot for it a long span of time. The procedure adopted, in general, was to study the weasels of one species from a given geographic area in so far as the material warranted, then lay this aside until additional critical material could be obtained, and finally, some months or a year later, complete the account. In this fashion the manuscript of the American weasels received my attention in each of the past twenty-five years (September, 1926 to date of publication). This is a confession of fact rather than a recommendation of procedure. This type of procedure unduly delays the diffusion of knowledge and for a variety of reasons justifiably annoys other students of the subject. Nevertheless, many gaps have been filled that otherwise would have remained open. Although specimens to solve several problems still remain to be collected and studied, it seems that a point of diminishing returns has now been reached, which, in fairness to all concerned, calls for publication of the results so far obtained.For assistance in the entire undertaking, I am more indebted to Miss Annie M. Alexander than to any other one person; she provided the means by which specimens from critical areas were obtained, made it possible to examine the European collections, and assisted in other ways. The late Professor Joseph Grinnell and Mr. Charles D. Bunker, among others, gave truly valuable encouragement and assistance.Collections containing weasels which were examined in the study here reported upon were as follows:Acad. Nat. Sciences of PhiladelphiaAmerican Mus. Nat. HistoryBaylor UniversityBerlin Zoological MuseumBoston Society of Natural HistoryBrigham Young UniversityBritish Museum of Natural HistoryCalifornia Academy of SciencesCarnegie MuseumCharleston MuseumCoe CollegeCollection of J. ArnoldCollection of Stanley C. ArthurCollection of Rollin H. BakerCollection of William BebbCollection of R. H. ColemanCollection of Ian McTaggart-CowanCollection of Stuart CriddleCollection of John CushingCollection of Walter W. DalquestCollection of William B. DavisCollection of J. M. EdsonCollection of Ralph EllisCollection of John Fitzgerald, Jr.Collection of Mr. GreenCollection of Ross HardyCollection of Donald V. HemphillCollection of L. M. HueyCollection of R. W. JacksonCollection of Stanley G. JewettCollection of E. J. KoestnerCollection of J. E. LawCollection of A. H. MillerCollection of Lloye H. MillerCollection of R. D. MooreCollection of J. A. MunroCollection of O. J. MurieCollection of Robert T. OrrCollection of Arthur PeakeCollection of Kenneth RaceyCollection of William B. RichardsonCollection Rocky Mt. Spotted Fever Lab.Collection of Victor B. SchefferCollection of William T. ShawCollection of O. P. SillimanCollection of W. E. SnyderCollection of Frank StephensCollection of T. C. StephensCollection of D. D. StoneCollection of Myron H. SwenkCollection of Joe and Dean ThiriotCollection of John TylerCollection of Jack C vonBloekerCollection of Alex WalkerCollection of Edward R. WarrenColorado Museum of Natural HistoryCharles R. Conner MuseumCornell UniversityDonald R. Dickey CollectionField Museum of Natural HistoryFlorida State MuseumFresno State Junior CollegeHumboldt State Teachers CollegeIllinois Natural History SurveyIowa State CollegeIowa Wesleyan CollegeKansas State Agric. CollegeLeland Stanford Junior UniversityLeningrad Academy of ScienceLos Angeles Mus. Hist. Art and Sci.Louisiana State UniversityMt. Rainier Nat'l Park CollectionMuseum of Comparative ZoölogyMus. Polonais d'Hist. Nat., WarsawMus. Vert. Zoöl., Univ. CaliforniaMuseum of Zoölogy, Univ. MichiganNational Museum of CanadaNaturhistoriska Ricksmuseum, SwedenNeuchatel University MuseumNew York State MuseumOhio State MuseumOklahoma Agric. and Mech. CollegeOttawa University, KansasParis MuseumProvincial Museum of British ColumbiaRoyal Ontario Museum of ZoölogySan Diego Society of Natural HistoryState Hist. and Nat. Hist. Soc. Colo.State Normal School, Cheney, Wash.Texas Cooperative Research CollectionUnited States National MuseumUniversity of ArkansasUniv. California Mus. Palaeo.University of IdahoUniv. Kansas Mus. Nat. HistoryUniversity of MinnesotaUniversity of Notre DameUniversity of OklahomaUniversity of OregonUniversity of South DakotaUniversity of UtahUniv. Washington Museum of ZoölogyUniversity of WisconsinUniv. Zool. Mus., CopenhagenThe largest single collection is in the United States National Museum, where the specimens of the National Museum proper and the United States Biological Surveys Collection, together, provide essential materials including a large share of the holotypes. Specimens in all of the North American collections including Canada and México have been made available, by loan, and in 1937 materials were examined in the principal collections of northern and central Europe. After the materials in North American collections were assembled, special effort, with considerable success, was made in each of several winters, to obtain specimens from areas not previously represented in collections.To the many persons who were in charge of the collections consulted, to those who at my request sought critical specimens, and to those who assisted in various stages of assembling data and in preparation of the manuscript, I am grateful indeed. Likewise, I am deeply appreciative of the grants-in-aid received from the Carnegie Institution of Washington, the University of California Chapter of Sigma Xi, the John Simon Guggenheim Memorial Foundation and the Kansas University Endowment Association. I am mindful also of an obligation to those who appropriated funds, by legislative action, for research use by The University of California and The University of Kansas.For assistance with the illustrations I am indebted to the late Major Allan Brooks forPlate 1, to Mrs. Mary Blos for figures 25-31, to Miss Ann Murray for figures11-13, to Mr. W. C. Matthews for all the photographs, to Mrs. Freda L. Abernathy for figures 2-9, 18-22, 24, and for retouching all the photographs except the following which were retouched by Mrs. Virginia Unruh: figs.dof plates2,3,4,9,10,11,16,17; figs.iof plates5,6,7; figs.h,j,kof plate7; figs.fandgof plates12and13; and figs.canddof plate14. To Mrs. Unruh I am further indebted for figures1,16,17and23and for much terminal assistance with preparing most of the illustrations for the engraver.
It was the recognition of this need for specimens from extensive areas from which no specimens previously had been collected that influenced me, approximately a year after the study was begun, to allot for it a long span of time. The procedure adopted, in general, was to study the weasels of one species from a given geographic area in so far as the material warranted, then lay this aside until additional critical material could be obtained, and finally, some months or a year later, complete the account. In this fashion the manuscript of the American weasels received my attention in each of the past twenty-five years (September, 1926 to date of publication). This is a confession of fact rather than a recommendation of procedure. This type of procedure unduly delays the diffusion of knowledge and for a variety of reasons justifiably annoys other students of the subject. Nevertheless, many gaps have been filled that otherwise would have remained open. Although specimens to solve several problems still remain to be collected and studied, it seems that a point of diminishing returns has now been reached, which, in fairness to all concerned, calls for publication of the results so far obtained.
For assistance in the entire undertaking, I am more indebted to Miss Annie M. Alexander than to any other one person; she provided the means by which specimens from critical areas were obtained, made it possible to examine the European collections, and assisted in other ways. The late Professor Joseph Grinnell and Mr. Charles D. Bunker, among others, gave truly valuable encouragement and assistance.
Collections containing weasels which were examined in the study here reported upon were as follows:
Acad. Nat. Sciences of PhiladelphiaAmerican Mus. Nat. HistoryBaylor UniversityBerlin Zoological MuseumBoston Society of Natural HistoryBrigham Young UniversityBritish Museum of Natural HistoryCalifornia Academy of SciencesCarnegie MuseumCharleston MuseumCoe CollegeCollection of J. ArnoldCollection of Stanley C. ArthurCollection of Rollin H. BakerCollection of William BebbCollection of R. H. ColemanCollection of Ian McTaggart-CowanCollection of Stuart CriddleCollection of John CushingCollection of Walter W. DalquestCollection of William B. DavisCollection of J. M. EdsonCollection of Ralph EllisCollection of John Fitzgerald, Jr.Collection of Mr. GreenCollection of Ross HardyCollection of Donald V. HemphillCollection of L. M. HueyCollection of R. W. JacksonCollection of Stanley G. JewettCollection of E. J. KoestnerCollection of J. E. LawCollection of A. H. MillerCollection of Lloye H. MillerCollection of R. D. MooreCollection of J. A. MunroCollection of O. J. MurieCollection of Robert T. OrrCollection of Arthur PeakeCollection of Kenneth RaceyCollection of William B. RichardsonCollection Rocky Mt. Spotted Fever Lab.Collection of Victor B. SchefferCollection of William T. ShawCollection of O. P. SillimanCollection of W. E. SnyderCollection of Frank StephensCollection of T. C. StephensCollection of D. D. StoneCollection of Myron H. SwenkCollection of Joe and Dean ThiriotCollection of John TylerCollection of Jack C vonBloekerCollection of Alex WalkerCollection of Edward R. WarrenColorado Museum of Natural HistoryCharles R. Conner MuseumCornell UniversityDonald R. Dickey CollectionField Museum of Natural HistoryFlorida State MuseumFresno State Junior CollegeHumboldt State Teachers CollegeIllinois Natural History SurveyIowa State CollegeIowa Wesleyan CollegeKansas State Agric. CollegeLeland Stanford Junior UniversityLeningrad Academy of ScienceLos Angeles Mus. Hist. Art and Sci.Louisiana State UniversityMt. Rainier Nat'l Park CollectionMuseum of Comparative ZoölogyMus. Polonais d'Hist. Nat., WarsawMus. Vert. Zoöl., Univ. CaliforniaMuseum of Zoölogy, Univ. MichiganNational Museum of CanadaNaturhistoriska Ricksmuseum, SwedenNeuchatel University MuseumNew York State MuseumOhio State MuseumOklahoma Agric. and Mech. CollegeOttawa University, KansasParis MuseumProvincial Museum of British ColumbiaRoyal Ontario Museum of ZoölogySan Diego Society of Natural HistoryState Hist. and Nat. Hist. Soc. Colo.State Normal School, Cheney, Wash.Texas Cooperative Research CollectionUnited States National MuseumUniversity of ArkansasUniv. California Mus. Palaeo.University of IdahoUniv. Kansas Mus. Nat. HistoryUniversity of MinnesotaUniversity of Notre DameUniversity of OklahomaUniversity of OregonUniversity of South DakotaUniversity of UtahUniv. Washington Museum of ZoölogyUniversity of WisconsinUniv. Zool. Mus., Copenhagen
The largest single collection is in the United States National Museum, where the specimens of the National Museum proper and the United States Biological Surveys Collection, together, provide essential materials including a large share of the holotypes. Specimens in all of the North American collections including Canada and México have been made available, by loan, and in 1937 materials were examined in the principal collections of northern and central Europe. After the materials in North American collections were assembled, special effort, with considerable success, was made in each of several winters, to obtain specimens from areas not previously represented in collections.
To the many persons who were in charge of the collections consulted, to those who at my request sought critical specimens, and to those who assisted in various stages of assembling data and in preparation of the manuscript, I am grateful indeed. Likewise, I am deeply appreciative of the grants-in-aid received from the Carnegie Institution of Washington, the University of California Chapter of Sigma Xi, the John Simon Guggenheim Memorial Foundation and the Kansas University Endowment Association. I am mindful also of an obligation to those who appropriated funds, by legislative action, for research use by The University of California and The University of Kansas.
For assistance with the illustrations I am indebted to the late Major Allan Brooks forPlate 1, to Mrs. Mary Blos for figures 25-31, to Miss Ann Murray for figures11-13, to Mr. W. C. Matthews for all the photographs, to Mrs. Freda L. Abernathy for figures 2-9, 18-22, 24, and for retouching all the photographs except the following which were retouched by Mrs. Virginia Unruh: figs.dof plates2,3,4,9,10,11,16,17; figs.iof plates5,6,7; figs.h,j,kof plate7; figs.fandgof plates12and13; and figs.canddof plate14. To Mrs. Unruh I am further indebted for figures1,16,17and23and for much terminal assistance with preparing most of the illustrations for the engraver.
The methods of study, after specimens were assembled, included first comparisons of specimens of like age and sex from each of several localities to ascertain the constant features by which full species were distinguishable, one from the other. For example, it was found that in every individual from Trout Lake, Washington, of the species here designatedMustela erminea, the postglenoidal length of the skull amounted to more than 47 per cent of the condylobasal length whereas it was less than 47 per cent in all individuals here designated asMustela frenata, from the same locality. Testing of specimens from other localities by means of this and other selected characters permitted the outlining of the geographic ranges of the full "species-groups." By comparing specimens of other nominal species and by examining specimens from localities geographically intermediate between the nominal species, I found intergradation and therefore arranged the nominal species as subspecies of a single species. Intergradation here is understood to be the result of crossbreeding in nature between two kinds of animals in the area where the geographic ranges of the two kinds meet. Presence of intergradation between two kinds of weasels was basis for according them subspecific rank. Absence of intergradation in nature at every place where the geographic ranges of two kinds met or overlapped, and absence of intergradation by way of some other kind, or chain of kinds, was basis for according each of the two kinds full specific rank. By thus applying the test of intergradation, or lack of it, I found that there were four full species of weasels, of the subgenusMustela, in all of the Americas.
Next, the specimens of one species were arranged in trays in a geographic sequence. The specimens from any one locality were segregated by sex and under one sex from one place were arranged from oldest to youngest, that is to say by age. The four series with the largest numbers of individuals of a given age were selected. Seventeen cranial measurements and three external measurements were recorded for each individual of each of these four series. For each measurement, the coefficient of variation, standard deviation and probable error were computed. The four samples subjected to such analysis were a series of adult males, one of adult females, one of subadult males and one of subadult females. Also, studies of each sex were made to ascertain seasonal changes in pelage. After data were obtained on ontogenetic (age) variation, secondary sexual variation, seasonal variation, and degree of individual variation by studying specimens in the manner described above, tests were made for subspecific (geographic) variation by comparing series of specimens of like sex, age and season, from different localities. For each one of several geographically variable features noted, a map was prepared for animals of each sex. When all the data thus obtained were codified, subspecific ranges were, in a sense automatically, obtained. On the resulting map showing geographic ranges of subspecies for a species, a type locality was accurately plotted for each name that had been applied to the species, and names then were applied in accordance with the international rules of zoölogical nomenclature.
The kind of variation which results from increasing age has been dealt with extensively for the skull (of the Old WorldMustela erminea) by Hensel (1881) and for the external features and to some extent for the skull by Hamilton (1933) in the North American formsM. erminea cicognaniiandM. frenata noveboracensis.
The young of bothermineaandfrenataare hairless and blind at birth. InM. frenata noveboracensis, the eyes open on approximately the 37th day. When 2 to 4 months old, the tail is pointed at the tip. This is because the terminal hair of the tail, including the black tip, is short and lies flat on the tail. In subadults and adults the hair on the terminal part of the tail is as long as that on the basal part, and the tail appears to be of uniform diameter all the way out to the end.
In the western subspecies ofM. frenata, and in its tropical subspecies, animals so young as to have pointed tails commonly have the underparts of the body more intensely colored than do adults. The young may have salmon-colored instead of yellowish fur on the underparts.
Otherwise, in animals that have attained approximately adult proportions—which appears to be at approximately 6 months of age in males—there are no variations which are ascribable to increasing age in the color-pattern or pelage that cause the systematist to confuse species or subspecies.
Of the several parts of the skull in juvenal animals, the braincase and width of the posterior part of the palate are most nearly of the size attained in the adult, the facial part of the skull at birth is the least developed, and the interorbital region is, in relation to its ultimate adult size, intermediate in stage of development. The permanent teeth are acquired when the animal is approximately eleven weeks old.
Four age groups, based on characters of the dentition and skull, have been recognized. They are:
Juvenile.—One or more deciduous (milk) teeth present. Birth to three months of age.Young.—Sutures widely open between the maxillae and nasals and between the premaxillae and nasals. Three to seven and a half months of age.Subadult.—Sutures between maxillae and nasals visible but indistinct. Seven and a half to ten months of age.Adult.—Bones of rostrum coalesced with no traces of sutures visible to the naked eye. More than ten months old.
Juvenile.—One or more deciduous (milk) teeth present. Birth to three months of age.
Young.—Sutures widely open between the maxillae and nasals and between the premaxillae and nasals. Three to seven and a half months of age.
Subadult.—Sutures between maxillae and nasals visible but indistinct. Seven and a half to ten months of age.
Adult.—Bones of rostrum coalesced with no traces of sutures visible to the naked eye. More than ten months old.
The skull as a whole increases in size until the animal is two-thirds of the way through the stage designated as young. After this time the width of the rostrum, as measured across the hamular processes of the lacrimals, increases until approximately a third of the way through adulthood. The interorbital breadth decreases from late subadulthood to adulthood and even in adults there appears to be a slight decrease in this part of the skull with increasing age.
The average zoölogist will readily distinguish skulls of juveniles and young from adults but usually fails to distinguish subadults from adults. Nevertheless, subadults must be distinguished from adults if geographic variation is to be measured accurately. The reason for this is that such differences in the form (not size) of the skull as result from increasing age equal and often exceed the differences of a geographic sort which serve for distinguishing subspecies that have adjoining geographic ranges. All sutures in the skull, except those between the tympanic bulla and the braincase, and those on the dorsal face of the rostrum, are obliterated while the animal is a subadult. Most kinds of mammals retain sutures throughout life or until the animals are well into adulthood. Therefore, skulls of weasels offer fewer features for estimating age than do those of most mammals and the skulls of weasels that are subadults or older are more difficult to classify accurately as to age than are the skulls of most other mammals. More reliance on shape of entire skull and less reliance on extent and shape of any individual bone is necessary in estimating the age of a weasel. Wright (1947:344) shows that the weight of the baculum (os penis) is a certain means of differentiating adults from males of lesser age. When approximately eleven months old,Mustela frenata oribasusof western Montana molts from the white winter coat into the brown summer coat. At that time spermatogenesis starts for the first time and the weight of the baculum increases from less than 30 milligrams to more than 52 milligrams.
In the autumn and early winter, most of the specimens are subadults. Ordinarily the few adults obtained in these seasons can easily be segregated from the subadults because ontogenetic development in the twelve additional months of life of each of the older animals has obliterated the sutures on the rostrum, heightened (vertically) and lengthened (anteriorly) the sagittal crest, widened the rostrum, and produced still other changes in form that are revealed by direct comparison of specimens of the two ages.
The secondary sexual variation, which has been detected, is in size of the animal, relative length of the tail and shape of the skull. The female is the smaller. In the smallMustela rixosaand apparently inMustela africanathe secondary sexual difference in size is relatively slight. InMustela frenataandMustela erminea, males are approximately twice as heavy as females, the degree of difference very definitely depending upon the subspecies. For example, inM. e. richardsoniithe recorded weights are 175 and 69 grams as opposed to 81 and 54 grams inM. e. cicognanii. In general, within one species the greatest difference in size of males and females is in those subspecies in which the animals are of large size. The secondary sexual variation in size is much more than the individual variation in either sex. The same is not true of secondary sexual difference in length of the tail (relative to the length of the head and body), which in eighteen subspecies ofM. ermineais from 1 to 7 per cent longer in males than in females. In two subspecies,M. e. haidarumandM. e. olympica, the tail is a fraction of a per cent the longer in females if we may rely upon the few specimens for which collectors' measurements are available.
In bothM. ermineaandM. frenatathe skull of the female is approximately 45 per cent lighter than that of the male, or put in the opposite way, the skull of the male is 83 per cent heavier than the skull of the female. The difference in this respect varies greatly depending on the subspecies. For example, the skull of the male is 127 per cent heavier than that of the female inM. e. richardsoniibut only 33 per cent heavier inM. e. anguinae. InMustela frenata, the subspeciesnoveboracensisshows most sexual dimorphism in weight of skull (3.6 and 1.7 grams) andolivaceathe least (5.3 and 3.8 grams). In general, the difference in this respect is less in subspecies the individuals of which are of small size.
Therefore, as might be expected, the secondary sexual variation in weight of the skull is less inM. rixosa, individuals of which are of small size, than inM. ermineaor than inM. frenata, in general of larger size. Nevertheless, inM. africana, in which the individuals are of large size, there appears to be less sexual dimorphism in weight of the skull than inM. frenataor than inM. erminea, although it should be remarked that there are too few data forM. africanato allow of forming a trustworthy conclusion concerning the amount of secondary sexual variation in that species.
The secondary sexual variation in shape of the skull consists of a slenderness in the female. In relation to the basilar length the spread of the zygomatic arches is more in males and, except in the one subspeciesM. f. altifrontalis, the rostrum is broader. Also the interorbital region is relatively broader in males of most subspecies. In most subspecies of bothM. frenataandM. ermineathe tympanic bullae are relatively (to the basilar length) longer in females. The maximum sexual dimorphism occurs inM. erminea arcticaand the minimum dimorphism inM. e. haidarum,M. e. anguinaeandM. e. muricus. Taking into account all of the subspecies of each of the North American species, the shape of the skull differs most inM. ermineaand least inM. frenata. In the latter species the greatest difference in shape of the skull, as was true also of its weight, is in the subspeciesM. f. noveboracensis. In these two subspecies,M. f. noveboracensisandM. e. arctica, in addition to the secondary sexual variation already mentioned in the skull, females have the braincase smoother and more rounded, the postorbital-, mastoid-, and lacrimal-processes relatively smaller, and the ventral face of the tympanic bulla at its anterior margin more nearly flush with the floor of the braincase.
In the weasels, subgenusMustela, the disparity in size of the two sexes is almost or quite as much as in any other fissiped carnivore. It is because of this large degree of difference that the skulls of the two sexes are described separately in the following systematic accounts. The need for such treatment was recognized by Reinhold Hensel (1881:127) more than sixty years ago when he wrote in the introduction to his "Craniologische Studien," ofMustela, as follows: ". . . die Geschlechtsdifferenzen am Schädel vieler Säugethiere . . . so gross sind, dass man diese wie Schädel verschiedener species behandeln muss, während in anderen Ordnungen (Rosores, Edentaten) die Schädel solche Unterschiede nichtzeigen." In the past, failure to appreciate the large amount of secondary sexual variation has resulted in erroneous deductions as regards characters of certain geographic races and has been the cause of some nomenclatural confusion, as for example, inMustela frenata macrura, where the female was named as a separate species (Mustela jelskii).
Individual variation is here considered to be the variation in one species which can occur between offspring of a single pair of parents, after variation ascribable to differences in age, sex, and season is excluded. Individual variation, therefore, is a term here used in a composite sense; it includes variations which probably represent different genetic strains within certain populations and variations induced within one generation by environmental factors.
In skulls of weasels, the individual variation in size is more than it is in relative proportions. Hensel (op. cit.) has stressed that weasels, like other carnivores, produced "dwarfed" individuals more than do herbivorous mammals. I cannot vouch for the accuracy of this view, but can say that individual variation is not greater than in some other fissiped carnivores. Impressions to the contrary probably result largely from failure to recognize age-variation. When skulls of a large series from any one locality are arranged first by sex, and under each sex according to probable age on the basis of extension anteriorly of the sagittal crest and of degree of postorbital constriction, individual variation is seen to be less than a cursory examination, even of only one sex, would suggest.
Study of a large series of one age of one sex of one species from one locality shows that some parts, of the skull for example, vary more than other parts. In illustration, among 22 male topotypes ofMustela frenata washingtonithe least interorbital breadth varied 25 per cent (9.0 mm. to 12 mm.) whereas the length of the tooth-rows varied only 13.3 per cent (15.6 mm. to 18.0 mm.). In color the individual variation definitely is more in areas of intergradation between subspecies than in other areas. Details of one such instance of intergradation are given in the account ofMustela frenata spadix.
Statements to the effect that there is much individual variation in the color of weasels, were made mostly fifty years or so ago by writers who had but few specimens from widely separated localities. Where marked climatic differences exist between localities only a few miles apart, marked differences occur in coloration of the weasels from the different localities. Much of what formerly was mistaken for individual variation now proves to be geographic variation. Individual variation actually is of slight amount in comparison with that in mammals generally. Differences in size and relative proportions of parts usually are correlated with geographic differences in color. The color does fade slightly in the period between molts. Also as a result of the seasonal color change, in autumn along the upper margin of the Austral Life-zone, some individuals become white whereas others become white on only the underparts, the upper parts changing only to lighter brown. Probably it would be correct to say that this variation was a combination of seasonal and individual variation rather than either one alone.
As might be supposed, individual variation is not the same in all species or subspecies. For example, p2 is always absent inMustela africanaand always present in certain subspecies ofM. frenata. In some other subspecies ofM. frenata, p2 is absent approximately as often as present. In the writer's experience, when only a few specimens are available for comparison, individual variation is more difficult to distinguish from specific and subspecific (geographic) variation than is age-variation or secondary sexual variation.
Among the larger series of specimens examined, only one instance of what might be called a mutation in the old sense of a large, sudden change, was detected. That was the loss of the second lower molar in many (less than a third) of the specimens from Newfoundland. The six instances of abnormal coloration described on pages 41 to 43, might be regarded as mutations of large magnitude but no evidence was found of repetition of an abnormality in any one population. Otherwise, in every instance where plotted, the manifestations of a variation arranged themselves about the mean in such a way as to form a smooth, unimodal curve.
When subspecific and specific variations are the objectives of study, seasonal variation must be understood, in order to be excluded from consideration, in the same way that variations ascribable to age, sex and individualism must be understood in order to be excluded from consideration. In weasels, change in color of the pelage is the seasonal variation most important for the systematist to understand. Other seasonal variations in the pelage are hairiness versus nakedness of the pads of the feet, length of the pelage on the body, and possibly the density of the pelage on the body. In the northern half of North America, roughly speaking, seasonal change in color is so pronounced (white in winter and brown in summer) as to be easily recognized. South of this area, in the Austral and Sonoran life-zones, the color of the winter pelage differs only slightly from that of the summer pelage. In these more southern latitudes the winter pelage in almost all subspecies is of lighter color than the summer pelage and has a smoky suffusion. With material of the two seasons in hand for comparison, close attention to the variation will permit the systematist to recognize the difference in shade of brown as seasonal variation and not geographic or specific variation. Farther south still, in the Tropical Life-zone, seasonal difference in color was not detected in the material studied. Seasonal change in color is discussed in the section immediately following.
In all American weasels (subgenusMustela) the color, at least in summer, is brown with more or less white or whitish on the underparts. In one species,Mustela africana, there is a longitudinal stripe of brown on the middle of the light-colored underparts; this stripe is absent in each of the other three American species. Two species,M. ermineaandM. frenata, always have a black tip on the tail. Of the other two species,M. africanalacks the black tip andM. rixosamay or may not have a few black hairs in the tip of its tail. White or light yellowish facial markings occur in subspecies ofM. frenatafrom the southwestern United Stated to Central America. Subspecies having the most extensive light-colored facial markings have the remainder of the upper part of the head black. In weasels without light facial markings the upper parts of the head all are brown. In the two species,M. ermineaandM. frenata, the extent to which the light color of the underparts extends down the insides of the legs and out on the underside of the tail, or the absence of light color on these parts, is a matter of geographic variation. The same can be said forM. rixosaexcept that first its tail is unicolored and second individual variation as well as geographic variation accounts for the color pattern on the underparts and legs in animals from the southeastern part of the range of the species.
The most remarkable feature of the coloration of weasels is the winter whitening. This occurs in the northern part of North America in each of the three species of weasels found on that continent. The black tip of the tail inM. ermineaandM. frenataremains black in winter. If an individual ofM. rixosahas black hairs on the tip of its tail in summer, there are thought to be black hairs there also in winter. Otherwise the winter pelage is all white in northern areas in each of the three species. In this white winter coat the animal is known as ermine.
The underlying cause seems to be protective coloration. At any rate, weasels are always white in winter if they are from areas where snow lies on the ground all winter, every winter, or almost every winter; and they are always brown if from areas where there is never, or rarely, snow in winter. The changes in color are effected by molt, one in autumn and one in spring. Animals that are brown in winter undergo the same two molts as do those that are white in winter. The capacity to acquire a white coat or a brown coat in winter is an hereditary matter just as one man grows red hair and another grows black hair. In the weasels, however, all individuals in the north turn white in winter and if one that was born there is kept through successive winters in the warmer south where there is no snow, he will still turn white each winter. A weasel born in a southern area, where all are brown in winter, molts into a brown (not white) winter coat even when kept in a cold, snowy, northern area where native weasels of the same species all turn white. Obviously, therefore, neither snow nor temperature is an immediate cause and, as we have said, the color in winter is a matter of heredity. The time of the molt, we now know, is determined by the amount of light. When nights grow longer and days shorter, a point is reached at which the lesser light received through the eyes causes the pituitary gland to cease producing a gonadotropic hormone. Directly or indirectly, the lack of this hormone stimulates molt and, probably enzyme action, or the lack of it, causes the melanoblasts of the cells in the hair follicle to be without pigment. Hence the hair grown from a follicle under such conditions lacks pigment (melanin) and is white. In spring, as the days grow longer and the nights shorter, the increasing amount of light received day by day through the eyes stimulates the pituitary gland to produce the gonadotropic hormone which directly or indirectly, stimulates molt and, probably by enzyme action, the melanoblasts are caused to be present in cells of the hair follicle and the melanoblasts provide granules of melanin pigment which are incorporated in cells of the growing hair. These granules of pigment give the hair its color.
Evidence in support of this hypothesis is given below.
Along the Pacific Coast from British Columbia southward,M. erminea(see fig.25on page95) is brown in winter. This is an area where snow rarely falls and the temperature in winter ordinarily is above freezing. In the remaining part of the American range of this species the temperature in winter is below freezing much of the time and snow remains throughout the winter or for long periods. In this colder part of the animal's range, only white coats occur in winter.M. frenatalikewise has a white coat in winter in the part of its geographic range where snow and freezing temperatures prevail throughout most of the winter and a brown coat in warmer, snowless areas to the southward and along the Pacific Coast. The third species,M. rixosa, exhibits a corresponding correlation between coat color and climate. On the Asiatic continent, several species, includingM. erminea, provide parallel correlations and nowhere are there any exceptions for the subgenusMustela. These data are an important part of the material on which we have based the induction that the underlying cause of seasonal change in color is a need for protective coloration.
As regards molt, most naturalists who have written upon the subject regard it as responsible for the change from the white winter coat to the brown summer coat. However, the change from brown summer coat to white winter coat has been thought by several writers to be effected by change in coloration of the individual hairs. Among those holding this opinion there may be cited Bell (1874:197) in reference toMustela erminea, and Coues (1877:123) in reference to American specimens to which he applied the same name. More lately Hadwen (1929) has taken this same view, and Gunn (1932) also discusses the possibility of the hairs changing color. Bachman (1839:228-232), Macgillivary (1843?:158), Audubon and Bachman (1851 (vol. 2):62), Schwalbe (1893:538), Pearsonet al.(1913:447), Miller (1930, 1931A), Hamilton (1933:300) and Rothschild (1942), among others, have been inclined to the opinion, or positively affirm, that the color change in autumn is the result of a molt. The papers cited above contain, in turn, references to many other printed accounts dealing with this question.
To my mind, it has not so far been demonstrated that the change in color of weasels in autumn is accomplished without a molt. Also so far as I am aware, no explanation has been given of how the pigment may disappear from the hair of weasels. Metchnikoff's (1901:156) idea that the senile whitening of the hair in man is accomplished by phagocytes which remove the pigment granules would hardly seem to explain the relatively sudden and complete autumnal change occurring in weasels. Anyhow, Danforth (1925:108), and some other students have thought that the action of these phagocytes was at most a factor of slight importance in the whitening of hair. Whatever be the complete answer to the question of how the weasel changes color in autumn, at least one specimen of long-tailed weasel, which is in process of color change in autumn, presents clear evidence of molt of the overhairs. This specimen ofM. f. longicaudais no. 188408, U. S. Nat. Mus., taken on November 12, 1897, at Rapid City, South Dakota. Other specimens ofM. ermineawhich were taken in autumn similarly show molt to be in progress. For these and other reasons, I am inclined to the opinion that the autumnal change in color, like the one in spring, is effected by molt. During the period of the autumnal color change, Noback (1935:27) had a captiveM. f. noveboracensisand, each morning, found clumps of brown hair on the floor of its cage; this was strong indication that molt was responsible for the color change in this instance.
However, I freely admit that the evidence does notprovethat the change from brown to white can be accomplishedonlyby molt; in the present state of knowledge it would be unscientific to deny that the change were possible of accomplishment by other means. Also, it is true that the fifteen specimens before me ofMustela frenata, subspecies included, in process of change from brown to white, with the exception of the one from Rapid City, South Dakota, if taken individually, do not, in macroscopic examination, show definite molt lines or other absolutely convincing evidence of molt. However, these same specimens, insofar as examined microscopically, do show overhairs all white, or overhairs pigmented throughout. The lighter color of the proximal parts of the overhairs in itself should not be accepted as evidence of color change, for in the fresh summer pelage, the same condition exists. Also, careful macroscopic examination suffices to show that in the transitional pelage of autumn, the brown overhairs generally are longer than the intermixed white overhairs.
Whether the underfur behaves in exactly the same way as the overhair, I have not myself definitely ascertained, but I assume that the underfur is molted twice each year, at least in the northern populations ofMustela frenataand in the other species of more northern distribution. Schwalbe's (1893) work, including sectioning of the skin and study of the hair follicles, led him to conclude that the underfur was molted twice each year inMustela erminea.
InMustela frenata noveboracensis,M. f. nevadensis, andM. f. nigriauris, measurements taken on adult males show the overhairs to be longer in the winter pelage than in the summer pelage of specimens from the same locality. For example, inM. f. nigriaurisfrom Berkeley, California, the overhairs of the summer coat (July and August) average 8 millimeters in length on the hinder back and 7 mm. on the belly, but average 9.5 mm. and 8 mm. respectively in January-taken specimens possessing the full winter coat. At Ann Arbor, Michigan, in the summer coat, the longest hairs on the hinder back average approximately 12 mm., and those on the belly, 9.5 mm., against 13 mm. and 9.5 mm. respectively in winter. Although general observations initially led me to believe that the black, terminal hairs of the tip of the tail are longer in the winter pelage than in the summer pelage, actual measurements fail to show a difference in length.
The change from one coat to the other in the long-tailed weasel has been described among others by Miller (1930, 1931A), Hamilton (1933) and Glover (1942) on the basis of captive specimens. In a general way, the progress of the molt in their specimens agrees with that which I have been able to make out from examination of skins taken in the wild. There is, however, this difference: Their specimens show a more spotted pattern when in process of hair-change than do specimens taken in the wild. Probably the more or less unnatural conditions under which these captive animals lived modified the normal progress of molt.
In wild-taken specimens of the speciesMustela frenata, subspecies included, the spring molt begins on the mid-dorsal line and proceeds laterally, producing, at almost any given time, a relatively sharp molt line separating the white winter hair from the incoming brown summer coat. However, in autumn the change takes place first on the belly, then on the sides, and finally makes its appearance over all the upper parts at about the same time, with the result that the upper parts have a salt-and-pepper appearance without at this time any sharply defined molt lines. In general, the molt pattern can be said to be reversed in the two seasons; in spring, it begins on the back and in autumn, on the belly. The difference in spring and autumn color pattern is better illustrated on plate39than by additional description. Swanson and Fryklund (1935:123) have observed that the "spring molt proceeds differently" than the fall one inMustela rixosa, and Barrett-Hamilton (1903:309) in commenting on the European hare (and the stoat?) remarks, "In spring the moult, and with it the brown colour, progresses in exactly the opposite order . . ." as compared with the white color of autumn, which that particular writer thought resulted from removal of pigment from the hairs rather than from molt.