Application of cranial morphology as a tool in science

1.2.1 Types of cranial characters

Two different types of skeletal characters are distinguished in morphological studies: metric (quantitative) and non-metric (qualitative) characters (Cheverud et al., 1979).

1.2.1.1 Metric characters

Conventionally, metric characters, which refer to overall differences in size and shape of bones, are measured with vernier and / or digital callipers (Perrin, 1975; Chen et al, 2008). Nowadays, the use of geometric analysis as a tool for morphometrical studies is becoming increasingly widespread (e.g. Marcus et al., 2000; Galatius et al., 2011; Goswami et al., 2010; Nicolosi & Loy, 2010). Geometric morphometrics is based on morphological landmark capture with, for example, a digitizer such as a microscribe, and subsequent data transformation and analyses (Klingenberg, 2011). Several precision estimates (including the mean absolute difference (MAD)) can be computed to assess measurement error (ME) and precision between linear measurements obtained through different data acquisition methods (Weinberg et al., 2004).

1.2.1.2 Non-metric characters

Since the late 1800s, minor variations in cranial and post-cranial skeletal traits such as teeth, ridges, fenestrae, and foramina (openings for blood vessels and nerves) have been increasingly included in anthropologic studies (Ansorge, 2001; Berry, 1975; Cheverud, 1979; Le Double, 1903). These discontinuous skeletal variants are known as non-metric characters

13| P a g e

and are coded by state, such as presence / absence, total number, or as relative position to another skeletal feature (Le Double, 1903; Berry, 1975; Perrin et al., 1982).

1.2.1.3 Cranial sutures and skull growth

Cranial bones are still unfused after birth and fuse with age (Rommel, 1990). In addition to the two skeletal characters aforementioned, degree of closure of suture lines is also frequently examined as part of morphological studies to infer a specimen’s age and assess maturity status (Van Waerebeek 1993; Gonzalez, 2002; Galatius et al., 2011; Chen et al., 2011). Advancement of suture closure is commonly assessed through scoring, where higher scores are given for more advanced stages of suture fusion (Van Waerebeek, 1993; Galatius et al., 2011). Growth of cranial bones and functional complexes can only be determined for individuals of known age (Perrin, 1975; Noldus & Klerk, 1984).

1.2.2 Addressing taxonomy and geographic variation through cranial morphology

1.2.2.1 Cranial maturity and age estimation

Only skulls from individuals that have ceased growth should be included when determining morphological difference between species, populations, and sexes; and

when assessing individual variation in delphinids, because the rostrum and several other cranial features are not fully developed in immature individuals (Perrin, 1993). Fusion of both epiphyses to the vertebral centra provides evidence for physical maturity (Calzada et al., 1997). In the absence of post cranial skeletal material, only cranially mature specimens should

14| P a g e

be included for morphometric examination (Perrin, 1975; Perrin & Heyning, 1993). A specimen is said to have attained cranial maturity when growth of the skull has ceased. Distal fusion of the premaxilla with the maxilla (refer to Fig. 1.7) has been determined as a reliable indicator for cranial maturity in several dolphin species (Dailey & Perrin, 1973; Mead & Potter, 1990; Calzada et al., 1997).

Figure 1.7. Schematic description of rostral fusion on a delphinid skull adapted from Amaha, 1994. Note: Rostral fusion occurs between the premaxilla and maxilla (indicated by horizontal black line) on

both sides of the rostrum. Fusion starts at the tip of the rostrum and progressively extends backwards. Distal fusion: Premaxilla-maxilla fusion up to the anterior end of the fusion between the left and right

premaxilla.

Distal fusion is, however, not a definite indicator of cranial maturity in some species, including the short-beaked common dolphin, D. delphis (Perrin & Heyning, 1993), and Pacific white-sided dolphin, Lagenorhynchus obliquidens (Van Waerebeek, 1993). Owing to this uncertainty, biological data such as age, total body length (TBL), and sexual maturity are considered, if available, when deciphering cranially mature specimens (Perrin & Heyning,

15| P a g e

1993; Murphy et al., 2006; Westgate, 2007). Condylobasal length (total skull length), premaxilla-maxilla fusion over at least 50% of the length of the rostrum, the overall degree of cranial fusion (e.g. based on a suture index), and developmental status of alveoli have also been included as a criterion for maturity assessment, especially in the complete absence of biological data (Van Waerebeek, 1993; Jefferson & Van Waerebeek, 2002; Murphy et al., 2006; Westgate, 2007; Tavares et al., 2010, Pinela et al., 2011; Juri et al., 2012).

1.2.2.2. Sexual dimorphism

Before addressing taxonomic issues or investigating geographic variation, it is necessary to test morphological samples for sexual dimorphism, as differences in absolute and relative size is evident in metric characters of the cranium in several vertebrate species, including the common dolphin (Samaai, 2005; Murphy et al., 2006). Sex related differences might also be present in non-metric characters (Berry, 1975, Perrin et al., 1994; Brasili et al., 1999).

1.2.2.3 Taxonomy

Historically, species and subspecies status of animals was predominantly determined from morphological and geographical data (Robineau et al., 2007). A quantitative guideline states that separation at the sub-species level is justified if 75% or more members of a population

can be distinguished from all (> 99%) individuals of the overlapping population (Amadon, 1949; Patten & Unitt, 2002). However, disagreement still exists about the number

of characters to examine when assessing subspecies level and the level of, for example, morphological differences needed that justify discrimination at the species and subspecies level (Patten & Unitt, 2002; Taylor, 2005; Robineau et al., 2007). It is generally accepted that

16| P a g e

one or more non-overlapping metric cranial differences validate a species status, while overlapping modal differences support a sub-species status (Westgate, 2007).

Nowadays, molecular analyses are increasingly being employed in taxonomic studies (Reeves et al., 2004). However, one limitation of molecular techniques is the fact that sub-species status is more difficult to determine with molecular markers in conditions of continued genetic exchange (Haig et al., 2006). Furthermore, phylogenetic history might not always be detected in a given genetic locus due to natural selection (Taylor, 2005). As a result, morphological studies continue to remain an important part in taxonomy (Yurick & Gaskin 1988; Heyning & Perrin, 1994; Perrin et al., 1994; Wang et al., 2000), because they complement results from genetic studies and can provide insights into mechanisms driving morphological differentiation (Adams et al., 2004; Natoli et al., 2006).

1.2.2.4 Geographic variation

Non-metric characters are believed to be largely under genetic control and less affected by selection pressure as compared to, for example, metric cranial traits related to the feeding apparatus (Pankakoski & Hanski, 1989). As a result, non-metric characters are regarded as good indicators of the degree of genetic exchange between populations and are widely used in anthropology (Berry, 1975; Cheverud et al., 1979; Brasili et al., 1999). Non-metric characters have also been deemed valuable in other vertebrate studies addressing geographic variation, including studies on cetaceans (Kinze, 1985; Van Waerebeek, 1993; Perrin et al., 1994; Wiig et al., 2012). A common method to determine the degree of separation between populations is to compute the mean measure of divergence (MMD), based on the frequency of trait expression of a set of non-metric characters (Sjøvold, 1977; Green et al., 1979). However, in

17| P a g e

odontocete studies, comparison between degree of discriminative power of non-metric and metric characters in geographic variation analyses has rendered different results. While metric characters were found superior in a study addressing geographic variation in the common

dolphin (Perrin et al., 1994), the converse trend applied to a study on narwhals,

Monodon monoceros (Wiig et al., 2012). Perrin et al. (1994) suggested employing both metric and non-metric characters in geographic variation studies.

1.3 The common dolphin, Delphinus sp.