Counting the number of "hickeys" on the body of stingrays? Scientists are not idle

Whenever the breeding season of manta rays comes, deep and shallow scratches will always appear on the bodies of female manta rays, and the edges of their pectoral fins are even more scarred, as if they have experienced several battles.

However, these are actually "strawberries" planted by males!

Female stingray: Tired | Jose Miguel Bosch Benitez / flickr

When it comes to leaving hickeys — or bite marks, really — on mates, male rays are obsessed with it.

The Bold Courtship of a Ray

In the vast ocean, "planting strawberries" is not a rare thing. Many fish of the subclass Elasmobranchia have this special habit. The subclass Elasmobranchia is one of the two subclasses of the class Chondrichthyes, including the order Sharks and the order Rays (that is, sharks and rays). The well-known manta rays, electric rays, stingrays and other flat fish are all groups under the order Rays.

Female sharks can also be “planted with strawberries” | References [1]

For rays, biting is just part of the foreplay. When a male ray finds a female he likes, he will follow her for a while. In the group of manta rays, multiple males will even pursue a female at the same time, forming a long line on the seabed, with up to 20 males swimming on the seabed.

Multiple male manta rays following the same female | youtube

When the time is right, the male rays following behind will swim above the female and bite the fan-like pectoral fins. Usually, one bite is not enough, and the male rays will start the "offensive" again and again.

Female stingrays are rejecting it in their hearts | youtube

The male manta ray uses frequent "kisses" to promote the female's willingness to mate. As long as the female is willing, the important matter of passing on the family line can be done naturally. During the mating process, the male manta ray will also keep biting the female's pectoral fin to stabilize himself.

The male will now turn over to the female's ventral surface | youtube

After mating, some males will still not let go and will bite the other and swim together for a while before leaving.

Why not go? | youtube

Male rays are able to implant genitals in females and hold them tightly for mating because they have a dense and sharp set of teeth. In some species, male rays only grow fangs during the breeding season, and at other times they have molar-shaped teeth like females.

The jaws of a male Atlantic stingray (Dasyatis sabina) are densely packed with fangs during the breeding season. A close-up of the teeth is shown on the lower left. | Reference [2]

However, female rays are not willing to be covered in wounds. In order to reduce the damage caused by their suitors, females have thicker skin. This is especially true in sharks (probably because shark teeth are more lethal). The dermis of female blue sharks (Prionace glauca) can be twice as thick as that of male blue sharks of the same size.

Although they have thickened skin, female rays have to face countless suitors who pounce on them and do not know how to take the consequences during the 7-month mating season, so their backs are often covered with scars, and even the edges of their pectoral fins are bitten and incomplete.

A female Potamotrygon scobina in captivity that the breeder forgot to separate from the breeding males. | rayman / monsterfishkeepers.com

Come, count the strawberries

How much harm does this crazy courtship method of male rays cause to their partners? Someone has actually studied it.

Stephen M. Kajiura of Florida Polytechnic University and his team conducted a year-long study on Atlantic stingrays (Dasyatis sabina), collecting several male and female Atlantic stingrays every month to count the number and shape of wounds on their backs.

Why do we need to count the strawberries on the males' backs? Could it be that... Yes, some males with really bad eyesight just rush in and bite several times before they realize that the object they are looking at is actually the same gender as themselves! Therefore, many male Atlantic stingrays are not spared in this courtship melee.

A male stingray (Bathytoshia lata) lies on top of a bruised female, but he also seems to have scratches on his body | youtube

Kajiura and his colleagues collected 75 female and 73 male Atlantic stingrays and counted 961 "strawberries", with the ratio of male to female wounds being about 3:1. They divided these "hickeys" into five types, and based on the morphology of the upper and lower jaws of Atlantic stingrays, they inferred how the different types were formed.

The first three types of wounds all had obvious teeth marks, which were left by sharp teeth scratching the skin during a "kiss". The proportion of these bite marks was no different between different sexes, indicating that this was an indiscriminate attack by male rays without distinguishing between males and females.

The first three types of wound morphology | References [2]

The fourth type of "edge abrasions" appears on the edge of the ray's body, with no obvious teeth marks. This may be caused by the male biting the other's pectoral fin for a long time during mating.

“Edge scratches” | References [2]

"Edge abrasions" accounted for 13% of the total wounds of female Atlantic stingrays, but only 3% of males. This also confirms that these wounds are caused during mating. After all, when the male finds that the other sex is wrong and is wrong after all, he can only leave disappointed and will not take further action.

The last type of "love scar" is even more brutal, with a piece of the victim's pectoral fin bitten off. This type of wound only occurs on males. Researchers speculate that this may be because the female's skin is too thick to be bitten off. The male Atlantic stingray who was misidentified for sex could never have imagined that in addition to sharks, he also had to be on guard against his own kind who were overwhelmed by the desire for love.

“Excision” type wounds | References [2]

In addition, Kajiura and others also found that the wounds of female Atlantic stingrays are mostly distributed in the back half of the body, especially the "edge abrasions", 96% of which appear on the back side; while the guys who "plant strawberries" on the same sex are not picky about the place, and on the males, several types of wounds are evenly distributed.

They speculate that this is because the cloaca at the back of the female body emits pheromones during the breeding season to show the male its reproductive status. In the vast ocean, the male Atlantic stingray finally caught the hormones of the opposite sex and immediately ran towards the source of the smell.

So, you can't just look at the face when you're looking for a partner, right? In the end, they all look the same after drying. | Facebook: @マリンワールド海の中道

Scientists count the "strawberries" on stingrays not because they have nothing to do. They can use this to understand the reproductive status of stingray populations, determine the start time, duration and intensity of mating activities, and better protect these unique animals.

References

[1] Ritter, EK, Amin, RW Mating scars among sharks: evidence of coercive mating?. acta ethol 22, 9–16 (2019). https://doi.org/10.1007/s10211-018-0301-z

[2] Kajiura, SM, Sebastian, AP & Tricas, TC Dermal Bite Wounds as Indicators of Reproductive Seasonality and Behavior in the Atlantic Stingray, Dasyatis sabina. Environmental Biology of Fishes 58, 23–31 (2000). https://doi.org/10.1023/A:1007667108362

[3]https://mantarayadvocates.com/how-do-manta-rays-reproduce/

Author: Jian Er

Editor: Mai Mai

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