Sexual dimorphism in the horn size of a pair-forming coral reef butterflyfish

Abstract

Sexual dimorphism is a common in the animal kingdom and is often linked to mate choice or competition for mates in polygynous mating systems. However, sexual dimorphism is less common in species that form heterosexual pairs and has not been recorded in pair-forming coral-reef fish. Here we demonstrate a pronounced morphological difference between males and females in the humphead bannerfish (Heniochus varius)-a pair-forming coral reef butterflyfish. Males of paired individuals collected in Kimbe Bay, Papua New Guinea had substantially larger hump and horn protrusions on their heads than females. Fish were also sexed, sized and aged to determine the reproductive and demographic basis of the pairing behaviour. H. varius pairs were exclusively heterosexual and were assorted strongly by total length and slightly less so by age. Females in pairs were generally the same size as male partners, but were frequently older by a year and sometimes more. Hump and horn lengths increased proportionally to body-size in both sexes, with horns growing at a greater rate among males. These findings suggest that H. varius form pairs primarily for reproductive purposes, with selection via a size-assortative process that likely also extends to selection for larger hump and horn protrusions among males. The larger humps and horns in males appear to be the first recorded example of a secondary sexual morphological characteristic in a pair-forming coral reef fish species.

Fig 1. Diagrams of Heniochus varius illustrating their cranial hump and horn structures.

The first two images show sexual dimorphism between the female (A) and male (B) within a size assorted breeding pair. (C) Shows the size measurements taken. (D) Shows an X-ray image illustrating the internal structures of the hump and horn features.

Fig 2

(A) Length vs Age and (B) Weigth vs Length relationships for 76 individual Heniochus varius (25 pairs as well as 26 single individuals where the other member of the pair was not caught). Males are dark blue and females are gold.

Fig 3

Correlations between males and females for: (A) age, (B) total length, (C) raw mass, and (D) raw body depth, in 25 pairs of Heniochus varius. Residual mass (E) and body depth (F) relationships are also shown, after accounting for relationships in the raw metrics to total length. Each point represents a pair, dotted lines indicates line of equality or the expected point if the male and female had an identical metrics, and solid lines represent linear estimates for statistically significant correlations (alpha = 0.05).

Fig 4

(A) Differences in hump and horn lengths standardized by total body length, for males and females in 25 pairs of Heniochus varius. Within-pair intersex comparisons of residual (B) hump and (C) horn length, after accounting for relationships to total length. Dotted lines are lines of equidistance.

Fig 5

Sex differences in the relationships between total length and (A) hump length, and (B) horn length of 76 Heniochus varius individuals sampled in Kimbe Bay, Papua New Guinea. Females are gold and males are dark blue.