ALLOMETRIC ANALYSIS OF CRANIAL PARAMETERS OF AMERICAN MINK, INCLUDING BONES OF MASTICATORY APPARATUS

Hubert Jakubowski¹, Marcin Komosa¹, Hieronim Frąckowiak^2
1 Department of Animals Anatomy, University of Life Sciences in Poznań, Poland
² Institute of Zoology, Department of Animal Anatomy, Poznań University of Life Sciences

ABSTRACT

The studies aimed at demonstration of uniqueness of American mink masticatory apparatus. The craniological relationships were defined in a sample of  25 skulls of adult minks (18 females, 7 males). The standard data, including  21 measurements, provided material for subsequent statistical analyses. The results demonstrated sexual dimorphism of some crania parameters. Most of characters of masticatory apparatus manifested high correlation with the total cranial length. In turn, equations of allometric analysis were constructed, in which the total cranial length was selected to represent the independent variable. Total lengths of dental rows, maxilla and mandible were markedly abbreviated, as compared to the total cranial length. In parallel, pronounced development was noted of the posterior part of mandible, which pointed to an extensive area for attachment of masticatory muscles. Length of hard palate exceeded markedly the length of dental rows. An isometry or proportional growth was noted  of the total cranial length and the length of hard palate.

Key words: craniometry, skull, allometry, American mink.

INTRODUCTION

Both morphologically and functionally, the skull is one of the most complex structures in the body [15]. Total skull length was considered to represent an independent variable, reflecting overall size [16,17,20]. Allometric relationships can be useful for evaluating body size on the basis of individual measurements of body parts [18].

Determination of a relation between increase in one parameter as related to the other ones is known under term of allometric scaling. In morphological sciences the scaling permits to describe proportions of animal body. The term of allometry is derived from the Greek word of  alloios, which means "distinct". Thus, allometry describes differences in relationships between studied parameters, including parts of animal body. The isometric growth, in which various body parts grow at the 1:1 ratio, or at the same rate, is relatively rare in living organisms.

Allometric analysis can estimate the covariation of characters [5] and provide a method to explicate the relationship between processes of growth and evolution [2]. Morphometric allometric relationships have been developed for bivariate allometric equations and for a multivariate generalization of the bivariate allometric equation. The formula of bivariate allometry [11] assumes a power function of the form y = ax^b  where x and y are measurements and the constant b is often called the allometric coefficient. The special case when b = 1 is called isometry [19].

The studies aimed at presenting specific nature of masticatory apparatus structure in the American mink and of metric relationships between craniological parameters, using allometric scaling.

MATERIAL AND METHODS

The studies were performed on  25 bodies of American mink, including 18 females and 7 males, obtained from a large breeding farm. The animals manifested a variable age.

Heads were isolated and subjected to thermic processing. Cranial measurements were taken using the standard technique of  von den Driesh [6]. The craniometry took advantage of slide caliper of accuracy up to 0.1 cm. The skulls were measured in two planes: the dorsal and the ventral one, the measurements were also conducted on bones of the mandible. The total of 21 measurements were taken, denoted by letter symbols (Fig. 1, Table 1). The numerical data were subsequently subjected to the following analyses: t test, correlation analysis and allometric regression (Fig. 2). The numerical material of cranial measurements was subjected to statistical appraisal using Statistica software, version 6.

Allometric regression was described by the following equation:  y = ax^b, equivalent to the logarythmic equation of
log y = log a + b log x

Where:
y – dependent variable
x – independent variable
a – constant (allometric coefficient)
b – allometric exponent  (points to a relationship between x and y variables; represents a slope of the straight line).

RESULTS

Evaluation of sexual dimorphism
Analysis using t test demonstrated sexual dimorphism within 8 parameters, including  cranial length (A-P), length of neurocranium (Ect-A), zygomatic width of cranium (Zy-Zy), the lowest height of mandible body between premolars 1 and 2 (HP₁-P₂₎, the highest length of mandible (Id-Goc), height of mandibular ramus (Gov-Cr), the highest width of sockets for first molars (Mol-Mol), the greatest breadth of the condyle process (BCP).

Values of the parameters allowed to conclude that female skulls of American mink are smaller than skulls in males. The most pronounced sexual dimorphism occurred in the features such as cranial length (A-P), length of neurocranium (Ect-A) and zygomatic width of cranium (Zy-Zy). These traits revealed maximum values higher by about 3 mm in males than females. The remaining features, as indicated by t test analysis, demonstrated insignificant differences between males and females.

Excluding the t test analysis, some of the cranial features could be concluded to demontrate values slightly higher in females than in males. These traits included length (St-P), greatest breadth of the palatine (GBP), alveolar length of premolar tooth row (LPTr), alveolar length of molar tooth row (LMTr), distance between sockets for mandibular canines (BC) and the greatest breadth of molar tooth row (BMTr). The traits mentioned above involved a relationship between teeth and the palate.

In parallel, the analysis distinguished 12 traits which manifested no sexual dimorphism. This allowed to maintain significant size of the sample (Table 2). At subsequent stages of the studies the traits were used for the group of males and females forming a single set.

Traits which demonstrated no significant inter-gender differences were subjected to analysis of linear correlation according to Pearson, in relation to the highest cranial length (Cb-P). The analysis distinguished 9 variables which manifested high correlation with the Cb-P variable (Table 3).

Variables which manifested low extent of correlation (< 0.40) were excluded from subsequent analyses. These traits involved lateral length of visceral cranium (Zl-P), alveolar length of molar tooth row (LMTr) and mandibular  bicanine breadth (BC).

Allometry
The model of Huxley [11] is the one of the most useful methods to study the allometry phenomenon. The subsequent stage involved analysis of allometric regression. For the analysis, the variables highly correlated with the maximum cranial length (Cb-P) variable were used. In the studies the maximum cranial length of Cb-P was used as  the independent variable.

Coefficient b value of 1 points to isometric growth of analysed parameters, value of b higher than 1 points to positive allometry while the values below 1 are typical for negative allometry. Individual values for the consecutive traits are listed in Table 4.

Most of the features directly related to the visceral cranium demonstrated the negative allometry. These features involved the maximum width of mandible (GBR), the maximum width of molar socket (BMTr), facial length (Ect-P), alveolar length of tooth row (LTr), alveolar length of premolar tooth row (LPTr) and the greatest breadth of the palatine (GBP).

Positive allometry was found in two features, including the maximum height of mandible body (HM1) and distance between canine sockets in maxilla (BB). Isometric growth occurred only in the length of visceral cranium base (St-P). Simultaneously, the neurocranium underwent intense extension.

DISCUSSION

The studies revealed that most of cranial parameters demonstrate larger values in males than females. The male skull of Mustela lutreola is characterised by a relatively high neurocranium, widely arranged zygomatic arches, a wide rostrum, and by wider auditory bullae and higher mandibles [1].

Masticatory muscles in carnivores manifest extremely vast development. In such animals the mandibulo-temporal joint moves mainly upward and downward, acting like scissors [7]. Radinsky [16,17] suggested that Mustelidae may manifest the strongest occlusion among all carnivores as they manifest relatively vast temporal muscles, strong maxilla and mandible.

Studies of Radinsky [16,17] and Greaves [8,9] revealed a general model for carnivorous mammals, which possess carnassial teeth. One of the arguments involved the fact, that the most pronounced shearing force developes at the carnassial tooth level, when the muscular resultant force acts one-third of the way along the mandible and the carnassial teeth are situated half-way along the jaw.

The animals prey on small mammals (musk-rats, hares, shrews) as well as on frogs, invertebrates and, equally frequently, on fishes and aquatic birds. Minks consume also eggs of birds [3,4].

This has been confirmed in present studies since maximum height of mandibular body demonstrated positive allometry in relation to the total cranial length. Undoubtedly, the traits make Americal mink a deft hunter.

The trait related to the length of visceral cranium base (St-P) revealed isometric growth in relation to maximum cranial length (Cb-P). In African wild dogs, similarly to domestic dogs, the length of mandible demonstrates an isometric growth in relation to cranial length [13].

Most of analysed traits manifested a negative allometry, which might be linked to abrupt growth of the total cranial length and of cerebral cranium in particular. This might reflect the fact that in prenatal and early post-natal periods nervous system and its surrounding structures grow much more rapidly than facial skeleton does [14].

Our studies revealed that some of the features related to teeth and the palate show larger metric values in females than males. Similar observations were made by Kieser and Groeneveld [13] who studied mandibulo-dental allometry in the African wild dog. However, their studies cannot fully explain this phenomenon, because this relationship has not occurred to a comparable extent in domestic dogs. One factor which might be of relevance is the increased dietary demand of pregnancy. It has been suggested that relatively larger teeth in females is an evolutionary consequence of the elevated masticatory demands placed upon them during pregnancy and lactation [10,12,21].

CONCLUSIONS

Positive allometry of the height of mandible body was decisively linked to growing surface area of attachment for the muscles of masticatory apparatus. Most of craniometric traits subjected to analysis of allometric regression demonstrated a negative allometric growth. The allometric growth was expressed in marked elongation of the entire skull, in particular in elongation of temporal fossae, providing vast areas for attachment of temporal muscles. The phenomenon of isometry was observed in the case of the total length of osseous palate (St-P), which was also typical of other carnivores. The characteristic traits included also the fact that length of dental rows manifested a negative allometry versus the Cb-P parameter. Moreover, dental rows in adcaudal parts of maxilla did not reach the level of choanae (posterior nares). In females certain traits linked to teeth and palate manifested values slightly higher than those in males.

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Accepted for print: 25.06.2008

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