C. K. Cornwallis
Unfamiliar male domestic fowl (Gallus gallus) engage in escalated aggressive interactions, sometimes lasting several hours, which result in the establishment of a relatively stable dominance hierarchy among males. Dominant males enjoy enhanced access to females and adopt a status-specific strategy of selective allocation of sperm to higher quality females when multiple females are available. Subordinate males allocate high numbers of sperm to initial copulations, regardless of female availability or quality. Where social status changes within a breeding season, males adjust their reproductive strategy accordingly.
FURTHER READING: Lee A. Dugatkin, Principles of Animal Behavior, 3rd ed. (New York: W. W. Norton, 2013), chap. 7, “Sexual Selection”; chap. 8, “Mating Systems”; chap. 15, “Aggression.” C. K. Cornwallis and T. R. Birkhead, “Social status and availability of females determine patterns of sperm allocation in the fowl,” Evolution 60 (2006), pp. 1486–1493.
Joris M. Koene
The mutual mating of hermaphroditic Japanese land snails (Euhadra subnimbosa) draws to a close, and the calcareous spear of one partner can be seen pulsating repeatedly in the center of the frame. Among the simultaneously hermaphroditic terrestrial snails and slugs, there are many species that reciprocally impale each other with calcareous spears or “love darts” toward the end of their courtship sequence. Debate has raged for years as to the function of these darts, though empirical research suggests that they serve to promote the storage and eventual use of donated sperm in fertilizing eggs, rather than as a nutritional resource or means of enforcing reciprocity in gamete exchange. Love darts thus play a role in post-copulatory sexual selection.
FURTHER READING: Lee A. Dugatkin, Principles of Animal Behavior, 3rd ed. (New York: Norton, 2013), Chap. 7, “Sexual Selection”; Chap. 8, “Mating Systems.” J. M. Koene, & S. Chiba, The way of the samurai snail, The American Naturalist 168 (2006), pp. 553–55.
Video by Anders Berglund
In the first sequence, a deepsnouted pipefish (Syngnathus typhle) female is seen feeding, without expressing any courtship signals. In the second sequence, the same female is seen dancing in the presence of a prospective male mate while displaying black-striped ornamentation along the sides of her body. The display persists throughout the course of copulation (third sequence), which culminates in the release of the female’s eggs into the white brood pouch under the male’s tail. The male then shakes the eggs down into the brood pouch, and he assumes a characteristic S-shaped posture as he fertilizes the eggs. The male then provides parental care for developing young until their emergence from the pouch as fry. In this and other sex-role-reversed species, access to males limits female reproductive success. Females in such species often mate with multiple partners, and they commonly compete for access to males both overtly and via sexually selected displays. In some insects and birds, females may even commit infanticide as a means of obtaining reproductive opportunities. While females in such species are typically less choosy than males, females remain discriminating in their choice of male mates.
FURTHER READING: Lee A. Dugatkin, Principles of Animal Behavior, 3rd ed. (New York: W. W. Norton, 2013), chap. 7, “Sexual Selection.” A. Berglund, M. Sandvik Widemo, and G. Rosenqvist, “Sex-role reversal revisited: Choosy females and ornamented, competitive males in a pipefish,” Behavioral Ecology 16 (2005), pp. 649–655.
Courtesy David Reby
A red deer stag (Cervus elaphus) with a harem of females nearby responds to the playback of conspecific male roars by issuing roars of his own. Intensive long-term research conducted by Tim Clutton-Brock and his colleagues on red deer inhabiting the Isle of Rum has revealed that male red deer employ ritualized roaring contests to assess potential rivals. The formant frequencies of roars are inversely correlated with the length of a stag’s vocal tract, and hence they encode information on the signaler’s body size. In a playback experiment manipulating the format frequency of roars, David Reby and his colleagues demonstrated that stags were more attentive, replied with more roars, and extended their own vocal tracts more in response to the playback of roars with lower formant frequencies. Red deer thus not only use information available in the format frequency of rival stags’ roars to assess the size of potential competitors, but they also adjust their own acoustic signals in response to those of their competitors. Selection favors such assessment (in that body size is a reliable predictor of competitive ability), and individuals attending to such cues can adjust their behavior accordingly, adopting a “hawk” strategy where they are likely to win an escalated contest, or a “dove” strategy where they have lower resource holding potential than their rival.
FURTHER READING: Lee A. Dugatkin, Principles of Animal Behavior, 3rd ed. (New York: W. W. Norton, 2013), chap. 7, “Sexual Selection”; chap. 13, “Communication”; chap. 15, “Aggression.” D. Reby, K. McComb, B. Cargnelutti, C. Darwin, W. T. Fitch, and T. Clutton-Brock, “Red deer stags use formants as assessment cues during intrasexual agonistic interactions,” Proceedings of the Royal Society of London, Series B, Biological Sciences 272 (2005), pp. 941–947.)