This week we are talking about the monk seals! Though there used to be three distinct species in the modern era, there are now two extant species left: the Hawaiian and Mediterranean monk seals, with the Caribbean monk seal now extinct. With the last known sighting in 1952, the Caribbean monk seal was found in (you guessed it) the Caribbean and surrounded waters until driven to extinction by humans for purposes of meat and oil. Little is known of the species because of the lack of information acquired before their extinction. The Hawaiian and Mediterranean monk seals on the other hand are still extant, though their populations are also in danger of extinction.
Beginning with the Hawaiian monk seal, an endangered seal with only approximately only 1,400 members left, this species is predictably found in and around the Hawaiian archipelago. With a grey coat and white bellow, this seal is fairly slender with small flat head and shorter snout compared to other species, a feature making it easier to identify. Hawaiian monk seals can grow 7-8 feet long (females growing slightly larger than males) and weigh up to 600 pounds! Pups are born around 30 pounds but, with the constant supply of milk from nursing mothers for 4-6 weeks, can grow to an astounding 150-200 pounds in that time frame.
The Mediterranean monk seal is very similar in appearance as the Hawaiian, growing up to 7 feet long and 700 pounds with the males tending to grow larger than the females in this case. Though the Mediterranean and Hawaiian appear very similar, there are some specific differences between the two. For example, the Hawaiian monk seal’s nostrils face forward, while the Mediterranean’s nostrils face upward! Perhaps the easiest distinction between the two however would be their coats. Males are colored black while the females are dark brown or grey, but their pups are the most distinguished. Female pups are born with a rectangular white strip on their bellies, while the males develop a butterfly shaped white stripe! Suffering with a smaller population across a larger range, the Mediterranean monk seals numbers have dwindled to less than 700 members. Ranging from a population of 250-300 animals in the Aegean Sea off Greece and 270 off the Western Sahara in the Atlantic Ocean, the seal is widely distributed with distinct larger populations and smaller groupings scattered within this range.
Monk seals are generalists – they aren’t picky eaters! Hawaiian monk seals eat many common fish, squid, octopus eel and crustaceans (though lobster in particular is not really part of their diet) that are near the seafloor and feed day and night. They prefer prey that hides in sand or under rocks – but they are careful and don’t seem to damage live coral they may be feeding around. They can hold their breath for up to 20 min and dive for up to 1800ft, but more commonly do shorter, shallow dives about 6 min and less than 200ft. They need to eat 4-8% of their body weight (or about 15lbs) daily! Interestingly, unlike other seal and sea lion species, they do not eat prey that are also targeted by fishermen. Mediterranean monk seals are similar, feeding on many fish (like eels, sardines, tuna, flatfish, mullets) and cephalopods (squid and octopus), but also lobster (unlike their Hawaiian cousins). These seals can dive up to 18 min and about 600ft, but mostly do shorter shallower dives at around 90 ft and may eat prey that is also targeted by fisherman.
Behavior & Breeding
Monk seals are usually solitary, although they will sometimes lie near each other in small groups they are careful not to touch each other, and may continually return to a particular beach. They spend most of their time at sea, about 2/3 of their time! They come to shore to rest (some will sleep for days on the beach in Hawaii), and to give birth. Females are sexually mature at 5-6 years, but it is unknown when males become sexually mature. Pregnancy lasts 9-11 months. Hawaiian monk seals give birth in March/April, and Mediterranean monk seals give birth in autumn – those for both species pups have been observed year round.
The biggest difference between these two is where they pup and how long they nurse. Hawaiian monk seals give birth on sandy beaches surrounded by shallow water, and nurse for only 4-6 weeks (the mother fasts during this time, and can lose a lot of weight) before sending the pup off to live on their own. The Mediterranean monk seal, after being hunted and populations decimated, needed to find a safe place to pup. They moved from pupping on beaches, to finding hidden caves often with underwater entrances (check out this article about newly discovered breeding caves!). These are safe from people, but still a danger to the pups if the water rises too high and washes the pup away. This would happen when the mother leaves the pup to go forage. She needs to do this as they nurse up to 4 months, 4 times as long as the Hawaiian monk seal, that is a long time to have to fast!
One last interesting behavior is that monk seal females have been known to foster offspring of other seals. This isn’t often see in nature, as that is a lot of work to put in to an offspring that is not your own (and isn’t spreading your genes into the population!). Thus it is interesting, and unique that these female monk seals choose to help these pups. Perhaps as we learn more about this species the why of this behavior might become clear!
The monk seal is the most endangered pinniped in the world! The Hawaiian monk seal is currently listed as Endangered under the Endangered Species Act, is covered under CITES Appendix 1, and is listed as “Protected” and “Depleted” under the Marine Mammal Protection Act (MMPA). The Mediterranean monk seal is listed as Endangered under the IUCN Red List. Although this is still a highly endangered population, they were actually reduced from “Critically Endangered” only recently in 2015, which is encouraging!
- Limited Food: Reductions in prey has negatively impacted both species of monk seal. As a relatively docile species, they may also be outcompeted by other large predators that may predate on the same food sources, leaving the monk seals with less. Particularly in the Mediterranean, seals also have to compete with commercial fishermen for much of their prey.
- Fishery Interactions: Speaking of fishing, another threat to monk seals (and especially the Mediterranean monk seals) is fishery interactions. This often means entanglement in fishing nets, marine debris, or abandoned gear, cases of which remain high today. Unfortunately for monk seals, fishery interactions also means the intentional killing of seals by fishermen who perceive them as pests or blame seals for stealing their catch. This is a huge source of mortality to monk seals in Greece and Turkey. However this isn’t the case everywhere: in Hawaii local fishermen have begun coming up with “best practices” to reduce bycatch of seals, and encourage local fishermen to use methods such as spear fishing that are less harmful!
- Intentional Killing: In addition to being killed by fishermen, monk seals are also at risk of intentional harm by humans for other reasons. Historically, their exploitation was far-reaching (as mentioned in the Introduction), and although commercial harvests of seals has ceased illegal killing for sport or other unknown reasons still occurs for both species.
- Habitat Loss: Monk seals need beaches and coastal areas, which unfortunately means they are at a greater risk of displacement due to coastal erosion, washing away of beaches during storms, and encroachment of industry and human development. Climate change plays a large part in this factor due to sea level rise and the increased threat of stormy weather, with the Northwest Hawaiian Islands being particularly susceptible to erosion due to storms.
- Male Seal Aggression: One little-known threat is actually other seals! Male monk seals have been reported to attack and even kill females and juveniles, which is a huge problem in an already depleted population. Although these cases seem to happen sporadically, NOAA Fisheries have taken action when instances occur to remove aggressive males and relocate them, or relocate young seals away from dangerous males. It is unknown why these normally placid seals would occasionally attack conspecifics.
- Predation by Sharks: Killer whales (orcas) and sharks are the two primary predators that seals are susceptible to, however in the Hawaiian Islands researchers have found that some Galapagos sharks have begun to specifically target new or recently weaned pups! These individual sharks will wait in the shallow waters near a beach where monk seals are pupping, grabbing the young seals when they enter the water. This is remarkable as the Galapagos shark typically does not predate on seals, but this has become a significant source of threat for the Hawaiian monk seals.
- Human Interactions: Although we mentioned some of the more drastic issues with human interactions (aka killing!), other less violent interactions can also be a source of threat. People approaching mothers and pups for pictures, swimming with seals, or disturbing them during haulout periods is a significant source of concern for both species of monk seal. Disturbing any wild animal during its period of rest is highly detrimental, but unfortunately many of the beaches frequented by these seals are also popular with people! This is also a concern where pets (especially dogs off leash) are concerned, as they may attack or kill seals on the beach, or may themselves be injured by a seal protecting her pups. As with any seal you encounter, if you happen across a monk seal please give them lots of space and leave them be!
- Disease: The final, and possibly most catastrophic, threat we will discuss is disease. Monk seals are particularly susceptible to several diseases including morbillivirus (i.e. phocine distemper virus, which jumped from canids into seals), West Nile virus (transmitted by mosquitos), leptospirosis and toxoplasmosis. Monk seals do not have natural antibodies to these diseases therefore have limited ability to fight them off. Biologists in Hawaii have actually begun to vaccinate wild monk seals against morbillivirus – check out more here, or listen to this podcast episode from our friend Michelle from the Aquadocs Podcast all about Hawaiian monk seal vaccination!
Toxoplasmosis is a particularly big problem for the Hawaiian monk seal. This disease is transmitted through feline excrement, which enters the marine system through incorrect disposal of kitty litter. As cats were introduced to the Hawaiian Islands and are not endemic, the monk seals have little to no immunity to toxoplasmosis now in their environment. Since 2001, there have been a minimum of 8 deaths per year due to toxoplasmosis in Hawaiian monk seals, a statistic which is likely to get worse as more people move to these islands.
- Where did the name “monk seal” originate? Well the name was suggested by naturalist Johann Hermann in 1779, when he published the first modern scientific description of the species after observing a captive seal that was in a travelling show in Strasbourg. Hermann remembered a paper he’d read that described a similar seal which was known in Marseille as “moine”; he assumed they must be similar species thus he suggested naming the animal Münchs-Robbe (Phoca monachus). Hermann also noted that the seals themselves resembled monks with their rounded heads and hunched shoulders when they came up to the pool edge, so he “judged it a reasonable name, and saw no reason to change it”!
- The Hawaiian name for monk seal translates as “the dog that runs in rough waters”.
- Monk seals are the only earless seals found in the Tropics.
- Monk seals can live to at least 30 years old.
- Like many phocids, monk seals shed their top layer of skin each year during what’s known as a “catastrophic molt”. It looks pretty unsightly with skin and hair sloughing off, but this is a normal process and the seals are absolutely fine.
- Although now extinct, Caribbean monk seals were known to have algae growing on their pelage, giving them a slightly green appearance! Similar observations have been made in Hawaiian monk seals as well.