Conservation through Research, Awareness and Education

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Sexual Selection

Leading the males on a mating train around the reef, the larger female trails a chain of eager males all vying for position closest to the female.

Like all elasmobranchs, but unlike nearly all the other fish in the sea, manta rays reproduce through internal fertilisation, which means male and female must come together to mate. They give birth to live young which are miniature replicas of their parents; ready to fend for themselves, they are completely independent from birth.

Courtship rituals and actual mating events are relatively infrequently observed and the only documented accounts of a birthing manta ray come from a single female which is housed in the Japanese aquarium at Okinawa. So many gaps still remain in our knowledge of the sex lives of these fantastic creatures, and what little we do know is based on very little hard scientific data…

Manta Ray Courtship & Mating

In the last few years scientists studying populations of manta rays have managed to document the courtship rituals, matings, pregnancy cycles and even the birthing (in captivity) of these animals. However, virtually all of these observations have occurred through research on only the smaller reef species of manta ray, with just one documented mating for oceanic mantas on record.

Just before mating takes place the male positions himself on top of the female’s back.

Around the world matings often occur at specific seasons and times of the year; in the Maldives courtship behaviour and matings are much more frequently observed during the months of October and November, and again in March and April, when the country’s two monsoons (seasons) transition from one to the other. Throughout the day manta rays spend a significant amount of their time visiting cleaning stations and just like humans, female manta rays spend much more time getting groomed than males, with female mantas often spending several hours each day cruising around a favoured cleaning site. The males know this, so during the mating season these cleaning stations become the focal point for the matings, with groups of males hanging around the site waiting for the females to turn up. It’s likely that the receptive females release a sexual pheromone scent into the water, signalling to the males their readiness to mate. The excited males home in on their potential mates, which are often the most heavily pregnant females, shadowing their every move they hope to be first in line once she has given birth and is ready to mate. It is not uncommon to re-sight a heavily pregnant female within just a few days, having already given birth she also bears the fresh scars from a recent mating event, she is usually still being followed by a chain of eager males. Exactly how soon after birthing the females in the wild usually mate is unclear, and where they give birth still remains a mystery. However, the female manta ray kept captive in Japan’s Okinawa Churaumi Aquarium, which has given birth to a single pup on three consecutive years, mated within hours of giving birth on all three occasions.

This heavily pregnant female attracts a train of male admirers who escort her wherever she goes as they wait for her to give birth and become receptive to mating.

The males often try their luck, testing the female’s receptiveness to their amorous advances by shadowing her movements, positioning himself directly on top of her back and using his unfurled cephalic fins to rub the top of her head. Most times these enthusiastic advances are met with rejection, as the female literally gives the male the cold shoulder, bucking and twisting her body upwards and away from the male in an attempt to dislodge him from her back.

Love Bites: Manta rays have no arms with which to hold onto their mates, so during mating the males actually engulf the female’s pectoral fin into his mouth, gripping it firmly.

Sometimes however, the female responds by leading the males on a dance around the reef known as a mating train… this is when the action really begins to heat up. As many as 25 males line up head-to-tail behind the female as she swims at great speeds around the reef, twisting and turning, even leaping from the water as she tests the suitability of the males. These ‘mating trains’ are spectacular to watch as the procession of mantas snake through the water, often rushing past like a freight train within inches of divers heads as the female tries to shake loose the less persistent of her amorous pursuers. Sometimes the chased female loops back on herself, as she begins to chase the tail of the mating train, while at other times she follows other females, who get caught up in the action.  Watch some wonderful courtship behaviour.

As one male attempts to mate with a female, biting hold of her left pectoral fin, another male attempts to dislodge his grip as they compete for the female.

Sooner or later the action begins to die down as the female, followed closely now (in most cases) by just a single male, slows down and ascends into the water column. Positioned on the females back, the male begins to slide his mouth down the top of the females left pectoral fin, using his cephalic fins to guide him until the female’s wing-tip enters his mouth. The male bites down hard on the end of the female’s wing, working as much as 3 ft (1 metre) of her pectoral fin deep into his mouth. Manta rays do actually have over a dozen rows of tiny teeth in the lower jaw of their mouth, with the male’s teeth slightly more cusp shaped than the females. It appears that mating is the only remaining functional use for these teeth, which are no longer needed for feeding, and that the males slightly more curve-shaped teeth help them to retain their grip when grasping hold of the female’s pectoral fin during mating.

During copulation the male flips his body underneath the female as the pair mate belly to belly.

Once the male has a good grip on the female’s pectoral fin he flips his body around and underneath hers, so that they are positioned belly to belly. The male then inserts one of his two claspers into the female’s cloaca and ejaculates his sperm. Copulation itself lasts for just a few minutes at most and throughout this whole process the female remains fairly still, while the male continues to beat his pectoral fins, causing the mating couple to spin around in the water column like a corkscrew. The pair also begins to fall towards the seabed during copulation because all manta rays are negatively buoyant and will begin to sink as soon as they stop swimming. This is the reason why the females usually choose to swim up towards the water’s surface before allowing the male to mate with her.

After a successful copulation has finished and the male has removed his clasper from the female, he continues to hold onto her pectoral fin with his mouth for a few seconds more, before the pair split and go their separate ways. At this point the male plays no further roll in the life of his offspring, leaving the female to nurture the development of the future embryo inside her.

Mating Scars

Examples of mating scars on the tips of mature female manta rays left pectoral fin tips.

During mating when the male manta bites hold of the female’s pectoral fin, they cause minor cuts and abrasions to the upper and lower surface of the female’s wing-tip. The male’s teeth, which are only in his lower jaw, often leave linear scrapes on the underside of the female’s fin, which appear as red scratches when fresh. These marks quickly fade against the white colouration of the wing underside and are often hard to see just a few weeks after mating has occurred. The male’s grip also causes scarring on the upper, or dorsal, surface of the female’s wing, leaving behind white and/or black circular marks as the upper layers of skin and pigmentation are scraped away during mating. These dorsal mating scars are permanent, and while not every mating results in scars, after a female has been through a few matings, most appear to have some permanent visible mark as a memento of their sexual exploits. These scars are therefore a very useful indicator, allowing manta scientists to gauge what percentage of the female population is comprised of fully mature females. The fresh mating scars also allow scientists to plot recent matings which can be used to track pregnancies and reproductive trends over the following years.

One other interesting observation of these mating scars is their distribution, with over 95% of mating scars present only on the female’s left pectoral fin. This trend for lateralisation is found throughout both species of manta rays around the world, and is also observed in a wide variety of other species, including humans, most of whom favour the right hand side of our bodies.

Gestation & Pupping

After a gestation period of just over a year, the female manta rays give birth to a single pup (although occasionally they may also give birth to twins), which measures roughly 1.5-2 metres (5-7 ft) across from wing-tip to wing-tip at birth. Manta rays reproduce via aplacental viviparity, meaning they give birth to live young which are hatched from an egg inside the female’s uterus. The pup, which is wrapped in a thin membranous egg case, hatches inside the mother’s oviduct and then feeds on the mother’s uterine milk called hisotrophe, until they are fully developed and ready to be born. In the wild the female probably selects a safe location, like a sheltered bay or atoll lagoon, within which to give birth. The miniature pups pop out of their mother’s uterus with their pectoral fins rolled up over their back. There is no evidence that female manta rays exhibit any parental care for their offspring once they have been born.

Near term pregnant manta rays can easily be identified as the unborn pup grows inside her bulging abdominal region.

While accurate gestation periods have been obtained from the female manta ray held in the Okinawa Churaumi Aquarium in Japan, a lack of consistent and continuous observations of the same individual manta rays in all but a few wild populations have made it extremely difficult to record accurate mating and birthing intervals of these wild populations. While scientists do at least have some data for the reef manta populations, there is virtually nothing on the gestation periods of the oceanic mantas, with just a handful of pregnant females ever recorded.  It is likely these giants have a similar gestation period, and pup birthing size, to that of the reef mantas.

As for how often wild populations of mantas give birth,long term studies in the Maldives suggest that there is an average of only one pregnancy every five to six years for each mature female. This is an extremely low reproductive rate, even for a large vertebrate animal, and this data has significant implications for the management and conservation strategies of this species, both within the Maldives and internationally.

Another interesting observation from these studies is the distribution of these pregnancies, which were heavily clustered together. Reproductive synchrony to this degree has never been documented for manta rays before, but it is none-the-less a fairly common phenomenon among a wide variety of animals and plants, including many species of elasmobranchs. Reproductive synchrony often makes sense in environments where seasonality makes certain time periods more favourable to the survival chances of the offspring, usually as a result of seasonal increases in food availability for the pregnant females or the newly born offspring. Synchronising reproduction can also help to reduce the overall predation pressures on a population’s offspring, therefore increasing each individual’s chances of survival.

Longevity & Maturity

While it has not proved possible to age manta rays using annual growth rings on their vertebra, which has been done in other species, re-sightings of wild individuals suggest that these animals live for several decades at least, and quite probably for more than 50 years. One large female manta ray at Lankan Reef in the North Male Atoll of the Maldives called Ping Pong (M12), has been seen regularly at this site for over 22 years, and continues to visit regularly throughout the Maldives S-W Monsoon.

Immature male: claspers are small and tucked inside the pelvic fins.

Mature male; claspers are enlarged, calcified and extend past the base of the pelvic fin.

Virtually all of the sightings of mantas at Lankan are of fully grown adults more than 3 metres (10 ft) in width, and from the image taken of Ping Pong back in 1989 she already appears to be a fully grown manta ray. Reef mantas are thought to take around 10-15 years to reach full size and sexual maturity, so that would make Ping Pong well over three decades old at least. However, until photo-ID studies like the one in the Maldives have been running for another 20 years we are unlikely to be sure exactly how long these animals can live. In the Maldives the female reef mantas mature at around 3m (10 ft), while the males, which are a little smaller, mature at around 2.7 metres (8 ft), attaining full size at just over 3.3 metres (12 ft) and 3 metres (10 ft) respectively. This is a little smaller than reef mantas in some other locations around the world, such as Mozambique in East Africa, where the big mature female reef manta grow to over 4.5 metres (15 ft). The oceanic mantas, which grow much larger, probably take a little longer to reach sexual maturity, which appears to be at around 3.5-4m (11-13ft) for the males and 4.5m (14-15ft) for the females. With female mantas attaining disc widths around one third larger than the males in both species, this sexual dimorphism is probably an adaptation to allow the females to carry and give birth to larger pups, which are more likely to survive than smaller offspring.

Female manta rays have no claspers, but they do possess two paddle-like pelvic fins, like the males.

As already discussed, the maturity of female manta rays can be ascertained in the wild through the presence of mating scars and visible pregnancies, but maturity of the male mantas of both species, like all sharks and rays, can be noted instead by the size and appearance of their claspers. When a male manta reaches sexual maturity his small, soft and pliable claspers become calcified and hardened for use in copulation. Furthermore, once a male has mated his claspers also become scarred through damage sustained during copulation; the female’s cloaca actually has barbs which hold the male’s clasper in place during copulation, but rip it as he pulls free afterwards (ouch!). This allows scientists studying these animals in the wild to record and document the male’s maturity as they develop from juveniles, to sub-adults, to mature males and fully mature males which have mated. These mature males also have visible clasper glands on the bellies just behind the claspers, which contain the sperm and seminal fluid.

 

© 2017 Manta Trust