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Monk Seal Fact Files

Mediterranean Monk Seal

(Monachus monachus)

Biology

External appearance and anatomy

Mediterranean monk seals have adapted well to life in their aquatic medium. Their body is torpedo-like, while the head is rounded, with a protruding muzzle.


Distinctive traits

A combination of specific external and internal characteristics makes Mediterranean monk seals unique and distinguishes them from other members of the order Pinnipedia. According to Lavigne (1998), these traits include:

  • The pelage of the species
  • Primitive skull features
  • Chromosomal constitution
  • The presence of 4 teats (in contrast to most pinnipeds that have only two)
  • The smooth vibrissae (whiskers)

Other authors have also noted the marked difference in size and weight of the species, and its dentition.


Size and weight

Often acknowledged as being amongst the largest species of “true” seals, adult Mediterranean monk seals average 2.4m in length nose to tail  and are believed to weigh approximately 250-300 kg (Marchessaux 1989, Boulva 1979).

Average length at birth is 94 cm (range: 88-103 cm Marchessaux 1989). The average weight at birth is 15 to 20 kg (Marchessaux & Pergent Martini 1991, Dendrinos et al. 1999). Up until weaning, growth is rapid, involving a significant increase in size within two weeks (Caltagirone 1995).

Despite an established average adult length ranging between 2-3 m, some older reports also record monk seals larger than 3 m (Carus 1893, Boulva 1979, Ibanez 1981, Reiner 1981, Smit & Wijngaarden 1981), with Reiser (1912) even suggesting an unlikely length of up to 4 m.

This significant variance in body length observed can be attributed to various factors, including errors in translation and differing measuring techniques (e.g. nose to tail versus nose to tip of extended hind flippers).

Long term studies at the monk seal colony of Cabo Blanco in Mauritania/Western Sahara indicate that males are only slightly larger than females (Samaranch & González 2000).


Flippers

Monk seal fore flippers have claws approximately 2.5 cm long on the first digit, decreasing in length towards the fifth digit. The claws of the front flipper are well developed, those on the back, however, are very small. With elongated first and fifth digits, the hind flippers are concave in shape (King 1983).


Vibrissae or whiskers

Highly sensitive, the whiskers or vibrissae of (monk) seals may help detect fish movements through the water and thus aid in hunting (Dunn 1978, King 1983).

There are 5 rows of vibrissae on each side of the nostrils, each row having from 2 to 8 vibrissae of various length and thickness. Their basic colour is light yellow to dark brown and towards the rear the colour becomes lighter receding to a straw-like yellow (Schnapp et al. 1962, Ronald 1973). The whiskers are smooth and oval in cross section (Ronald 1973).


Pelage

Apart from the pups, which possess a soft and woolly pelt or ‘lanugo’, juvenile and adult Mediterranean monk seals have very short and bristly hair (about 0.5 cm long; the shortest hair amongst pinnipeds, Ling 1970), which lays close to the animal’s body, thus forming a close-cropped pelt (Ronald 1973, Boulva 1979). Scars, which are distinctive of adult seals, are the result of interactions with other individuals and the environment (Forcada & Aguilar 2000). Dorsal scars are more frequent in females, suggesting that they are inflicted by males during the mating season (Grau et al. 1994, Samaranch & González 2000). Ventral scars, in contrast, especially in the area of the neck, are more frequently observed among males and appear to result from fights during the mating season.

The Mediterranean monk seal is characterised by marked variations in external appearance between different development stages. González et al. (1996) and Samaranch & González (2000) distinguished six such types:

  1. Large Black Males: As the name suggests, this group consists exclusively of large black adult males. Their overall black pelage, which appears around the age of 4, is interrupted by a white belly patch near the umbilicus, a whitish well-developed throat and numerous scars. The patch may sometimes stretch to the dorsal part of the body.
  2. Large grey seals: A uniformly greyish to brown pelage with numerous scars on the back is usually characteristic of large adult females. On occasion, however, some males that later moult into large black males may also be included in this category.
  3. Medium sized seals: The colour of the pelage of this group, which includes mainly medium sized sub-adults of both sexes, is variable and has fewer scars than the two previous groups.
  4. Juveniles (7-23 months): With a brown to dirty greyish pelage and few visible scars, members of this developmental type have a small, thin and elongated body.
  5. Youngsters (70 days - 9 months): Pelage is similar to juveniles in colour (light grey). However, ‘youngsters’ are usually smaller than, or as large as, juveniles, with a roundish body.
  6. Pups (0-70d): Mediterranean monk seal pups have a black pelage and a white or yellowish patch, large and squarish, around the umbilicus, which is distinctive of the species and is not present in the other two monachine species. The hair of the pelage is soft and woolly and approximately 1.0-2.0 cm in length (Boulva 1979). The patch is often marked by black spots (Dendrinos et al. 2000) and may vary in shape, size and position between different individuals and according to gender (Badosa et al. 1998). Such differences in the patch, combined with differences in the colouration of the fur, body form and size, enable field researchers to identify and closely monitor monk seals in the early stages of their life (Dendrinos et al. 2000).


Dentition

The dentition of the Mediterranean monk seal comprises four incisors, two canines and ten molars in each (upper and lower) jaw (Ranzani 1823, Carrucio 1893).

The incisors are characterised by their large size and a small ridge located at the rear of the tooth (Duguy & Marchessaux 1992), whereas the milk dentition differs through the absence of two molars (Ronald 1973, Boulva 1979). Compared to the Hawaiian monk seal, dental development in Mediterranean monk seals is delayed, starting at the age of 2-3 weeks, does not follow a well-defined tooth eruption pattern and does not appear to be associated with the health or nutritional condition of the newborn (Androukaki et al. 2002).


Internal anatomy

Aristotle is the first known figure in history to provide information on the anatomy of the Mediterranean monk seal. His detailed descriptions in the fourth century BC, considered generally accurate to this day, suggest that he studied specimens with care (King 1956, Johnson & Lavigne 1999a).

The species’ anatomy has generally received only little or fragmented scientific interest in more recent years. Apart from a general overview contained in King’s 1956 monograph on the genus, such information has been provided by following authors:

  • Alessandrini (1819) provides a detailed description of the anatomy of the birth tract.
  • Dieuzeide (1927) gives a thorough account of the species’ skeleton, muscles, digestive system, breathing and cardiopulmonary system.
  • Marcoci & Popa (1957) provide an overview on the internal anatomy of the Mediterranean monk seal.
  • Ronald (1973) provides measurements and describes the location of the internal organs of a dead juvenile monk seal.
  • Schnapp et al. (1962) give measurements and descriptions of the liver, brain and kidney of a dead monk seal from the Black Sea.
  • Cebrian et al. (1990) describe the digestive system of a dead monk seal from Santorini, Greece.
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