Boldness and reproductive success in zebrafish

Animals in general react in similar ways to stressful stimuli. There are two basic mechanisms in which animals react to stress. some animals react by being aggressive and actively try to attack the thing that is perceived stressful by the individual while other animals react by being passive and show low levels of aggression. The aggressive individuals have a so called proactive coping style while the inactive non-aggressive individuals have a reactive coping style. Proactive animals often are dominant and reactive animals subordinate in a social interaction. The coping style an animal will show can be determined by many factors for example genotype or environmental factors. The differences between proactive and reactive individuals is not only visible in the behavior but also in their neuroendocrinology. In vertebrates stress is regulated through the so called Hypothalamic-Pituitary-Adrenal Axis (HPA-Axis) or as it is called in fish the HPI axis (where I stands for interrenal, which have the same function as the Adrenals in mammals). The regulation of this axis differ between proactive and reactive individuals where proactive individuals show a lower activity of the axis than reactive individuals. 

The zebrafish, which is a commonly used experimental animal, is a fish that lives in shoals of approximately 10-20 individuals. Zebrafish has its native range in India, Myanmar and Bangladesh where it lives in small streams and flooded plains. Being a social fish species they interact between each other a lot. However how the boldness of an individual zebrafish contribute to the social hierarchy in the group had until recently not been investigated. At the day of my birthday this year my former supervisor published a paper with me among others as a co author (Dahlbom et al. 2011) where we investigated if one could predict social status by testing for boldness in individual zebrafish i.e. if boldness correlates with coping style.

To test this we first tested the boldness of individual zebrafish and then put the fish in a tournament where they met conspecifics in two rounds to isolate the extremes in dominant and subordinate individuals (see Dahlbom et al. 2011 for details). What we could see was that individuals that proved to be bold in the initial boldness screen actually more often ended up dominant (proactive) in a encounter with another zebrafish and thus boldness seem to correlate with coping style shown in the social encounter. 

Recently the first paper citing our paper (Ariyomo and Watt, 2011) was published. They further investigated the effects of boldness and aggressiveness in zebrafish but in the area of reproductive success of male zebrafish. Could it be that bold and proactive males had higher reproductive fitness than shy subordinate males? They saw that if a female was paired with a male it didn't make any difference between bold and shy males on the number of laid eggs by the female. However bold and aggressive males succeeded better with fertilizing eggs. Thus, more eggs in the tanks with a bold aggressive male were fertilized than in thanks with a shy non-aggressive male. 

So to conclude bold zebrafish seem to be more likely to become dominant and be proactive than shy zebrafish. Bold zebrafish males also have a higher reproductive success than shy males by being more successful in fertilizing eggs. Then you might ask, why isn't all zebrafish bold and proactive, why is there still reactive shy individuals? This could be explained by the fact that the different coping styles actually both have their benefits, however in different environments, and thus are both types maintained in the population. 

References

Dahlbom SJ, Lagman D, Lundstedt-Enkel K, Sundström LF, & Winberg S (2011). Boldness predicts social status in zebrafish (Danio rerio). PloS one, 6 (8) PMID: 21858168


Ariyomo, T., & Watt, P. (2011). The effect of variation in boldness and aggressiveness on the reproductive success of zebrafish Animal Behaviour DOI: 10.1016/j.anbehav.2011.10.004