Hooked On a Feeling: Fish Hormones and the Nature of Love, by Cassie Davis

     A documentary released in 2003 revealed the plight of an Amphiprion ocellaris trying to save its offspring from captivity. The A. ocellaris went through a battle of emotions and adventure tied to the rescue of his offspring much like one a human would endure in the same situation. Pixar’s Finding Nemo is a heart-warming family film, and although fictional, strikes up very real scientific questions. The musical artist known as Haddaway put it best in the song “What is Love, baby don’t hurt me.” When this question is framed in the context of Ichthyology it becomes “Do fish feel love?” Are there certain “love” hormones that enable fish to feel deep affection or sexual attachments? Scientifically speaking, love is a jumble of chemicals in the brain that help form a strong, constant, and emotional feelings affection or dedication to something. Although there is no research proving the existence of feelings of affection in fish through which the term “love hormone” would suggest, plenty of research can confirm that social interactions between fish in the teleost group are affected by select hormones such as isotocin (oxytocin) and arginine vasotocin.

Who Can Explain the Love Between Marlin and Coral? Finding Nemo (2003)

There are multiple cases of lifelong monogamy within the animal kingdom, and fish are no exception. Even though monogamy among fish isn’t that common but some, such as the French angelfish (Pomacanthus paru) and the clownfish like in Finding Nemo have been observed to spend an extended period of time with a partner (Frost 2013). There is ample evidence that suggests that lifelong partners are more successful at producing offspring, which encourages monogamy. Is this “love” the result of hormones that cause strong affection and dedication or merely a tactic to successfully reproduce that has been reinforced through natural selection and social environments? To explore this, one must delve into the inner chemical working at play. There are many hormones that impact social interactions between teleosts but isotocin (the fish version of oxytocin), and arginine vasotocin stand out among the rest when it comes to affection or “love”. These chemicals combine to account for the fuel behind most social hormones in fish.

Oxytocin is a hormone found throughout the animal kingdom in various forms that stimulates sociability. In fish, oxytocin takes a different form called isotocin, but performs the same social function. Many studies have shown that isotocin makes individuals more sensitive to social information, whether it’s the size of one’s opponent or clues about the social hierarchy.  Adam Reddon, a researcher at McMaster University, performed a study on the effects of isotocin in fish, specifically daffodil cichlids (Neolamprologus pulcher). Daffodil cichlids live in permanent groups, making them prime subjects for observation as they must be sociable. Reddon started by injecting a group of daffodil cichlids with isotocin or saline (control) before setting them up for duels, and then monitored their behavior. He found that with isotocin, the cichlids would behave differently depending on how big their opponent was. If they perceived their opponent as larger than themselves, they became more aggressive. He then injected a group of cichlids with saline and set them up for duels. Not surprisingly, they behaved based on their intrinsic level of aggressiveness rather than the size of their opponent. Moving from duels to a social context, researchers found that cichlids injected with isotocin produced more submissive gestures when challenged by a higher-ranking member of the group (adopting a head-up posture and quivering its body) (Reddon et al. 2012). Both of these experiments helped to demonstrate the effects of isotocin on social interaction between fish living in a group, identifying this hormone as a major factor in fish sociability, but what about affection and finding a lifelong mate? What hormone addresses “love” and mating specifically?

Daffodil Cichlid.   Photo by John Robertson 

Arginine Vasotocin (AVT) is responsible for regulating many aspects of fish physiology and behavior, including those of a sexual or social nature. Arginine Vasotocin is therefore the main hormone involved in “love” and mating specifically (Balment et al. 2006). Katherine Semsar studied the effects of AVT in Bluehead wrasse (Thalassoma bifasciatum.) With males, she found that this hormone would shift their demeanor to the opposite of what it was before the injection. Territorial males would start displaying behaviors of non-territorial males, increasing their courtship of females and decreasing their aggressiveness. In non-territorial males, actions of territorial males would emerge after AVT injection such as an increase in chasing, increase in courtship, decreased feeding, and other behaviors rarely seen in non-territorial males. In both cases the manipulation of AVT in males shifted their social status to that of their counterpart, however it also increased the courtship rate among both groups. This study shows that Arginine Vasotocin is hormonally one of the main contributors for prolonged partnerships among Bluehead wrasse. Semsar then went on to test the AVT levels in females. She noted that dominant females were found to have a higher concentration of AVT than subordinate females. These dominant females would display the same characteristics of dominant male behavior. This experiments suggest that though AVT impacts individuals differently, it is vital in establishing dominance in a social hierarchy. Through both of these experiments Arginine Vasotocin was shown to play a major role in prolonged partnerships and social interaction (Semsar 2003).

Bluehead Wrasse. Photo by Evan D'Alessandro, courtesy Oregon State University Source 

  

Through these studies we have been given a small glimpse into the physiological reasons behind fish behavior in sexual and social settings. However, in order to support these findings further with more solid evidence, future studies should expand to other fish species not in the “bony-fish” category like sharks, skates, and rays to see if the presence of AVT and isotocin is a common thread in the very diverse fish world. Although there is not currently any research proving the existence of feelings of affection in fish that could be attributed to one specific “love hormone” the research presented here can begin to confirm that hormones such as isotocin and arginine vasotocin play a major role in social interactions and prolonged periods of mating between teleost fish.

References

Balment, R.J., W. Lu, E. Weybourne, & J. M. Warne. 2006. Arginine vasotocin a key hormone in fish physiology and behaviour: A review with insights from mammalian models [Abstract]. General and Comparative Endocrinology, 147(1), 9-16.  

Frost, E. 2013. Is It Love? Why Some Ocean Animals (Sort Of) Mate For Life. Available: http://www.smithsonianmag.com/science-nature/is-it-love-why-some-ocean-animals-sort-of-mate-for-life-16907109/ (April 2017)

Reddon, A. R., C.M. O'Connor, S.E. Marsh-Rollo, & S. Balshine. 2012. Effects of isotocin on social responses in a cooperatively breeding fish [Abstract]. Animal Behaviour, 84(4), 753-760.  

Semsar, K. 2003. Hormonal Mechanisms Regulating Alternate Phenotypes (Doctoral Dissertation at North Carolina State University). Available: https://repod.lib.ncsu.edu/bitstream/handle/1840.16/4853/etd.pdf?sequence=1&isAllowed=y (April 2017)