Polygynous mammalian mating systems are quite diverse, varying in how females and territories are guarded, as well as the uses of resources amongst other things. Only around 5% of mating systems in mammals are monogamous (unlike the diversity of avian mating systems where 90% were monogamous). The majority of monogamous mammals are canine; around 50% of canine species are monogamous. The result is that many mammalian mating systems are actually polygynous.
A possible explanation for the vast difference in mating system strategies is that; male parental care is much more effective in birds when compared to mammals. Also whilst birds are fairly dependent on their parents following birth, mammals are often born with reasonable independence and are able to fend for themselves, requiring less parental care. This means a single parent does not greatly affect the survivability of the offspring.
During pregnancy, the benefit of male care in mammals is low; it is the female who invests all the time and resources into gestation.
The reason why monogamy is not generally prevalent in mammals is due to the fact that male parental investment rarely offers any benefits to the male. Therefore by engaging in monogamy with a female, he is essentially wasting opportunities to mate. The females are also in no position to force monogamy upon the males.
The classic image of a polygynous mating system is that, a single dominant male will defend a group of females, all of which are potential reproductive partners. There are however, many more examples of polygynous systems:
- Roving males – e.g. polar bears, males will wander around, between territories, travelling to ovulating females. When arriving at a female, they will copulate and soon move on in search of new females to mate with
- Single male with a permanent territory and a small group of females – The individual male will defend the females of his group from incoming males
- Multiple males with permanent territories and a larger group of females – The males defending these females from incoming males are usually related
- Males with permanent territories located at a resource – These males do not protect a group of females, but instead guard a territory (such as a watering hole) mating with females as they arrive to use the resource
- Males with only a permanent territory – No resources or females are located within the territory, however, females travel to the male territory with the sole purpose of mating
- Single or multiple males defending a group of females within a non-permanent territory – Harem systems such as these occur when a group constantly moves around, such as herds of wildebeest
The type of polygynous mating system depends on the dispersion of females, for example, a system such as roving males is more likely to evolve in an area where females are widely dispersed. Polygynous mating systems are simply a case of males trying to monopolise access to females, this is because male fitness depends on the number of mates he has and therefore the number of related offspring produced. Female fitness depends on the access to /quality of resources, this is important for successful survival and reproduction.
Determining the Type of Polygynous Mating System
- If females are solitary and widely dispersed, the males will be travelling long distances between mating. A roving males system develops (e.g. polar bears)
- If females are solitary, but close, then it is more likely that a territorial defence system will develop. (e.g. voles)
Defendable Female Groups
- If the female group is fairly small, it is relatively easy for a male or a 2-3 related males, to guard the group against intruding males. In this situation, a territorial defence system is adopted. (e.g. colobus monkey)
Female Groups with Non-Permanent Territories
- The grouping of females is often observed in mammals, but they do not always have a permanent territory. In such a case, the males travel with and defend the females in a harem system (e.g. baboons).
Non-stable and Non-defendable Females Groups
- In situations where females do not have a permanent territory and are also not defendable, it is much simpler for the males to predict the movement of females, and guard resources and areas used by the females on a regular occasion. Resource based territorial defence such as this occurs in the waterbuck
The Effect of Group Size on Male Numbers
In mating systems where groups of females are to be defended, the number of males can vary from a single male to a small group. The number of males depends on the group size of the females.
- Small females groups are typically guarded by a single male, the male therefore has ‘exclusive’ access to all the females.
- In larger female groups however, it is necessary for more males as a single male would be unable to defend the whole group. In groups like this, the males (typically related) will compete with each other for access to the females in oestrus. Groups of 5 females or more will typically require multiple males for protection.
Lek Mating Systems
A lek is a tightly packed territory with small amounts of resources present. Leks are an unusual mating system because females prefer to mate with the male(s) on the territories closest to the centre. Because of this there is constant competition between the males for who has the desired central territories. This means there is a large skew in the reproductive success of males; those on the outer boundaries of the lek engage in no or very small amounts of copulations, moving towards the centre the number of successful copulations per male increases dramatically.
Lek mating systems may have developed as a means for females to become protected from predators. A predator is much less likely to strike at the centre of a lek due to the large number of males. The females therefore prefer the aggregation of males as it means they are safer, larger leks have a smaller risk of predation.
Males will gather and form condensed leks in areas that females visit frequently, in an effort to increase their chance of reproductive success. Also due to the large variation in male quality, lesser quality males are sometimes observed aggregating towards higher quality males in the hope that the females will copulate with them.
Monogamy in mammals occurs in around half of all canid species (i.e. monogamy amongst mammals is most common in canines). Monogamy also occurs in high amounts in primates (around 15% of primates are monogamous).
The importance of parental care is often the deciding factor of whether a polygynous or monogamous mating system develops. When biparental care is important, monogamy usually develops – as by both parents investing care into their offspring, they are maximising their survival. In mammals (canines) it has been shown that biparental, monogamous family groups have higher milk energy outputs and overall higher litter growth rates.