Polyandry

Definition and Evolutionary Context

Polyandry, derived from the Greek words poly (many) and andros (man), describes a mating system in which a single female mates and often forms pair bonds with multiple males during a single breeding season or reproductive cycle. This system stands in contrast to polygyny, where one male mates with multiple females, and monogamy, where individuals form exclusive pair bonds. From an evolutionary perspective, polyandry is particularly intriguing because it reverses typical sex roles observed in many species, where males are often considered the more competitive and promiscuous sex, and females the more selective sex, due to the higher energetic investment in reproduction (Trivers, 1972).

While polyandry is common in some insect species (e.g., honeybees, some butterflies) and fish, it is exceptionally rare among mammals, with only a handful of well-documented cases. Its existence challenges straightforward applications of Bateman's principle (1948), which posits that male reproductive success is limited by access to females, while female reproductive success is limited by resources and gestation/lactation. Polyandry suggests scenarios where females may benefit from multiple partners, or where ecological pressures constrain male reproductive opportunities, leading to a reversal of typical sexual selection dynamics.

Types and Mechanisms of Polyandry

Polyandrous systems can be broadly categorized based on the nature of the female-male relationships and parental investment:

Cooperative Polyandry

In cooperative polyandry, multiple males assist a single female in raising her offspring. This form is most commonly observed in species where parental care is arduous and requires more than one attendant, or where resources are scarce. A classic example is the cooperatively breeding marmosets and tamarins (Hrdy, 1981). In these New World monkeys, females typically give birth to twins or triplets, which represent a substantial energetic burden. Multiple males (often brothers or close kin) assist the female in carrying, provisioning, and protecting the young. The males gain indirect fitness benefits by helping kin, or direct benefits if they are among the fathers, increasing the survival chances of the offspring.

Another example is the Tasmanian hen (Tribonyx mortierii), where a female mates with two males, and all three participate in incubating eggs and raising chicks. The males are often brothers, suggesting inclusive fitness benefits for the non-siring male.

Sequential Polyandry

In sequential polyandry, a female mates with one male, lays a clutch of eggs, and then leaves that male to incubate and care for the young while she mates with another male and repeats the process. This system effectively allows the female to increase her reproductive output by producing multiple clutches in a single breeding season. This form is characteristic of some birds, such as phalaropes and jacanas. In these species, males typically perform all or most of the incubation and chick-rearing duties, freeing the female to produce more clutches. The ecological conditions that favor sequential polyandry often include abundant food resources, which allow females to quickly replenish energy reserves, and high predation pressure, which makes multiple small clutches more advantageous than one large one.

Other Forms

• Fertility Insurance Polyandry: Females may mate with multiple males to ensure fertilization, especially if the primary partner has low fertility or if there's a risk of sperm depletion. This is more common in insects and fish.
• Good Genes Polyandry: Females may mate with multiple males to acquire genetically superior sperm, even if the primary partner is already adequate. This could lead to increased offspring viability or attractiveness (e.g., "sexy son" hypothesis).
• Resource Acquisition Polyandry: In some cases, females may mate with multiple males to gain access to resources (e.g., food, nesting sites) or protection that different males can offer. This is particularly relevant in species where males control valuable territories or resources.

Human Polyandry

While extremely rare globally, polyandry has been documented in certain human societies, primarily in harsh, resource-scarce environments. The most well-known form is fraternal polyandry, found historically in parts of Tibet, Nepal, and among certain groups in India. In this system, a woman marries a set of brothers. The children are considered to be the offspring of the household, and paternity is often not strictly assigned to a single brother, or the eldest brother is considered the social father.

Evolutionary explanations for fraternal polyandry often center on ecological and economic factors (Goldstein, 1976). In environments with limited arable land, fraternal polyandry prevents the fragmentation of family land holdings through inheritance, thus maintaining a viable economic unit. It also limits population growth in resource-poor areas. By sharing one wife, brothers avoid dividing the family estate, allowing them to pool labor and resources, which can lead to a higher standard of living than if each brother married separately and established his own household on smaller, less productive plots of land. This system also reduces the number of women who reproduce, which can be adaptive in environments with scarce resources.

Critics of purely economic explanations for human polyandry point to the complex social dynamics and cultural norms that also play a role, arguing that while economic pressures may provide a context, cultural evolution shapes the specific manifestation of such systems (Levine, 1988). The psychological adaptations that might underpin or respond to human polyandry are a subject of ongoing anthropological and evolutionary psychological inquiry.

Critiques and Open Questions

The study of polyandry continues to challenge and refine evolutionary theories of sexual selection. A key question is why polyandry is so rare, especially among mammals, given the potential fitness benefits for females. The costs associated with polyandry, such as increased risk of sexually transmitted infections, potential for male-male conflict, and the complexities of shared parental investment, may often outweigh the benefits.

Furthermore, the precise mechanisms by which females choose multiple mates and manage their relationships remain areas of active research. The genetic consequences of polyandry, such as increased genetic diversity among offspring or the potential for cryptic female choice, are also subjects of ongoing investigation. Understanding polyandry requires a nuanced approach that integrates ecological, social, and genetic factors to explain its sporadic occurrence and diverse manifestations across species.