Polyandry without Polygyny

There are instances of several males in a species mating with only one female during a breeding season. This phenomenon has proven enigmatic to sexual selection theorists. Female mechanisms that regulate sperm competition make it advantageous for males to cooperate by banding together to keep other males out of scarce breeding areas. These male territory guardians have equal opportunity to inseminate the single female.

The Sandpiper: Near Complete Sex Role Reversal

Female sandpipers of certain species have been studied extensively to yield information on polyandry and sex role reversal. The female sandpiper will take the lead in courtship, grow larger and more violent than male sandpipers, and arrive on breeding grounds first (males traditionally arrive first in most other species). Upon arrival on the breeding grounds, females compete for territory (see image below). As a result of this role reversal, female sandpipers manage to attain a higher reproductive success rate than their male counterparts.

It has been suggested that polyandry is adaptive in sandpipers because of the variety of unusual ecological features:

• adult sex ratio is biased toward males
• spotted sandpipers nest in areas where there are large numbers of mayflies (nutrition)
• a single parent can care for offspring as well as two can (young are precocial)

The female sandpiper defends her mating area from a competing female. (Alcock 498)

There are a variety of mechanisms for females to acquire the sperm and parental care for offspring of many different males, the sandpiper is only one example. The benefits that the females are rewarded with, however, are fairly consistent. See Table 1.