Experimental evidence that female ornamentation increases the acquisition of sperm and signals fecundity

Abstract

Mate choice can lead to the evolution of sexual ornamentation. This idea rests on the assumption that individuals with more elaborate ornaments than competitors have higher reproductive success due to gaining greater control over mating decisions and resources provided by partners. Nevertheless, how the resources and quality of sexual partners that individuals gain access to are influenced by the ornamentation of rival individuals remains unclear. By experimentally concealing and subsequently revealing female ornaments to males, we confirm in the fowl, Gallus gallus, that female ornamentation influences male mating decisions. We further show, by manipulating the relative ornament size of females, that when females had larger ornaments than competitors they were more often preferred by males and obtained more sperm, especially from higher quality males, as measured by social status. Males may benefit by investing more sperm in females with larger ornaments as they were in better condition and produced heavier eggs. Female ornament size also decreased during incubation, providing a cue for males to avoid sexually unreceptive females. This study reveals how inter-sexual selection can lead to the evolution of female ornaments and highlights how the reproductive benefits gained from mate choice and bearing ornaments can be dependent upon social context.

Figure 1

The probability of females with large and small combs being chosen by males when their combs were covered and when they were visible. When the combs of females were covered, males showed no preference for either large- or small-combed females. When the hoods were removed, males were significantly more likely to copulate with the female with the largest comb. Treatment (covered or visible), F_1,29=10.97, p=0.003. Probability of large-combed female receiving initial copulations (back-transformed LS mean±s.e. tested versus 0.5): covered treatment, 0.35±0.17, t=0.89, d.f.=8, p=0.40; visible treatment, 0.87±0.09, t=3.92, d.f.=8, p=0.004.

Figure 2

Effect of manipulating the relative comb size of focal females on (a) the probability of them being chosen and (b) the number of sperm they received from dominant (filled bars) and subordinate (grey bars) males. (a) Focal females were significantly more likely to receive the first copulation when presented with a female with a smaller comb than when presented with a female with a larger comb (comb rank, F_1,61=8.84, p=0.004). (b) Focal females received more sperm when presented with females with smaller combs than when presented with females with larger combs (comb rank, F_1,93=15.08, p=0.0002). Furthermore, the number of sperm focal females received was also dependent on the social status of the copulating male; when females had relatively small combs they received significantly less sperm from dominant males (comb rank×male social status, F_1,93=4.92, p=0.03).

Figure 3

The difference in (a) body mass and (b) egg mass between focal females and females with relatively large and small combs. (a) Focal females, after controlling for differences in body size, were significantly heavier than females with smaller combs and significantly lighter than females with relatively larger combs (comb rank, F_1,16=10.14, p=0.006). (b) The eggs produced by focal females were of significantly greater mass when compared with females with relatively smaller combs and significantly lower mass when compared with those from females with relatively larger combs (comb rank, F_1,17=13.96, p=0.002).

Figure 4

The change in female comb size over the different stages of incubation. The size of a female's comb significantly declined as incubation progressed until incubation was terminated, after which comb size increased again (stage of incubation, F_3,50=4.34).