![]() ![]() Females are often found with multiple severed arms 3, suggesting there is extreme variance in reproductive success among males, another great example supporting Bateman’s principle. 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 copulation 3. ![]() This arm autotomy allows the blanket octopus to distract potential predators and swim away from danger 2. 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!). 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. The female blanket octopus can be as long as 2 m and thus can produce a large quantity of eggs, maximizing her reproductive success. Weighing in at up to 40,000 times the weight of her mate, the female blanket octopus demonstrates that, occasionally, size does in fact matter 3. 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. 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 weight 3. At first glance, being a dwarf may appear to be a disadvantage. 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 animals 1. The difference in body size ratio in male and female blanket octopuses can likely be attributed to sexual selection 3. 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 dimorphism 3. 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 offspring 2. 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. 3Now, I know what you’re thinking: something can’t be right here. Norman et al (2002) photographed and described the first live male specimen of the elusive blanket octopus males are only a fraction of the size of females. ![]() 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 meters 2! These pelagic species spend their entire lives in the middle of the open ocean, making them both difficult to access and to study 4. In fact, the first male to be observed alive in the wild was described and photographed in 2002 by researchers conducting night dives 3. This amazing animal is colloquially known as the blanket octopus and includes four species in the genus Tremoctopus. ![]() One particularly interesting case of sexual size dimorphism can be found in a mollusk species found in tropical and sub-tropical oceans. 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. There are vast differences in the roles of males and females that facilitate maximizing the reproductive success of a pair. ![]()
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