New research into mbuna aggression

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Scientists have conducted research on the aggression of four mbuna species, both in the aquarium and in the field.

The species in question were Maylandia (or Metriaclima) callainos, M. aurora, M. zebra, and M. benetos, in the aquarium and at two sites in the field, Thumbi West Island (the first two species) and Mazinzi Reef (the other two).

They identified two different types of rocky habitat: bedrock, where only fractures in the rock provide hiding-places, and cobble, defined as boulders with interstices. 

They found that territorial males of M. callainos and M. zebra had a preference for cobble, and M. aurora and M. benetos for bedrock. Territorial males were identified and observed, and their interactions with other mbuna were documented.

Aquarium experiments involved males of M. zebra and M. benetos being placed alone in a tank with a flowerpot cave and females in sight in an adjacent tank to stimulate territoriality. Other males (of the same two species) were then introduced, in a clear protective container, to stimulate aggression, which was again recorded, in terms of biting and quivering.

The field results are particularly interesting. The two bedrock species were more aggressive than the two cobble species, with M. aurora the most aggressive and M. callainos the least. 

Not only did the bedrock species perform more aggressive acts in general, but their aggression appeared to be aimed mainly against non-mbuna (many of which sometimes feed in the rocky habitat even if they don’t breed there); they also attacked non-territorial mbuna and conspecific males. 

By contrast the cobble species made more attacks on mbuna than non-mbuna. M. zebra launched more attacks against heterospecifics, making M. callainos the only species to make more attacks on its own kind. The authors postulate that the overall high incidence of attacks on heterospecifics was simply because more of them came within range.   

The laboratory results paralleled those from the field, with the bedrock species used in the experiments (M. benetos) performing more aggressive acts than the cobble species (M. zebra).

The authors conclude their paper by examining how their results relate to various factors previously put forward as reasons for male aggression in mbuna: competition for females, access to breeding territories, competition for food resources, sexual selection for male vigour, and response to habitat complexity and increased/decreased exposure to predation or other interference.

While it may appear obvious to the aquarist familiar with these fishes that all of these factors probably play a part, science needs evidence to support such hypotheses before regarding them as proven. Hence this is valuable research, albeit only a drop in the overall ocean of mbuna behaviour. A very interesting paper.

For further information see: Danley, P. D. (2011) Aggression in closely related Malawi cichlids varies inversely with habitat complexity.  Environ. Biol. Fish. DOI 10.1007/s10641-011-9838-7

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