For some, today might be an annoying reminder of your relationship status that you wish would just be over already. On the other hand, maybe your mind is filled with thoughts of the lasting bond shared between partners. Either way, Valentine’s Day is upon us, and it seems impossible to ignore. While scrolling Twitter and Facebook you might even see posts and articles like these:
Every year it seems like there is some version of these articles circulating around. Here at Science Snapshots, rather than making our own “animal relationships” list, we decided to dive into a recent study of fish that create long-lasting partnerships to demonstrate how scientists approach interesting animal relationships and investigate why they occur.
Biologists refer to a long-term partnership between two individuals as a pair bond. While most people typically think of humans when talking about long-lasting relationships, pair bonding has evolved multiple times across the animal tree of life. Pair bonding usually occurs in species in which both parents contribute in some way to raising offspring . This ensures more offspring survive than if there were no pair bond. However, pair bonding has also been observed between individuals that are too young to reproduce, between individuals of the same sex, and in species that don’t have any form of parental care at all. Coral reef butterflyfishes in the genus Chaetodon from the Great Barrier Reef fit the latter description. These fish don’t provide any care to their young, yet they form pair bonds that can last up to seven years! Why they create these long-term partnerships remains a mystery. Because butterflyfish pairs aren’t caring for offspring, there must be some other benefit to this behavior. A group of scientists recently set out to test if food might explain why these relationships occur in two species of Chaetodon butterflyfishes: oval butterflyfish (C. lunulatus) and eastern triangle butterflyfish (C. baronessa).
One hypothesis for pair bonding in animals without parental care is that pairing up helps organisms defend high value resources, like food or shelter, which directly benefits both partners. The two species of butterflyfishes in this study feed almost entirely on coral. Since coral is a food that can’t run away, defending a good patch of reef with lots of coral to feed on is worthwhile, but defending that patch might be hard for a fish to do alone. Researchers, led by Dr. Jessica Nowicki (now a postdoctoral researcher at Stanford University), aimed to test if pair bonding in these butterflyfish species led to cooperative defense of coral resources and if that benefitted one or both partners. To answer these questions, Dr. Nowicki and her team conducted a series of observations and experiments in the wild near Lizard Island on the Great Barrier Reef. Scuba diving researchers followed fish around, observing and recording the amount of aggression butterflyfish pairs showed to other butterflyfish, as well as the level of coordination the partners showed by swimming close together and in synchrony. They also measured how frequently the fish in a pair fed on corals. To determine the benefits of pair bonding, these measurements were compared to those recorded for butterflyfish that did not have partners. For both the oval and the eastern triangle butterflyfish, paired individuals fed on coral more frequently than individuals that lacked a partner. Spending more time eating suggests that paired fish have more energy reserves than solitary fish. So being in a relationship is better than the single life, at least for these butterflyfish.
Interestingly, there seem to be differences in the way these partnerships work in the two species. For oval butterflyfish, partners spent most of their time swimming in sync with each other and both partners were similarly aggressive to neighboring butterflyfish. However, eastern triangle butterflyfish partners spent significantly less time swimming together because males were patrolling the borders of their territories while females spent most of their time feeding on their favorite types of corals in the heart of their territory. While patrolling, male partners of this species were more aggressive toward neighbors than their female partners. In fact, eastern triangle butterflyfishes generally showed higher levels of aggression than oval butterflyfishes. Although both species benefit from pair bonds, they do so slightly differently, with oval butterflyfish partners contributing equally to defense of their coral resources while male eastern triangle butterflyfish take on the bulk of resource defense.
After recording data on coordination, aggression, and feeding in naturally occurring butterflyfish pairs, the researchers then ran an experiment in which they removed one fish, “breaking-up” the partners to see if there were differences between established relationships and newly formed ones. It turns out butterflyfish get over break-ups really easily; all individuals in the study had new partners within 18 hours of their original partner being removed! While all fish made new partnerships quickly, there was a cost to these new relationships, at least at first. Compared to the original pairs, new butterflyfish pairs were overall more aggressive, both towards neighboring butterfly fish and towards their own partner. All this fighting was likely responsible for the lower feeding activity that the researchers also observed in the new partnerships. While these new partnerships got off to a rocky start, after four days the fish seemed to get used to their new partners, and aggression and feeding behavior gradually returned to levels observed in the original pairs. The researchers conducting this study concluded that one reason these butterflyfishes form long-lasting pair bonds is to avoid the conflict associated with new partners. Despite eventually working out their differences with a new partner, if butterflyfishes routinely changed partners, in the long run the initial conflict period of each new relationship might outweigh the feeding benefits of sharing a territory.
Next time you see one of those “animal relationship” posts, you might think about the science behind this knowledge. If it’s anything like this butterflyfish study, researchers may have spent hundreds of hours observing animals, all in order to increase our understanding of the natural word. And perhaps we can also gain some personal inspiration from our animal neighbors. Oval butterflyfishes show that sharing the work between partners can have benefits for everyone without all the aggression. Or maybe, if you’ve recently gone through a break-up, you can take some encouragement from this study: there are always more fish in the sea.
This Science Snapshot is a post highlighting a recent study from another university: Pair bond endurance promotes cooperative food defense and inhibits conflict in coral reef butterflyfishes
This post was written by Ryan Greenway. Ryan is a biology Ph.D. student that uses fish to study how ecology and evolution interact to create the diversity of life on our planet.