Download Sexual Selection – Courtship and mating rituals

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Sexual Selection
Like all elasmobranches, but unlike nearly all the other fish in the sea, manta
rays reproduce through internal fertilization, which means male and female
must come together to mate.
They give birth to live young that are
miniature versions 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.....
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.
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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.
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
Just before mating takes place the male positions himself
on top of the female's back.
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
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.
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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.
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.
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.
Sooner or later the action begins to die down as the female, followed closely now (in most
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's, gripping it firmly.
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
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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 females pectoral fin during mating.
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
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.
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
During copulation the male flips his body underneath
the female as the pair mate belly to belly.
mate with her.
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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
During mating when the male mantas bite hold of the female’s
pectoral fins, 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 fins, 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
Examples of mating scars on the tips of mature female manta rays left pectoral fin tips.
female’s wing, leaving behind white and/or black circular marks as the upper layers of skin and pigmentation are scrapped away during mating. These
dorsal mating scars are permanent, and while not every mating results in scars, after a female has been though 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.
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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 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
Near term pregnant manta rays can easily be identified as the unborn pup
grows inside her bulging abdominal region.
evidence that female manta rays exhibit any parental care for their offspring
once they have been born.
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
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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's 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 elasmobranches. 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. Synchronizing 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
Mature male; claspers are enlarged,
calcified and extend past the base of the
pelvic fin.
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
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continues to visit regularly throughout the Maldives S-W Monsoon. 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)
Immature male: claspers are small
and tucked inside the pelvic fins.
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.
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 scared through damage
Female manta rays have no claspers, but
they do posses two paddle-like pelvic fins,
like the males.
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
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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.
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