Sure it is common that in various animal species, there is a phenomenon that once a male and female are done mating, they are to go their separate ways. But there is an interesting practice amongst the cephalopods family where after the female and male mate, the female consumes her male partner. This occurrence is called sexual cannibalism, which more specifically means (in context of mating or courtship), the killing and consumption of a mate (actual or potential) before, during, or after sexual intercourse. In order to further explain and provide evidence for this strange cephalopod act, the natural behaviour of six O. cyanea Gray octopuses were observed, having 21 matings amongst them.
In this study, the scientists wanted to test a hypothesis they thought could be called an individual octopus’s strategy: to mate or attempt to mate with any conspecific octopus encountered because there might be a chance another one might not be met. Although this is not true for all the octopus species, it is a generalization for those species who might not have many encounters, unlike the O. cyanea Gray octopuses. The O. cyanea Gray species was chosen because there are more encounters of them found near Turtle Cove in the southern portion of the Palau Archipelago, in western Micronesia (where this study was conducted). The habitat in this location could be described as having rubble (rough) coral and large stone boulders in the backreef, in addition to some places having live coral. The observational study began from midday April 18, 1995 to midday April 21, 1995, and used the technique of long-term daily focal sampling of female octopuses in the daylight hours. A total of 14 SCUBA divers videotaped the behaviors as the octopuses foraged throughout the day.
The octopuses were distinguished by their size and the marks on their skin. A den which belonged to a large female octopus was marked with a buoy, as it was checked frequently for 2.5 days. The females weighed about 1 to 2 kg, while the males weighed about 0.5 to 0.7 kg. Before getting to the results, it is important to understand the two ways octopuses can mate. The first way is when the male mounts the female, and the second way is when he remains ‘‘at a distance’’ and extends his arm to her mantle cavity.
The results of 21 matings were witnessed with and without videotapes, where 13 were caught on tape on April 20th, one on April 21st, and the seven others were without video seen on April 18th and 19th. On April 20th with the 13 matings, the duration of each (in minutes) were the following: 1, 2, 0.5, 1.5, 14, 5, 7, 1, 23, 12, 5, 5 and 2. The matings were of the second way (as mentioned in the paragraph above) “at a distance” where the male remaining distant from the female and extending his third right hectocotylus arm and inserting it into the female’s mantle cavity.
On April 20th, the female around 5:30 hour became alert by her den and 30 minutes later she began to forage over the reef about 70 meters away from her den. At 7:00 hour she spotted a male, about half her size and they mated 13 times. Twice amongst the 13 matings the female “rejected” the male when he extended the arm, but throughout the 13 occasions, the male had a sudden “jump back” due to sensing aggression from the female. Interestingly, on the 11th mating, the female became diverted by the attention of another small octopus swimming past. Luckily for the small octopus, it was able to escape by inking and jetting swiftly away from the assertive female. The original male came back to the female and continued mating while she was foraging for about 70 seconds. Then at the 10:25 hour, the female moved slowly towards the male and suddenly swept him and with force knocked his head backwards against the ledge of the coral as he tried to ink and escape. Unfortunately for the male, the female engulfed him in her arms and took him back to her den, and for the next 24 hours without foraging, she spent her time consuming the male. Yum.
The moral of the story is that amongst the O. cyanea Gray family, if you are a small male and mate with a larger female, you are in a great risk of cannibalism. The benefits for the female was that she was able to gain food and spermatophores, while the male tried to gain fertilization and tried to escape an aggressive female. In another observational study, a male octopus selected a smaller female to mate with, but he created a “ double home” (a hole next to the female’s den) in order to maintain enough distance to be able to touch the female. With the smaller females compared to the males, it was unlikely to see the act of cannibalism. Recent work has shown that in the context of reproductive behavior, cephalopods use chemical signals. When a female was in the process of consuming her mate she captured a day before, another male came and used a different method of mating, lasting for three hours. The female’s behavioral change is still difficult to explain however, some suggestions could be due to her state of satisfaction or nutrition level. Overall, in the process of mating, if the female is larger, the male is better off remaining at a safe distance, and if the male is larger, then the female might be at risk to be consumed. Scientists say that the female octopus may think, “If the male is not suitable as a mate, attack and eat him.”
References
Hanlon, R. T., & Forsythe, J. W. (2008). Sexual cannibalism by Octopus cyanea on a Pacific coral reef. Marine & Freshwater Behaviour & Physiology, 41(1), 19–28. https://doi.org/10.1080/10236240701661123
Kate Ellis
October 15, 2018 — 4:06 am
Oh wow, never thought that this kind of stuff will happen to its own species. Glad that humans don’t have this kind of thinking…
Yasmine Green
October 16, 2018 — 4:57 pm
I was unaware that it was size that played a factor in who gets eaten in octopus sexual cannibalism. I always thought it was the male’s inability to satisfy or perform that made the female eat her potential partner. I also was unaware that males will eat the female for the same reason. This seems to go against the species’ overall fitness because eating the female means there is one less female to pass her genes along. In future will this lead to larger octopus, and in that same vein, what is the advantage a larger octopus have besides sexual selection success?
Dennisha King
October 16, 2018 — 7:29 pm
This is very interesting that the female would consume the male after mating. If the male was smaller than the female, then wouldn’t this mean that the offspring would likely come out smaller, so why would the larger female want to mate with someone smaller than them? Although it is important that the female receives this form of direct benefit because they would no longer need to forage for the time after. I think this behavior may have started through individual learning where one female octopus had the idea to eat the smaller male and then it continued because the behavior does not seem to have any evolutionary benefits on its own.
Erika McDonnell
October 17, 2018 — 12:02 am
This is so interesting! I had heard of sexual cannibalism in some insects, like spiders, but I didn’t know that it existed in larger animals like the octopus. Also, I wasn’t aware about the benefits of it to the female. I wonder if larger female octopi will be decreasing in number if males are less willing to mate with them due to the risk of cannibalism?