Sexual Selection

The thorn in Darwin’s side

Types of Sexual Selection#1 - Intra-sexual competition:

Males compete with males for mating partners

Females passivePolygamy common

Types of Sexual Selection#2 - Inter-sexual competition:

Females select males with direct natural selective advantages for their offspring.

Once the preference exists, those males will be more fit -> leads to an ever-increasing force towards exaggeration of trait.

Male Competition or Female Choice?

#1 - Barrette and Vandal (1990) studied sparring in caribou. Of 713 matches between males of different antler size, males with smaller antlers withdrew 90% of the time.

Male Competition or Female Choice?

#2 - Eberhard (1979,1980) studied stag beetles in the use of horns "in beetles, finding that they tend to be used either to pry a rival off his site or to lift and drop the rival to the ground.

Male Competition or Female Choice?

#3 - Male birds of paradise exhibit a highly exaggerated and flashy tail. Males with more exaggerated tails attract more predators, but also have more mating success.

Male Competition or Female Choice?

#4 - During mating season, the pouch of the male frigate bird becomes conspicuously inflated and changes in color from orange to a distinctly brilliant shade of red.

Male Competition or Female Choice?

#5 - Pleszczynska (1978) showed that male lark buntings with more nest cover in their territory attracted more females, and these females had higher breeding success.

Male Competition or Female Choice?

#6 - In a recent experiment, zoologist Marion Petrie of Oxford University placed a different peacock in each of eight pens containing a small number of randomly chosen peahens. She then charted the progress of the 350 chicks that hatched. Chicks that grew the fastest and survived the longest, had fathers with trains that had the largest eyespots

Male Competition or Female Choice?

#7 - Male spring peepers calls draw a lot of attention- they can be heard from a mile away. Gerhardt and Klump’s study (1987) suggest that if a male cannot call above the chorus call then he will not attract females.

Male Competition or Female Choice?

#8 - Male elephant seals can be as large as 450 cm and weigh 1,800-2,700 kg. Females can be 360 cm long and weigh 900 kg. Males have a pronounced proboscis, corrugated neck shields.

Male Competition or Female Choice?

#9 - Humans…. what do you think? Do males compete for females, or do females choose their mate?