Targeted territoriality
in Dusky Gregories (Stegastes nigricans)
Gabrielle McLaughlin, Louise Barbe & Pierre Courrier
University of Neuchâtel, 30th August 2019
Abstract
Figure 1 : Dusky Gregory
Dusky Gregorys (Stegastes nigricans) are algae farming damselfishes that have been described to exhibit high territoriality. While territoriality may be beneficial in terms of acquisition of resources such as food, shelter and mates, it can also be costly. In this study we observed Dusky Gregories on coral reefs in the Red Sea off the coast of Egypt to determine whether these fish might discriminately attack territory intruders based on the time of day and/or intruder diet (whether or not an intruder feeds on algae). While our results determined no significant difference for either factor, there was a trend to target herbivore intruders over non-herbivore intruders. Due to limited data collection, to better understand whether Dusky Gregories make discriminatory decisions when attacking intruders, future studies should further investigate these and other factors to gain further knowledge on these territorial interactions.
Introduction
Territoriality is a widespread behaviour in animals that may serve multiple functions such as access to food, shelter or mates (Jan et al., 2003). While variations of the definition exist, at its core it is described as the defense of an area (Threshner, 1976). Such behaviour is often observed in areas where species have overlapping ecology, such as in coral reef habitats. In this study, we focused on Dusky Gregories (Stegastes nigricans). S. nigricans (fig. 1 and 2) are algae farming damselfishes that have been described to exhibit high territoriality (Hamb, 2011; Johnson et al., 2011). These fish may be found living in reef flats, bays or lagoons ranging from the Red Sea to French Polynesia, Ryukyus, Mauritius and New Caledonia (Lieske and Myers, 2004). The presence of Dusky Gregories can benefit reef habitats by promoting coral and algal growth and diversity through their farming and territorial behaviours (Hamb, 2011; Lieske and Myers, 2004; Huisman and Wilson, 2012).
Figure 2 : Dusky Gregory
However, despite the many benefits of holding a territory, it can also be costly (Jan et al., 2003). Along with this knowledge, we gathered some information during preliminary swims on a coral reef located in the Red Sea off the coast of Egypt. We noticed that Dusky Gregories remain in areas within a patch and appear to defend it from a variety of fish species. We began to wonder if Dusky Gregories defend their patches from fish in a discriminatory manner, possibly to reduce on costs such as energy expenditure. For instance, it could be beneficial to target herbivorous fish that threaten their food resources rather than spending energy and risking injury by defending a carnivorous fish that poses no threat to this resource. From here we focused on two questions: Is the defence of the Dusky Gregory's territory linked to the diet of actively hunted intruders? Are Dusky Gregorys more aggressive in the late afternoon than in the morning or vice versa?
From these questions we were able to make our hypotheses and predictions:
H0 : Time of day (morning or evening) and diet type (herbivore or non-herbivore) do not affect Dusky Gregorys defense of territory.
Prediction : Dusky Gregorys will defend their territory at the same frequency in the morning and the evening against intruders of all diet types.
H1: Time of day (morning or evening) and/or diet type (herbivore or non-herbivore) affect Dusky Gregory territory defense.
Prediction: Dusky Gregories will selectively defend their territories from herbivores rather than non-herbivores (or vice versa) and will do so more aggressively in the morning than in the evening (or vice versa).
Materials & Methods
Figure 3 : Ducks Diving Center in Mangrove Bay, starting point of our observations
Observations were made on a reef located in the Red Sea, off the coast of El Qousseir, Egypt (fig. 3). The reef located on the coast of the Mangrove Bay Resort was in good health. This is most likely due to the reef being located in a small bay that allows for protection from wind and wave damage. Further, the reef is under active protection by members of the Ducks Diving Center at the Mangrove Bay Resort.
A total of 18 observations were carried out in the in the field between April 8-11, 2019 : 9 in the morning (between 8:00 and 10:00am) and 9 in the afternoon (between 5:00 and 6:00pm). Each observation was made on a new patch, for a total of 18 different patches. The patches were situated at approximately 30 centimeters (cm) to 1,5 m below the surface and had a surface area of approximately 2 to 4 m^2. Patches were observed from approximately 2 meters (m) away for a duration of 30 minutes. Some observations were made by humans while others were performed using video recordings.
Observations made by video recordings were performed using a GoPro camera that was secured to a box filled with stones, which held it on the ground.
Each patch was occupied by several Dusky Gregories (3 to 12). The objective of these observations was to count the number of intruders Dusky Gregories would defend from their territory. A defense of territory is defined by an increase in speed towards an intruding fish with or without contact. A fish was considered as an intruder when it came within approximately 1m of the patch. Each intruding fish was counted and identified by family. For the analysis, the different families were also categorised according to their diet types: herbivores or non-herbivores.
Data was analyzed using the software RStudio. A generalised linear model (GLM) was performed using compiled data for each family. While compiling the data has the disadvantage of making the model less representative of reality, it allows for a more robust model.
Results
The statistical analysis did not demonstrate an interaction between the fixed factors "time of the day" and "diet type" (p = 0,59). Nor did it detect an effect of the time of the day on the frequency of Dusky Gregory's defenses (p= 0.93, fig.4).
While the results show no significant effect of diet type on the Dusky Gregory’s defence of territory, with a p-value of 0.09, there is a tendency for attacks towards herbivores over non-herbivores. Due to the small amount of data, these results are to be taken with a grain of salt.
Figure 5 : Likelihood of Dusky Gregory territory defense based on intruder diet type.
Figure 4 : Effect of the time of day on Dusky Gregory attack ratios.
Also, a slight difference can generally be observed between herbivores (green) and non-herbivores (yellow) regarding Dusky Gregories defense behavior.
For example, Angelfishes, Squirrelfish and Wrassefish appear to be less chased off than Parrotfish and Damselfish but again, nothing is significant.As shown (fig. 5), Soldier fish seem to be completely ignored by the Dusky Gregory, however this could be explained by the fact that only 6 Soldier fish were observed.
Six data points do not allow arbitrary conclusions to be drawn.
Discussion and Future directions
Confirming our prediction, our model shows that time of day (morning and night) has no effect on Dusky Gregory territory defence. This result seems logical because defending the patch for only part of the day would not properly protect resources. The other fish could then eat all the algae that has previously been cultivated. However, the factor of time may differ depending on the season, for instance Dusky Gregories may potentially increase defence towards carnivores during the breeding season in order to protect their eggs. Also, the reproductive activity increases in summer and could influence the antagonistic behaviour of the Dusky Gregory because of displays, egg-guarding and spawning (Letourneur 1992). Furthermore, the temperature of the water, depending on the day or the circadian cycle of the fish, could have caused a difference but we did not notice anything significant. The other prediction claiming that the Dusky Gregory would repel herbivores more than non-herbivores is still verified with a small tendency to attack herbivores. However, here again the natural environment has been greatly simplified with the statistical model and the data taking. Indeed, we simply analyzed the different fish families while some species of fish eat more or less algae within the same family. This could explain the result of only obtaining a "tendency" and not a "significance".
Many variables were able to bias observations, therefore finding a sufficiently significant factor was complicated, especially due to the small number of observations. One of the major biases is whether an intruder that arrives in the patch comes to feed on algae or something else; the Dusky Gregory could attack the intruder thinking that it would come to eat the algae while the intruder is only in fact interested by nearby coral or other food. As noted by R.E. Tresher (1976), Dusky Gregories may also defend their patch for other reasons; they may protect their eggs by attacking intruders, or defend the territory from other fish that could be seeking refuge, which can strongly influence the herbivore / non-herbivore analysis.
Additionally, we sometimes noticed a group effect when several fish, sometimes very numerous, attack a patch at the same time. The Dusky Gregories are then outnumbered by other fish and cannot all push them back at the same time. This group effect also adds difficulty in data accuracy (for instance by attempting to count 20+ fish at once), so systematic video use may be useful in the future for a more detailed analysis.
Future directions
We didn’t analyse the influence of the size of the fish because previous publications mentioned that it has only a minimal effect on the likelihood of attack by the Dusky Gregory (Tresher, 1976), however it could be interesting to look at the difference of the juvenile versus adult fish in the attack frequency, as well as the aggression toward inter and intra-specific species.
More observations would have been necessary in order to obtain more meaningful results closer to reality. Moreover, since the observations were made near the patches, the human presence could influence the behavior of the Dusky Gregory and distort the results. The use of video would again be an advantage to get closer to natural conditions, and to compare with our previous results. Additionally, we sometimes thought to observe a competitiveness between Dusky Gregories of the same patch, and so it could be interesting to focus on the nature of this phenomenon to know more.
Conclusion
Despite our results showing no significance for either factor, we did find that Dusky Gregories have a tendency to attack herbivores more than non-herbivores. It seems that the time of day has no influence on the attack frequency by DuskyGregories. Many approximations were made during the statistical analysis as well as the data taking, which leads us to be careful when considering the results.
References
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