Social competition and selection in males and females

Abstract

During the latter half of the last century, evidence of reproductive competition between males and male selection by females led to the development of a stereotypical view of sex differences that characterized males as competitive and aggressive, and females as passive and choosy, which is currently being revised. Here, we compare social competition and its consequences for selection in males and females and argue that similar selection processes operate in both sexes and that contrasts between the sexes are quantitative rather than qualitative. We suggest that classifications of selection based on distinction between the form of competition or the components of fitness that are involved introduce unnecessary complexities and that the most useful approach in understanding the evolution and distribution of differences and similarities between the sexes is to compare the operation of selection in males and females in different reproductive systems.

Keywords: dominance status; mating systems; sex roles; sexual selection; social competition; social selection.

Figure 1.

Modes of reproductive competition in males and females. Fights among males are frequent and severe in many polygynous species, as in chacma baboons (Papio ursinus, (a), picture credit: Elise Huchard) but also occur among females, as in meerkats (Suricata suricatta, (b), picture credit: Andrew Young). Male infanticide is common in many mammals, including many polygynous primates, such as chacma baboons ((c), picture credit: Ryne A. Palombit), whereas female infanticide is also common in other species, including meerkats ((d), picture credit: Andrew Young). (Online version in colour.)

Figure 2.

Ornaments and armaments in shorebirds. Males are more highly ornamented in many polygynous shorebirds where they compete with each other on leks in order to gain sexual access to females, as in ruffs (Philomachus pugnax, (a), illustration of a lek by Johann Friedrich Naumann), whereas females are more highly ornamented than males in some polyandrous species where they compete with each other to gain access to paternal care, as in the painted snipe (Rostratula benghalensis, (b), illustration by S. Herbert). Similarly, armaments (here wing spurs highlighted by circles) are more developed in males than in females in polygynous species, as in the masked lapwing (Vanellus miles, (c), picture credit: Gary Stockton/CC BY-NC-ND 2.0) but are more highly developed in females in some polyandrous species, as in the northern jacana (Jacana spinosa, (d), picture credit: Benjamin Keen/CC-BY-SA-3.0). (Online version in colour.)

Figure 3.

Physiological suppression of reproductive function in males and females. The physiological suppression of reproductive function is common among males, even in species where males are solitary, as in orangutans (Pongo pygmaeus), where large males with pronounced secondary sexual characters ((a), picture credit: Michael Malherbe) suppress the development and reproductive function of younger males ((b), picture credit: Michael Malherbe) living in overlapping homeranges or in elephants (Loxodonta africana, (c), picture credit: Elise Huchard). Reproductive suppression is also common among females in singular breeders like Damaraland mole-rats (Fukomys damarensis, (d), picture credit: Markus Zoettl). (Online version in colour.)

Figure 4.

Horn length and fighting success in male and female Ontophagus beetles. (a) Relationship between male fighting success and horn length in Ontophagus taurus (n = 27 contests). Graph adapted from Moczek & Emlen [235], fig. 3. Picture credit: Tom Murray. (b) Relationship between relative horn length and relative fitness of competing females in O. sagittarius. Positive values on the x- and y-axes represent cases where, in pairs of competing females matched for body size, the focal female had a larger horn and produced more broods relative to her competitor. Graph reproduced from Watson & Simmons [236], fig. 3. Picture credit: Udo Smidt. (Online version in colour.)