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, 272 (1559), 149-57

Antlers Honestly Advertise Sperm Production and Quality

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Antlers Honestly Advertise Sperm Production and Quality

Aurelio F Malo et al. Proc Biol Sci.

Abstract

Evolutionary theory proposes that exaggerated male traits have evolved via sexual selection, either through female mate choice or male-male competition. While female preferences for ornamented males have been amply demonstrated in other taxa, among mammals sexual characters are commonly regarded as weapons whose main function is to enhance male competitiveness in agonistic encounters. One particularly controversial hypothesis to explain the function of male sexual characters proposes that they advertise male fertility. We test this hypothesis in red deer (Cervus elaphus), a species where sexual characters (antlers) reach an extreme degree of elaboration. We find that a global measure of relative antler size and complexity is associated with relative testes size and sperm velocity. Our results exclude the possibility that condition dependence, age or time of culling, drive these associations. Red deer antlers could signal male fertility to females, the ability to avoid sperm depletion throughout the reproductive season and/or the competitive ability of ejaculates. By contrast, male antlers could also signal to other males not only their competitive ability at the behavioural level (fighting ability) but also at the physiological level (sperm competition).

Figures

Figure 1
Figure 1
(a) Measures recorded to analyse relative antler size and complexity. (i) Length of the main beam from the burr to the furthermost point of the crown; (ii) length of the frontal point; (iii) length of the middle point. The three diameters were calculated by measuring two transversal diameters at a given level of the antler. (iv) Burr diameter; (v) beam diameter was taken halfway above the frontal tip and below the middle point (lower diameter); (vi) beam diameter was taken halfway above the middle point and below the crown ramification (upper diameter); (vii) number of crown points; (viii) number of total points (only points larger than 2 cm were considered). Given that all variables were correlated with body length, residuals were used. (b) This figure shows five of the six parameters that determine sperm velocity. The one-way grey arrows graphically define the three sperm velocities: curvilinear velocity (VCL), straight-line velocity (VSL) and average path velocity (VAP). The two-way arrow shows amplitude of lateral head displacement (ALH). Circles show beat crosses that define beat cross frequency (BCF). Linearity (LIN), defined as the ratio of distances of straight-line track length and actual track length, is not shown. Actual track, solid line; smoothed track, dotted line; straight-line track, dashed line.
Figure 2
Figure 2
(a) Relationship between relative antler size and complexity (FS relative antler size) and relative testes size (n=75). (b) Relationship between relative antler size and complexity (FS relative antler size) and sperm velocity (FS sperm velocity) (n=73).

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