Quite a lot is known about basking shark courtship but very little is certain about their gestation or where they give birth
No scientist has ever dissected a pregnant basking shark and only one live birth of six 1.5-2m long pups has been observed. The terminology surrounding basking shark reproduction can be quite confusing but this should clarify it:
- They are live bearing (this is called being viviparous)
- The young hatch from eggs inside the uterus (this is called being oviparous)
- The developing embryos nourish themselves by eating eggs in the uterus (they are oophagous)
Courting basking sharks often congregate in groups of 2 to 12 off the West coast of the Isle of Man. You may be lucky enough to see courting behaviours such as paired swimming, following nose to tail, close following, or, if you’re really lucky, breaching (leaping out of the water). Picture: Pauline Oliver.
Scientists had assumed that basking sharks fed indescriminately but Dr Sims and his team found that basking sharks foraged preferentially and for longer in areas where their favourite shrimp prey species, Calanus helgolandicus, was more abundant than other species in the plankton. C.helgolandicus is a species of small (about 1-2mm long) shrimp called a calenoid copepod. It is about the same size as as half a gain of uncooked rice. When the scientists looked at the type of plankton present where basking sharks were feeding they found more and bigger plankton, with more of their preferred prey species, C.helgolandicus, than they did when sampling plankton where sharks were just swimming, not feeding.
Basking shark courtship behaviour and mating knowledgebase
It is wonderful that something as exotic as basking shark courtship behaviour can be seen from the shore on the Isle of Man. You will find an eyewitness account in the Isle of Man basking shark stories section of this website.
Dr David Sims team has provided a marvellous up to date account of basking shark courtship behaviour . Dr Sims’ team based their scientific paper on the courtship behaviour of basking sharks on observations of 25 separate episodes that occurred over a 5-year period. They concluded that courting behaviours occur along oceanographic fronts off Plymouth from May to July while the basking sharks are feeding off the rich plankton found at these fronts. The same seems to be true off the Isle of Man coast. Another group of scientists, travelling in a Coastguard helicopter off the coast of Novia Scotia, had a wonderful bird’s-eye view of 13 basking sharks engaged in courtship behaviour and of a possible mating. The sighting only lasted 5 minutes but they had video footage and photographs that they were able to analyse later.
Click on the links below to find out more about basking shark courtship, mating, pregnancy and much more.
- When do basking sharks become mature?
Previous to work done by Dr David Sims and his team it was thought that the female did not become sexually mature until 7-9m[17,21] and males were estimated to become mature at 4.6-6.1 m. Dr Sims and his team found that sharks exhibited sexual courtship behaviour when over 5m in length. Smaller individuals who were present did not engage in such behaviour.
- What is basking shark courtship behaviour?
Dr Sims describes basking sharks nose-to-tail following, close following, approaching one another’s flank, close swimming, parallel swimming, echelon swimming (where sharks follow one another but are slightly offset, not in a straight line) and breaching (leaping out of the water). They describe how only large basking sharks in groups of 3 or more breach repeatedly, leaping clear of the water. They propose that breaching is being used as male-male competitive behaviour during courtship displays. However, the only shark that they were able to definitely sex was a large female that breached 7 m from their boat, which must have been interesting! They propose the alternate idea that female basking sharks breach to signal their readiness for mating. The scientists in Novia Scotia describe similar courtship behaviour to Dr Sims’ team. They also mention that the basking sharks swam faster than usual and exhibited circling behaviour by up to 10 sharks at a time.
The Manx Basking Shark Watch Courtship Study 2008-2010
When the Manx Basking Shark Watch (MBSW) public sighting scheme began in 2005 it soon became apparent that there is a lot of social swimming, putative courtship behaviour, in Manx waters. Much of it is located in June and July off the West coast of the Isle of Man. MBSW has undertaken to study this behaviour. Here are some excellent photographs taken by local people who have observed these behaviours. IF you see any of these behaviours please leave the area as not only are you likely to disturb this important part of this endangered animals life cycle but it is also possible that one might breach, landing on your boat. this would be bad for you and bad press for the basking shark, which is only doing what comes naturally!
These basking sharks are close nose-to-tail following. We see a lot of this behaviour. Sometimes there are many animals in a line: Photo by Mal Kelly.
This photo was taken by Dr Fiona Gell of DAFF. It shows two sharks, one shark swimming parallel to the other but slightly behind the other. They often slip from parallel swimming to this and back again.
This photo by Adrain Corkill shows two sharks that are swimming in circles after one another.
This great photo of courting circling basking sharks was taken by Adrian Corkill. It would be easy to assume that this group of sharks is feeding in a circle because they have found a good spot of plankton. However, if you look closely you will see that some of them are VERY close together. If they stay this close for more than a minute or so assume that this is an intense courtship event and leave the area. At the very least drop back 100m or more to watch. Do this very carefully and slowly as there may be many more sharks feeding below your boat.
This great basking shark courtship photo was taken by Dermot Shimmin. Three sharks are swimming in VERY tight formation. Two are parallel swimming with the third shark forming a circle with the other two by nose to tail following.
This photo by Dr Fiona gell shows two basking sharks VERY close together indeed. These sharks had previously been identified as a male and a female. Unfortunately we did not get any underwater photographs of this. Colin, the male, was tagged with an archival MK10 PAT tag shortly after this event.
This wonderful photograph was taken by Adrian Corkill off Bradda Head. It shows in a courtship display, two basking sharks parallel swimming. Note exactly HOW close they are to one another. Sharks swimming parallel to one another but far apart are not courting like these are. You may see parallel swimming sharks get very agitated and start tail thrashing at this stage. Again, leave the area asap!
Again, these two sharks are close parallel swimming. Photo by Mal Kelly. Sometimes they are almost stationary in the water when they do this, then one starts tail thrashing.
On a lighter note this cartoon by Derek Pitman illustrates the problems of basking shark group behaviour. Although it is very nice seeing lots of sharks together it can become very tricky driving your boat. If you see one basking shark assume that there are many more around. there often are. It means that you must drive very very slowly with a look-out watching for sharks on or just below the surface. Remember it is against the law to harm a shark by careless driving.
- Basking shark mating
There has only been one scientific account of a possible basking shark mating but we have our own possible Isle of Man mating sighting documented in our ‘Isle of Man Basking Shark Stories’ section on this website! The MBSW scientific investigation of basking shark courtship hopes to film mating one day.
The scientists in the Novia Scotia helicopter saw possible mating behaviour where 2 sharks body curves matched one another. They saw one shark holding on to another by biting down on its pectoral fin. They also observed white-coloured marks on the dorsal area of the pectoral fins and behind the dorsal fin. They interpreted these as being bite marks made by males on females during mating. Although basking sharks are filter feeders they do have an impressive array of small teeth (see the section of feeding for more details). It would seem likely that these teeth are primarily used for holding on to their sexual partner during mating, as there is no evidence of basking sharks using teeth for catching any food items. When Dr Matthews dissected recently mated basking sharks he found many abrasions on the genital cloaca regions of sharks that were known to have been recently mated. He surmised that the claw on the males’ claspers caused abrasions on and in the females and that abrasions on both sexes were caused by the abrasion of the sharp sharkskin one against the other. Evidently basking sharks are not gentle with one another! We often see rubbed noses on mature basking sharks. This is probably caused by sharks rubbing against one another during courtship.
It is possible to identify the sex of a basking shark if you can see its ventral region (underneath). Dr Matthews found that the male has two long scroll-like claspers, which are used to introduce about 4 gallons of sperm into the female during mating. He dissected one female that had recently mated, which is how he knew how much sperm was used. He measured one clasper as being 1.09m long. It would seem that everything about basking sharks is done on a grand scale!
- How do basking sharks nourish their unborn young?
Is not clear how the basking shark mother nourishes her young so it is worth explaining some of the fascinating different methods that other shark species use to nourish their live-born young. Sharks are a very diverse group and they exhibit many different forms of reproduction. Some sharks lay eggs, others bear live young that are nourished inside the mother by egg yolk only. Others provide egg yolk and a rich milk-like substance called uterine milk. Some shark species are oophagous, the mother ‘lays’ many eggs into her uterus and her developing young eat them. Other shark species cannibalise their womb-mates; this is called adelophagy and can do little to improve the reputation of sharks! Some shark species exhibit a combination of strategies, the live-born young having been nourished by egg-yolk, intrauterine milk and by eating eggs whilst in the uterus. Finally, some sharks have placental structures that directly nourish their young.
When Dr Matthews dissected 6 female basking sharks he found that only the right ovaries were functional. He didn’t understand why the half-metre across ovary had 6 million eggs ready to be spawned when a female would only have a few live births in her lifetime. It now appears that this is because the eggs are used to feed the developing young. This is probably a rich eggy soup like some other shark species. This may explain the curious structure of the basking sharks nose (see section below with photographs and links to research suggesting this). The grooved nose may act like a straw helping the unborn shark to suck up the soup.
Dr Matthews also found that the uterus was lined with structures called trophonemata or ‘feeding threads’. Trophonemata are associated with shark species such as the butterfly ray in which these long nutritive threads feed the developing sharks directly into their oesophagus. It is possible that basking sharks adopt more than one strategy. Basking sharks might use a combination of nutrition from oophagagy (eating eggs) AND trophonemata (feeding threads) to nourish their unborn, developing young. As discussed before, other species of sharks use a combination of methods to nourish their unborn young but until a pregnant basking shark is examined this will remain a matter for conjecture.
- How long is a basking shark pregnancy?
It is not clear how long the gestation period of the basking shark is. Gestation in some other viviparous (live-bearing) sharks is known to be about 22 months. There have been various suggestions about how long basking shark gestations are. Some suggest 20 months others basing their estimate on an analysis of the age/length data suggest a gestation period of 3.5 years, another equally prominent scientist concluded that a 1-year gestation was more probable. The conclusion? It is not known how long a developing basking shark spends in its mothers uterus!
Where are basking sharks born and how big is a newborn basking shark? There are only three accounts of female sharks being found with live young in them[22,17,36], and one record of a basking shark birth. Given how many basking sharks have been butchered for their livers the lack of stories about pregnant basking sharks suggests that they might stay in deeper water or somewhere that is not normally targeted by fisheries. Dr Sims and his team reasoned that as young-of-the-year, (size 2-3m) do not appear inshore until mid-June that it was probable that young of 1.5m length were born offshore in deep water in May and June.
The oldest record of a pregnant basking shark female is by a T. Pennant writing in 1769 who wrote “ They are viviparous, a young one about a foot in length being found in the belly of a fish of this kind”. Dr Matthews also describes how a fisherman told him that when he opened a female he found a young one 6 foot long inside. A Norwegian fisherman described catching a female basking shark that gave birth to 5 live young and 1 stillborn which he estimated to be between 1.5 and 2m long.
When Dr Matthews dissected basking shark females he found that they had a 1-2m long uterus. The uterus size tallied with the very few accounts of the size of basking shark young found inside female sharks. The smallest newborn basking sharks are about this size so this is quite plausible. We have been extremely lucky to get a picture of a newborn basking shark that was seen off the West coast of the Isle of Man during July 2005. The picture shows the remarkable snub nose, a bit like a little pig.
This snubby pig-like nose with the underset lower jaw is so unlike the adult that unless it was so closely associated with the group of adults and so clearly feeding on plankton with them you would think it was a different species! Picture Shane Stigant July 17th 2005 Four miles off Dalby.
Armelle Jung of the French shark research Team APECS has very kindly let us use their picture of a very young basking shark that was taken off France. Note the similarities with the Manx picture.
here are some excellent photographs of a 2.9m long male basking shark at http://homepage.mac.com/mollet/Cm/CSteinbergImages/BaskingShark_js_ss.html (broken link)
These photographs of a small dead basking shark clearly show the same features in the ‘nose’ area. A most interesting feature is a small ‘nostril-like’ hole below the pointed peak of the nose area.
According to www.elasmo-research.org/education/ecology/rocky-basker.html (broken link) this is a physical adaptation to help them suck up the ‘eggy soup’ that nourishes them in their mother’s womb. The following text is pasted and copied from their site. Their copyright applies:
“Juvenile Basking Sharks offer yet another mystery: they have a peculiar elongated snout tip that looks remarkably like a wavy carrot. In some individual Baskers the carrot-tip waves upward, in others downward. This peculiar snout shape disappears by the time a Basking Shark reaches a length of about 16 feet (5 metres). A recent study examined the rostral (snout) cartilages of an 8.5-foot (2.6-metre) female Basker from Japanese waters. It revealed that the undersurface of the snout of a juvenile Basker has a groove that is continuous with the palate. This groove becomes very faint in specimens longer than about 10 feet (3 metres) and disappears completely by a length of 30 feet (8.5 metres). This grooved, elongated snout may help channel food into the mouth of very young Basking Sharks. Like other lamnoid sharks, Baskers supplement their diet during fetal development by imbibing a thick ‘soup’ containing thousands upon thousands of tiny, unfertilized eggs produced by the mother. The grooved, elongated snout may help channel these eggs into the mouths of fetal Baskers as they develop in their mother’s womb. Therefore, the peculiar snout of near-term and new-born Basking Sharks may help support their energy needs during their period of most rapid growth.”