Many animals can forego an immediate reward for a bigger or better reward in the future. The ability to ‘delay gratification’ has been demonstrated in chimpanzees, dogs, monkeys, crows, cockatoos, pigeons, and rats. Earlier this year, the ability was demonstrated in cuttlefish. Now researchers have found a new addition to the club – the cleaner wrasse.
In humans, delayed gratification is synonymous with the Marshmallow Test – an experiment designed at Stanford in the 60s by the late Walter Mischel. In the experiment, kids were sat in a room and given a marshmallow. A researcher then told them that they could eat it immediately or if they held off until the researcher came back into the room, eat two instead.
There was considerable variability in the time that children managed to go without eating the marshmallow. The experiment became more popular as Walter Mischel subsequently published studies tying the results of the marshmallow test to achievements of these kids in later life (however, many of the of the reported correlations have failed to replicate).
Humans are willing to wait months and forego an immediate reward if the delayed reward is a larger sum of money. Most animals wait less than a minute before they eat their immediate reward (however, chimpanzees are more patient than humans when it comes to delayed food rewards). This begs the question – is the ability to delay gratification limited to mammals and some birds?
Researchers Melisande Aellen, Valerie Dufour, and Redouan Bshary tested the Bluestreak Cleaner Wrasse (L. dimidiatus), on two delayed gratification tasks. This small fish lives near coral reefs and is known to remove ectoparasites from “client” fish.
In one of the tasks (“quantitative task”), the fish was given more of the same food (a bigger reward) if it did not eat the immediate reward, similar to the Marshmallow Test. In the other (“qualitative task”), the delayed reward was a more sought after food item (a better reward) of the same quantity.
Like the kids in the marshmallow test, there was significant individual variation in the period of time fishes were willing to wait for their delayed reward. When the delayed reward was bigger (two, four or eight times more), the fish were willing to wait a maximum of 8 minutes. According to the researchers, the performance of wrasse in the quantitative task is comparable to that of monkeys.
“Comparing our results to the existing literature, it appears that the wrasse generally outperforms corvids, matching the performance of monkeys. The highest performing cleaner fish, L. dimidiatus individuals, delayed gratification on a level that is similar to those of chimpanzees and dogs.”
When it was a better quality reward, fishes waited only up to 7 seconds before devouring the immediate reward (the fishes were not starved and the preference for the higher quality reward – ‘prawn’ as compared to the lower quality item – ‘fish flakes’ had been established prior to the experiment)
“The cleaner L. dimidiatus only waited up to 7 s for a higher quality food reward, which contrasts with their performance in quantitative versions of the task.”
The researchers also tested other types of wrasse – the Bicolor Cleaner Wrasse and two non-cleaner wrasse (Three-Spot Wrasse and Checkerboard Wrasse). They had hypothesized that cleaner wrasses would do significantly better at the tasks due to their ecology. The relevant aspect of cleaner wrasse ecology is this – although they feed on ectoparasites of client fish, cleaner wrasse prefer to eat their client’s mucus.
But, mucus-eating (understandably) is a deal-breaker for the client fish, who chase away the cleaner wrasse or just leave. To continuously feed on ectoparasites, wrasse have to control their urge to eat clients’ mucus.
Researchers hypothesized that since self-restrain is key to the cleaner wrasse’s business, they will do better at the delayed gratification task compared to the non-cleaning wrasse. However, non-cleaner wrasse performed equally well in the tasks. According to the researchers, this suggests that the ability to delay gratification is a lot more common in fishes like the wrasse than we imagine. Also, advanced cognitive abilities only present in mammals and birds may not be necessary to delay gratification.
“given that Labroides (cleaner wrasse) and Halichoeres (non-cleaner wrasse) are separate genera within the Labridae family, it appears that the ability (to delay gratification) is generally present in wrasse and hence potentially also at higher taxonomic levels within fishes.”
Read more about the study here.
