Function of nuchal humps of a cichlid fish from Lake Tanganyika: inferences from morphological data

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Abstract

Nuchal humps occur in various fish groups, but their functions are rarely studied. Both sexes of Cyphotilapia gibberosa, a cichlid fish endemic to Lake Tanganyika, possess nuchal humps. This study morphologically analysed the humps of this species to assess the primary factors responsible for hump development. Semi-landmarks showed that the size of male humps positively correlated with body condition, and thicknesses of hypodermises implied that the humps were larger in males than in females. These facts suggest that sexual selection on male humps is intense. Semi-landmarks showed that the humps were less prominent in females than in males, and that the transformation of nuchal humps with growth (becoming more prominent) ceased at medium body size in females. These facts suggest that sex recognition likely plays a role in the evolution of the uniform humps of large females, because, according to this hypothesis, a morphologically moderate hump may be favoured. If male humps also function as a sex recognition trait, the transformation of nuchal humps is expected to cease at a certain body size, as did female humps. However, the male humps became continuously more prominent from the smallest to the largest individuals examined. The body size at which hump transformation stops or at which extreme hump shape interferes with sex recognition may be outside of the size range of the present samples. A prominent nuchal hump may also function as a species recognition trait, because sympatric cichlid species do not develop nuchal humps as prominent as this species. In short, the present morphological analyses do not contradict the hypotheses that C. gibberosa individuals recognise conspecifics and their sex from the shapes of the nuchal humps, and that females prefer males with larger humps.

Keywords

Cyphotilapia gibberosa Lipid storage Sexual selection Sex recognition Species recognition 

Notes

Acknowledgements

We thank H. Phiri, M. Mbewe, T. Banda, L. Makasa and the other staff members of the Lake Tanganyika Research Unit in Mpulungu for support in the field. We are also grateful to M. Hori and M. Kohda for providing research equipment for the fieldwork, and to K. Tsuji for advice on the manuscript. This study was partly supported by a Grant-in-Aid for Scientific Research (B) (No. 26291078) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan.

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Copyright information

© The Ichthyological Society of Japan 2018

Authors and Affiliations

  1. 1.Institute of Natural and Environmental SciencesUniversity of HyogoSandaJapan
  2. 2.Division of Nature and Environmental ManagementMuseum of Nature and Human ActivitiesSandaJapan

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