Paper: Hooper, A. K., Wegener, B. J., & Wong, B. B. M. (2016). When should male squid prudently invest sperm ? Animal Behaviour, 112, 163–167. doi:10.1016/j.anbehav.2015.12.005
The driving purpose of living organisms is to pass on their genes, at least from an evolutionary perspective. Squid are no exception. Animals are programmed to maximize successful mate pairings in hopes of producing strong offspring that will survive to further pass on genes. We generally assume that females choose the strongest, flashiest males and when given a choice, males will also choose the highest quality female. This study explores our assumptions and tests whether males put more effort into mating with higher quality females or if our expectations are misplaced.
When encountering multiple potential mates at once, males can scope out all the options and pick the best one. The southern bottletail squid (Sepiadarium austrinum) doesn’t have this luxury and often only run into females one at a time. The squid must decide if the female is worth mating with without knowing its future options. This study investigates how the reproductive effort of males varies with females of different sizes if they are encountered one at a time.
The researchers used sperm quantity as a proxy for mating effort. Males must invest energy and resources into creating sperm, so it is in limited supply. The southern bottletail squid transfers sperm to females in bundles called spermatophores. Multiple sperm bundles are transferred during each mating event. These bundles can be easy measured after a mating event. Females store spermatophores near the mouth where other males can remove them or females can eat them. The sperm is therefore a high energy cost and also a high risk as most sperm will never be used to create offspring.
The researchers went SCUBA diving and caught wild squid to bring to the lab. Males of equal size were presented two females for mating, one after another thirty minutes apart. Some squid were presented a large female, followed by a smaller female; and others a small then larger female. Larger females are thought to be higher quality in this species as they have the potential to produce more offspring.
After matings, squid were euthanized and spermatophores were removed and counted- both those transferred to the females and those remaining in the male spermatophoric organ. To analyze sperm quality, five spermatophores per female were analyzed for number of sperm and viability.
Results and Significance
The number of sperm in each spermatophore was consistent throughout the experiment regardless of size of female or mating order. The number of spermatophores was higher for the first mating whether the first female was large or small. Two matings used about half of the available spermatophores in a male. The study also confirmed the assumption that larger females are more fertile by showing that they carry significantly more eggs than small females.
This study confirms that female squid vary in size and quality as potential mothers. But, the results go against the idea that males will invest the most energy into mating with higher quality, larger females. Mating order appears to be the best indicator of how much reproductive effort a male will put into the mating. In a species that may not encounter many options for mates it might pay off to go big every time, even the female looks a little small.