Environ Biol Fish DOI 10.1007/s10641-011-9838-7 Aggression in closely related Malawi cichlids varies inversely with habitat complexity Patrick D. Danley Received: 23 September 2010 /Accepted: 2 May 2011 # Springer Science+Business Media B.V. 2011 Abstract Within the past 2 million years, the cichlids strongly influenced by habitat type. The aggressive of Lake Malawi have diversified into well over 500 behavior of one sympatric species pair, Maylandia species resulting in one of the worlds largest lacustrine benetos and Maylandia zebra, was observed under fish radiations. As a result, many of the habitats within controlled laboratory conditions. Laboratory results the lake support a high diversity of species. In these support field observations: the bedrock associated highly species rich communities, male cichlids must species performed more aggressive acts and aggressive acquire and defend a territory to successfully reproduce. acts were directed equally at con- and heterospecifics. Within the rock-dwelling cichlids of Lake Malawi The results of this study suggest that habitat complex- (mbuna), this has resulted in the formation of poly- ity plays a larger role in shaping aggressive behavior specific leks on the heterogeneous rocky benthos. than other suggested factors such as competition for Aggression is fairly common in these leks and has been resources. tied not only to individual reproduction but to the larger phenomenon of community assembly and the mainte- Keywords Mbuna . Aggressive behavior . Speciation . nance of biological diversity. In this study, I examined Habitat utilization . Lake Malawi . Metriaclima . the patterns of aggressive acts of four species within the Pseudotropheus mbuna genus Maylandia at two locations in the southern Lake Malawi. The number of aggressive acts Abbreviations of two sympatric species was examined at each Mc-cobble Maylandia callainos location. At each site, one species defends territories Ma-bedrock Maylandia aurora over bedrock and the other over cobble. The number of Mz-cobble Maylandia zebra aggressive acts across the four species was compared. Mb-bedrock Maylandia benetos The influence of habitat type on male aggression was examined and the targets of male aggression were identified to evaluate several hypotheses concerning the evolution of male aggression. The results show that Introduction aggression quantitatively varied among species, was largely directed towards heterospecifics, and was The cichlid fishes of Lake Malawi have undergone an extraordinary radiation since the formation of the lake P. D. Danley (*) 1–2 million years ago (Meyer 1993; Albertson et al. Department of Biology, Baylor University, 1999). Adaptations to the different macrohabitats One Bear Place #97388, Waco, TX 76798, USA (Fryer and Iles 1972) and different diets (Liem e-mail: [email protected] 1991) have played an important role in the early Environ Biol Fish diversification of Lake Malawi cichlid fishes, while aggressive behavior may play in structuring species the evolution of reproductive characters, specifically rich communities (Mikami et al. 2004). To this end, female mate preferences, male color pattern, and male this study examines the aggressive behavior of four reproductive behavior, are thought to have played a closely related species of Lake Malawi’s rock- primary role in the diversification of the most closely dwelling cichlids. Through an analysis of their related extant species (Danley and Kocher 2001). behavior, I address several hypotheses concerning Male aggression is one aspect of cichlid reproductive the evolution of aggressive behavior in the mbuna. behavior that is gaining greater attention. Aggression is These include several competition for resources hypoth- necessary for males to acquire and retain breeding eses (mates, space, food). I find that the most compre- territories (Holzberg 1978). Within the rocky habitats, hensive hypothesis takes into account the complexity of this results in the establishment of year long poly- the habitat in which males establish territories. specific leks. Over and within the heterogeneous rocky benthos, males from as many as 40 species compete with each other to establish a territory from which they Methods may approach and display to passing females. In most species of mbuna, if the male successfully attracts a Lake Malawi mbuna female, she will follow him back to a breeding area typically located in the center of the male’sterritory. Mbuna communities tend to be highly diverse; at this While there appears to be some species specificity to study’s sites, territorial males could potentially interact type of substrate males of different species utilize with at least 10 other mbuna species. In these highly (Danley 2001), the highly heterogeneous environment diverse communities, males must acquire a territory to of the benthos dictates that heterospecific males successfully breed (Holzberg 1978). Within any given utilizing different rock types may occur in close territory, the physical features of the substrate define proximity to one another. This may lead to potentially multiple breeding areas and, generally, the male will high levels of interspecific competition and aggression. choose one such area as the focus of his territory. In While male aggressive behavior may have arisen doing so, the male prevents both con- and hetero- from the need to establish a platform from which males specific males from utilizing many of the remaining, can court and mate, male aggression may serve several unoccupied breeding areas within his territory. The additional functions. Male aggression may be used to necessity of male territory acquisition, the high reduce the number of potential competitors for females diversity of these communities, and the presence of by discouraging the presence of similarly colored males multiple potential breeding sites per territory provide (Seehausen and Schluter 2004). Heavily defended conditions that may generate interspecific aggression. territories develop ‘algal gardens’ which might provide trophic resources for females (Genner et al. 1999) Field analysis thereby increasing the male appeal. Alternatively, male aggression may be a secondary consequence of Study species selection on male vigor (Borgia and Coleman 2000) or may evolve in response to physical complexity of This study focused on the behavioral ecology of two the habitat (Jensen et al. 2005). Physically simple species at each of two separate communities (Fig. 1): habitats provide little cover for breeding and may Thumbi West Island (14° 01’27.58” S34°49’ 25.55” E) expose a breeding pair to higher rates of predation, egg and Mazinzi Reef (14° 7’55.98” S34°56’38.87”E). predation, or disruption from interlopers. Males in such These sites are located in the southwest and southeast habitats may need to increase their aggressive behavior basins of the lake, respectively. Mbuna communities at to successfully secure their breeding territory relative to each location are extremely diverse. Ribbink et al. males breeding over more complex habitat. (1983) identified 43 species of mbuna at Thumbi West Understanding the evolution and diversification of Island and at least 10 mbuna species occur at Mazinzi aggressive behavior of the mbuna will facilitate our Reef (Danley, pers. obs.). At each location, a number of understanding of the creation and maintenance of different habitat types can be identified (fields of biological diversity while informing us of the role that boulders >2 m in diameter, areas of rock-sand interface, Environ Biol Fish Fig. 1 Map of Malawi and inset of study area etc.) Of these habitat types, the study species typically Data analysis occupied habitats dominated by either a bedrock or cobble. Bedrock habitat consisted of flat stretches of Counts of aggressive acts were log10 transformed to continuous bedrock occasionally pocked by dislodged achieve a normal distribution. Bartlett’s and Levene’s strata. Cobble habitat contained areas of rocks 20– Tests were used throughout to confirm normality and 100 cm in size. Bedrock is a relatively simple habitat; it is homoscedacity of the data (data not shown). Non- generally uniform save for the closed ended caves parametric test were used when the data was not created by the dislodged strata. Cobble, in contrast, is normally distributed after transformation. An analysis topographically complex and possesses ample interstitial of variance (ANOVA) was performed to determine if areas beneath and between individual rocks. Behavioral significant differences existed in the number of observations at Thumbi West Island focused on May- aggressive acts performed by each species. A Tukey’s landia callainos (N=20, designated Mc-cobble), which post hoc test was used to identify significant differ- occupy territories over cobble, and Maylandia aurora ences between individual species. Wilcoxon tests were (N=18, Ma-bedrock), which tend to mate in cracks in used to compare the number of aggressive acts directed the bedrock. At Mazinzi Reef, behavioral observations towards conspecifics versus heterospecifics as these focused on one cobble dwelling species, Maylanida data were not normally distributed after transformation. zebra (N=15,Mz-cobble), and one bedrock inhabiting Similarly, Wilcoxon tests were used to compare the species, Maylandia benetos (N=15, Mb-bedrock). At number of aggressive acts directed towards mbuna each location, each focal male defended a territory versus non-mbuna species. An analysis of covariance