There are vast differences in the roles of males and females that facilitate maximizing the reproductive success of a pair. Whether the differences lie in shape, size, parental investment or a combination of all three, this sexual dimorphism allows different species to successfully mate and raise offspring to reproductive competence.
One particularly interesting case of sexual size dimorphism can be found in a mollusk species found in tropical and sub-tropical oceans. This amazing animal is colloquially known as the blanket octopus and includes four species in the genus Tremoctopus. These pelagic species spend their entire lives in the middle of the open ocean, making them both difficult to access and to study4. In fact, the first male to be observed alive in the wild was described and photographed in 2002 by researchers conducting night dives3. Believe it or not, a female blanket octopus can grow a few orders of magnitude larger than the male blanket octopus, with males measuring up to 2.4 cm and females measuring up to 2 meters2!
Now, I know what you’re thinking: something can’t be right here. How could a pair of individuals with sexual size dimorphism so extreme possibly find a way to mate? Well, like all things in life, if there is a will, there is a way. In this case, the evolutionary rationale for this extreme difference in size is that the female blanket octopus can maximize her own reproductive success by producing a large number of offspring2.
The blanket octopus is not alone in its incredible sexual divergence in morphology; it belongs to one of four different ocean-dwelling octopus families that exhibit this same extraordinary size dimorphism3. The difference in body size ratio in male and female blanket octopuses can likely be attributed to sexual selection3. Through sexual selection, the characteristics of a larger female and smaller male have evolved so dramatically that the male octopuses are considered to be dwarf animals1. At first glance, being a dwarf may appear to be a disadvantage. However, it has been suggested that this decrease in size may allow males to maximize the number of tentacles with which to defend themselves, without carrying any excess weight3. Additionally, large size can serve as an advantage in the female as well; the larger the female, the larger number of small eggs she can develop and carry until hatching. Weighing in at up to 40,000 times the weight of her mate, the female blanket octopus demonstrates that, occasionally, size does in fact matter3.
In addition to the tremendous differentiation in size and weight of the male and female blanket octopus, they have also evolved the fascinating ability to autotomize their arms. Autotomy is a behavioral strategy that some animals use whereby they self-amputate appendages to escape predation (you may have seen this in lizards that drop their tails for example!). This arm autotomy allows the blanket octopus to distract potential predators and swim away from danger2. However, an even more fascinating adaptation is that male Tremoctopus have a modified reproductive appendage (hectocotylus) containing sperm that is severed and transferred to the female; the male is unlikely to survive past this first and last act of copulation3. Females are often found with multiple severed arms3, suggesting there is extreme variance in reproductive success among males, another great example supporting Bateman’s principle. In other words, a large proportion of males may not even contribute to fertilizing eggs harbored by a female.
There is still a need for research examining the sensory capacity of males and their ability to locate females, despite their minute size. A magnificent creature, the blanket octopus has developed both exaggerated and diminished external traits that allow it to thrive in the ocean environment. So the next time you think about gender differences in height in humans, imagine going on a date with someone the size of a pea. One cannot help but appreciate the beauty and irony of mother nature.
[By: Allison Essary]
Allison Essary is a fourth-year undergraduate student studying Animal Science with an emphasis in Animal Behavior. Her focus lies in conservation and welfare of exotic animals through the understanding of animal behavior.
1Hoving, Henk-‐Jan T., et al. “The Study of Deep-‐Sea Cephalopods.” Advances in Cephalopod Science: Biology, Ecology, Cultivation and Fisheries Advances in Marine Biology, 2014, pp. 235– 359., doi:10.1016/b978-‐0-‐12-‐800287-‐2.00003-‐2.
2Mangold (1922-‐2003), Katharina M., Michael Vecchione, and Richard E. Young. 2016. Tremoctopodidae Tryon, 1879. Tremoctopus Chiaie 1830. Blanket octopus. Version 16 November 2016. http://tolweb.org/Tremoctopus/20202/2016.11.16 in The Tree of Life Web Project, http://tolweb.org/
3Norman, M. D., et al. “First Encounter with a Live Male Blanket Octopus: The World’s Most Sexually Size-Dimorphic Large Animal.” New Zealand Journal of Marine and Freshwater Research, vol. 36, no. 4, 2002, pp. 733–736., doi:10.1080/00288330.2002.9517126.
4Shapiro, Leo. “Common Blanket Octopus -‐ Tremoctopus Violaceus -‐ Overview.” Encyclopedia of Life, 14 Apr. 2012, eol.org/pages/492783/overview.
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