Throwback Thursday

(This report is a summary of a scientific paper written by Clinton Duffy and Scott Tindale, scientists from the NZ Dept of Conservation and Auckland Museum)

For the first time the courtship behaviour of the giant devil ray (Mobula mobular) has been witnessed and recorded in New Zealand’s northern waters. On March 4th last year two scientists watched and videoed a full-term pregnant female with up to four males in hot pursuit, for nearly 2 ½ hours. Clinton A. J. Duffy1,2 and Scott C. Tindale3

Not much is known about the courtship behaviour of rays but Scott Tindale is now able to report these have similarities to those of other large mobulids, and provide the strongest evidence to date of reproduction by this species in New Zealand waters.
The scientists did not see the female being bitten as has been reported for similar species elsewhere, possibly due to the way she used the sea surface to prevent males positioning themselves above her. Nonetheless each time she did that the lead male pressed her abdomen from beneath.  

Rarely seen

Rarely seen Seeing mating behaviour of this sort is rare, particularly for the highly mobile and highly specialised pelagics such as manta and devil rays which are found around the world in both tropical and temperate regions.  

"…The female swam rapidly ahead of the pursuing males, often banking
steeply or swimming in an inverted
loop to get behind them.  She would
also stop suddenly forcing the
following males to overshoot and
circle back…"

They are also increasingly at risk due to fishery practices, pollution and habitat loss. Rays are taken for their meat, skin and branchial filter plates and as by-catch though where there are resident populations or predictable seasonal aggregations they have considerable eco- tourism value. But their resilience to fishing is low due to their very low population growth rate; better knowledge of their reproductive behaviour can shed light on their numbers and prospects

The IUCN Red List of Threatened Species lists several of their kind as vulnerable and/or endangered.

What happened

Photo 4: Possible attempt by a male giant devil ray to grasp the left pectoral fin tip of a pregnant female

At around midday on March 4th 2017 two groups of rays were first seen. Scott Tindale and Clinton Duffy used GoPro5 and Contour 1700 HD digital video cameras attached to a camera pole to capture the images.
The seas were calm, conditions clear and sunny. The sea depth was 165 m with the sea surface temperature 21.8o C. Underwater visibility was estimated at 20m.

A first group of three rays between 2.0 to 2.5 m wide swam rapidly in straight lines, changing direction frequently. The sex of the rays in this group could not be determined though the largest of them looked to be pregnant with a definite bulge mid drift.

" …The closest male would approach from below and behind,
and gently press the female’s
abdomen, underside of her head,
and pectoral fin with his  
cephalic fins… "

The second group consisted of a full term pregnant female three metres wide, (Photo 2) pursued by four others. Two were 2.5 to 2.8 m wide, with the other two smaller. These four were judged to be mature males based upon the size of their claspers. No clasper scarring was seen which suggests none of the males had mated recently.  

Throughout the two and half hour period the female swam rapidly ahead of the pursuing males, often banking steeply or swimming in an inverted loop to get behind them (Photo 3). The female would also stop suddenly forcing the following males to overshoot and circle back.

From time to time she would stop swimming just below, or at the surface with her back and one or more wing tips clear of the water. When this occurred, the closest male would approach from below and behind, and gently press the female’s abdomen, underside of her head and pectoral fin with his cephalic fins and the top of his head (Lead photo & Photo 7).  

The female would tolerate this briefly before accelerating away, or turning almost 180° on top of the male, and swimming off in the opposite direction, accompanied by a lot of splashing as the female’s pectoral fins slapped the surface.
On three occasions while the female was at the surface the male appeared to try and grasp the female’s pectoral fin tip (Photo 4). These were the only attempts to bite the female observed. No mating scars were noted on the female. No aggression or interference was observed between the pursuing males. But the two largest were always the closest to the female.

When the female slowed at the surface only the closest male would make contact with her. The other large male would circle the pair, sometimes joined by one or both of the smaller males.

At other times the smaller males would circle below them, often out of shot, hen rise rapidly to rejoin the train as soon as the female began to move off again.

At about 3.15 pm all five rays followed the female in a downward spiral from the place where they were first seen.

Weighing up the encounter

The courtship behaviour witnessed appears similar to that of other large rays involving prolonged close pursuit of females by multiple males in mating trains.
The female avoidance involves faster than usual swimming speeds and veering, circling and looping behaviour.

Copulation in other large rays (M. alfredi and M. birostris) involves the male grasping the female’s pectoral fin tip, usually the left, with its mouth and positioning itself abdomen-to-abdomen with the female.

Most descriptions of courtship behaviour in mobulids do not involve obviously pregnant females, but others elsewhere have witnessed males pursuing pregnant females on several occasions. What was seen on this occasion were males repeatedly pushing against the female’s underside, including the distended abdomen, courtship behaviour which has also been observed in bat rays (Myliobatis californica) where males closely follow and ‘bump’ or push against the female’s underside and cloaca.

Male rays appear able to distinguish between reproductive and non-reproductive females in the same area. The male of one species (Dasyatis Americana) has been shown to be strongly attracted to females that have recently given birth.

Seeing a mating train, including a full-term pregnant female, is strong evidence that rays are indeed breeding off Northland, New Zealand, and as observed elsewhere, it takes place during late spring and early summer based on clasper condition, and the presence of semen in the claspers.

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