Fascinating Courtship of Gentle Ocean Giants
What animal spends hours, if not days in courtship, followed by nearly 13 months of gestation to give birth to a single fully formed mini-me rolled up like a burrito? The majestic manta ray!
The Mating Games. Credit: BBC One
Growing up to seven metres from wing-tip to wing-tip, these ocean giants have a fascinating reproductive strategy that was showcased in the latest episode of the new BBC One series ‘The Mating Games: Out of the Blue’ narrated by Sir David Attenborough and also available on Discovery Channel. The episode, featuring humpback whale heat runs, penis fencing nudibranchs and beach battling sea lions also follows the courtship dance of elegant reef manta rays.
To help document the elaborate mating ritual of these gentle giants, Bristol-based production company Sliverback Films Ltd. enlisted the help of the Manta Trust, a UK registered charity dedicated to the research and conservation of manta rays and their relatives.
Courtship events between manta rays are rare to witness but if you spend enough time around manta ray cleaning stations there is a good chance you catch a parade of mantas on heat. Cleaning stations are special areas of coral reef where aquatic life congregate to be cleaned by picked clean by cleaner fish and shrimp. A male manta ray will often try his luck, testing the female’s receptiveness to his amorous advances by shadowing her movements. Positioning himself directly on top of her back he uses his unfurled cephalic fins to rub the top of her head. Most of the time these enthusiastic advances are rejected; the female literally gives the male the cold shoulder, bucking and twisting her body upwards and away from the male to dislodge him from her back. Sometimes, though, the female responds by rapidly accelerating forward, leading the male around the reef at high speed. This behaviour soon attracts the attention of more amorous males, which follow in line behind the female to form a manta Conga dance – the ‘courtship train’ has begun!
As many as thirty males line up head-to-tail behind the female as she swims at great speeds, twisting and turning around the reef, even leaping from the water. These courtship trains are amazing to watch as the procession of rays 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 pursuers.
Even rarer is to witness actual mating events themselves. These activities have only been recorded a hand full of times before. And still, despite their size and popularity amongst scuba divers, the natural birthing of a manta ray pup has never been recorded in the wild!
Images above: Reef manta ray courtship train, copulation, pregnancy bulge and a newborn individual.
Like all elasmobranchs (sharks, rays, and skates), but unlike most other fish in the sea, mobulid rays reproduce through internal fertilisation; males and females must physically come together to mate. Armed with little more than a tiny row of sandpaper-like teeth, and ridged cartilage on the top of their mouth, the male manta ray bites onto the pectoral fin of the female and rolls belly to belly to insert his claspers into her cloaca with copulation lasting only thirty seconds.
Being negatively buoyant, during reproduction manta rays sink and researchers often observe individuals with scrapes and scratches from crashing into the sharp coral reef. Another tell-tale sign that a female has reproduced is the so-called mating scars left when the male manta ray grasps hold on the female’s wing tip with his mouth. These love bites allow manta scientists to gauge what percentage of the female population consists of fully mature females, an important metric for studying population demographics.
After a gestation period of just over a year, the female reef manta ray gives birth to a single pup (although occasionally they may also give birth to twins) which measures roughly 1.5 metres (5 feet) across from wing-tip to wing-tip.
Manta Trust researchers at the University of Cambridge are using a novel, state of the art, contactless underwater ultrasound unit to scan female manta rays in the wild. Ultrasound examinations will allow to determine the fecundity of the Maldivian manta population more accurately as well better establish the maturity status of non-pregnant manta rays which is crucial in population vulnerability studies.
Research using contactless underwater ultrasound unit © Andy Ball.
Many parallels can be drawn between the life history strategies of manta rays and some large marine mammals; particularly the incredible humpback whale heat runs which also features in ‘The Mating Games: Out of the Blue’. Unlike their warm-blooded cousins, however, female manta rays do not exhibit any parental care for their offspring once they have been born.
The Climate Crisis is a serious concern for the survival of this species. The largest known population of reef manta rays in the world is found in the Maldives and numbers only around 5,000 individuals.
“In the Maldives, we are starting to witness the degradation of the seasonal monsoon winds which drive productivity in the region. Worryingly, years of weak monsoon winds coincide with years of minimal reproductive activity. In 2010 and 2011 our team recorded no pregnant females at all despite seeing many of the regular females around” - Dr. Guy Stevens (Manta Trust CEO)
The working theory is that when times are tough and food is less abundant, manta rays will focus on foraging rather than reproducing. If these bad spells of productivity become more frequent this could spell disaster for a species that reproduce on average every two to seven years.
With the UN Climate Change COP26 conference just around the corner, this year has been billed as the last opportunity to prevent a 2-degree Celsius rise in global temperatures. This will be imperative to prevent long-lasting and irreversible changes to the natural world. We must act now to secure a future for these dancing rays, and for all other life on Earth.