In a typical monogamous system, the male will build a nest within his territory; he will then attempt to attract a female to this nest. Upon attracting the female to the nest the pair will engage in ‘social monogamy’. True monogamy is not observed due to the occurrence of extra pair copulations (where the male or female engages in copulation with other males/ females). However, they still remain in the nest with their partner in a socially monogamous system. Biparental care is observed in nearly all monogamous species
Polygynous mating systems sometimes consist of biparental care, in cases where biparental care does not occur however, it is only typically the female who provides parental care. Below are a few examples of different polygynous mating systems:
1. A single male will attempt to attract multiple females to a single nest within his territory
2. A single male will visit multiple female nests
3. A single male will build multiple nests in different territories and attempt to attract females to them
Polyandry is a rarely observed mating system. Care is often biparental or if not, male only. Polyandry consists of the female building a nest within a single territory. She then attempts to attract multiple males to the different nests. This is essentially a role reversal of the second example of polygyny.
Why are birds mostly monogamous?
(Lack, 1968) – Males don’t want to attract more than one female. This is because reproductive fitness of the males is essentially reduced by an increasing amount of females.
Males and females will have more offspring if they share in the raising of a brood; this is because biparental care has a profound effect on fledgling survival.
The problem is that birds are endothermic and lay eggs, they must therefore keep the eggs constantly warm, but at the same time they must forage for food which would leave the eggs cold and exposed to predation. Biparental care allows one parent to keep the eggs warm whilst the other goes foraging for food.
Raptors (large sea birds) have long travel distances when searching for food. Because of this, single parent Raptor species have 0% chick survival. The long periods of time the parent must spend searching for food leaves the eggs/chicks open to predation and exposed to the cold for too long to survive.
Advantage of Male Parental Care
The survivability of chicks decreases if the male dies. The larger the male of the species is, the more profound the decrease in survivability is. The survivability of the chicks can vary greatly dependant on species however. For example, in species where the male is extremely large (such as the Raptor birds of prey) if the male dies, the chick survivability decreases to 0. In smaller species, the survivability may remain above 50%. Because of this, when survivability is above 50%, it is beneficial for the male to attract multiple females as the females are less dependent upon him.
Monogamy as Female Decision
Females lose out in polygynous situations. This is because absence of the male e.g. Death, decreases fledgling success. But also because the energetic cost of raising the young is increased for the female as she has to raise them alone. This affects her future production of offspring.
In an experiment by Wright and Cuthill (1989), they attached a weight to the male of a monogamous couple to simulate the male engaging in multiple parental care roles. The weight reduced his activity around the nest and as a result the female compensated by increasing her workload around the nest. The female suffered in terms of reproductive success next winter as a result.
Female Resistance against Later Female Arrival
The level of female aggression against later female intruders directly relates to the future mating status of the male. Males which mated with passive females (who showed little aggression against female intruders), were more likely to engage in polygynous mating activity. Males which mated with aggressive females were much more likely to remain in a monogamous relationship. This was proven by an experiment conducted by Sandell (1998) on starlings, using caged female intruders.
Diversity of Monogamous Systems
Within monogamous systems it is possible to find sexual infidelity (promiscuity). Therefore these monogamous systems are not truly monogamous – monogamy is only expressed in the way that the offspring are brought up i.e. biparental care.
Extra Pair Copulations
Sexual infidelity that occurs in monogamous relationships is known as extra pair copulation. This is where mating occurs outside of the monogamous pair bond whilst the partner is away and is common in many species e.g. Red winged blackbird.
The result of extra pair copulations can be observed by the varying rates in extra pair paternity. There are varying rates of chicks from other males (extra pair paternity) and chicks raised by other males between species.
Female Gain from EPCs
The benefit to males from extra pair copulation is simply that he is able to spread his genes around, however it is a little less obvious why EPC is beneficial to females, below are some examples:
1. Sterile Males – A female may want to mate with more males as there is a possibility that the initial male was sterile. If this was the case and the female did not mate with another male she would have essentially wasted a reproductive year. It is therefore beneficial to engage in EPCs to ensure she is fertilised.
2. Harassment – Some believe that females engage in EPCs simply to reduce the amount of harassment she receives from other males attempting to copulate with her. But this doesn’t have to be the same in humans. We have to realise that we can get legal help if you get sexually harassed.
3. Genetic Benefits – The female may attempt to get ‘good genes’ from another male, rather than the male which she has paired with i.e. the male who will provide parental care for the offspring.
4. Mating with non-related males – By participating in many EPCs, the female increases the chance of having offspring which are not from a related male.
EPC Reduction of Male Fitness
Female EPCs reduce the resident paired male’s genetic fitness (as he is less likely to be the father). To reduce the impact of EPCs, the resident male may:
1. Engage in frequent copulation to ensure sperm competition.
2. Perform retaliatory copulation i.e. forced copulation when the male realises that the female has engaged in an EPC.
EPC Attempts by Males
In an experiment conducted to observe how males responded to the threat of EPCs, a caged or uncaged paired male was present, with many neighbour males.
When the paired male was caged (i.e. No defence abilities), neighbouring males made more attempts to engage in EPCs with the female. When the male was uncaged however, there were no attempts by the other males to engage in EPCs with the female. This shows that if there is an opportunity to engage in EPC, males will take advantage of it.
There are three main reasons for why polygyny may occur:
2. Other factors important in fitness
3. Excess resources
Polygyny – Deception
The male will attempt to deceive the female into thinking that he is single, allowing him to get more than one mate.
An example of this is the polyterritoriality in the pied flycatcher:
- The male keeps two nests 200m-2km apart to which he attracts females.
- The second female suffers as the male is pair bonded to the initial female and spends the majority of time at that nest where he invests more parental care. The second female produces around 40% fewer fledglings
- From observation it is suggested that she cannot detect that the male is paired
- The large distance between the nests prevents the correct recognition of pairing.
Polygyny – Other Factors
The degree of male parental care is not the only determinant of female fitness; there are other factors which matter too:
Example 1 – Group defence in yellow rumped caciques
In order to protect themselves against other avian predators, this species forms communal nesting areas. The large number of birds helps protect them against predators. (This is a typical mammalian strategy)
Example 2 – Quality of territory
Male territories can vary in quality and therefore it is sometimes better to mate with a polygynous male with a good territory, rather than a monogamous male with a very poor territory.
Males can become polygynous when:
- The cost of polygyny to females is not too high (for example loss of progeny production/ survival)
- There is a patchily distributed, defendable resource that females require
For example – Great Reed Warblers
Males defend reed bed territories, where more dense reed provide better protection. Females survey the territories and pick which is the best. However, if all the monogamous male and good territories are gone, the female may pick a polygynous male with a good (dense) territory. Quite often these females who pick the polygynous males do not fair worse than those who pick the monogamous males with poor (sparse) territories.
Example 3 – Excess resource
During times when resources such as food are sparse, selection may occur for the males/females with the largest/ highest quality resources. However, when these resources are in excess it may be that there is much less pressure put on selection, as a result the mating status of the male is less important. For example, Weaverbirds exhibit polygyny during periods where there is an abundance of resources available.