Question 4 (6 marks) The dunnock is a songbird known for the variation of its mating systems. Explain the conditions (ecological or social) under which the mating system of the male dunnock might be variously described as i) monogamous, ii) polygynous, or iii) a case of polygynandry.
Either social or ecological factors can be evoked; you need not deal with both. Essentially the ecological basis of different mating systems in this species is the necessary foraging area (= female home range).
If food is moderately abundant then a single female will forage over a relatively limited area which is defensible by a male. In other words because food is available in sufficient density females move within a smaller area from which it is possible to exclude rivals. Thus the male can be effectively monogamous with that female.
If food is even more abundant in a given season then it may be that the feeding ranges of two females are both small enough in area to be defended by a single male, i.e. both ranges are encompassed by his territory. So the male is able to monopolize the sexual favours of both females and he practices polygamy.
If food is scarce females will have to range more widely to meet their requirements and the area is simply too large to admit of excluding other males. In effect other males may be able to steal copulations. In some instances the female may be able to mate with several males whose territories lie within her home range. In each case the male will continue to provide parental care and the female is effectively polygamous with 2 or more males (polygynandry).
The above is really invoking only ecological conditions but social conditions could influence the sexual pairings of the dunnock: female dunnocks mate frequently and by 'flirting' with males other than their primary mate they can obtain help from these males in the raising of a second clutch. Or females can behave aggressively toward other females to preclude a male from being polygynous.
The following are answers that received 6/6; they are not necessarily perfect, e.g. the writer may have omitted a proper species common name etc.; so sometimes I have added to the answer in square brackets:
#"Female dunnocks eat small invertebrates and seeds. If their food is abundant and concentrated in a small region, it is easy for males to control females. Under such conditions of good food supply, male dunnock can be monogamous or even support two or more females and become polygynous. However, if the food is in short supply and therefore scattered, it is hard for the male to control even one female. Thus ecological factors can affect the mating system.
Also, females sometimes mate with the subordinate males when living in a dominant male's territory (obviously when the dominant male is not around). If she mates with the subordinate male [a] sufficient number of times, she can get parental care from the subordinate male and the dominant male. Thus under appropriate social factors, polyandry in female dunnocks increases her fitness."
#"In the dunnock the female has a home range where she feeds. Depending on food distribution or availability her home range may change in size. Males keep territories in which they exclude other males and will mate with any female within his territory. His territory however doesn't change in size. When only one female's home range is entirely within the male's territory they are monogamous. If the food is more readily available and the females keep smaller home ranges, more than one female could be within the male's territory and he would be polygynous. If the female's home range overlaps the territories of more than one male, she will mate with all of them and be polygynandrous." [this answer takes the prize for brevity with everything present]
Question 5 (6 marks) Choose two particular animal species and use their mating behaviour to illustrate the difference between a lek mating system and one described as a 'scramble competition polygyny'.
There are several similarities between these types of mating system: they both involve polygynous males and in both there is an absence of any material resources (food/shelter) given by the male to the female. In both systems females tend to live most of the time dispersed over wide areas. In part because of this dispersed female distribution alternative ways of monopolizing females (e.g. harems, defending resources) are unworkable.
Along with some details about the chosen species, the best sort of answer should have presented explicit differences between these two systems:
An explosive breeding assemblage is a type of SCP, the difference being a reduced time window that makes it impractical to exclude rivals through aggression rather than spatial dispersion of females.
The following are answers that received 6/6; they are not necessarily perfect, e.g. the writer may have omitted a proper species common name etc.; where necessary I have added to the answer in square brackets:
#"The mating systems lek and 'scramble competition polygyny' may appear on the surface somewhat similar but they are very different. In a lek mating system males of the species gather around and congregate in a particular place where females come to choose mates. This type of mating has been shown by a species of deer [Uganda kob]. Males will gather in one place and the females will go there when they need to mate. The place where the males gather has changed over the years showing that the females do go out looking to find these leks. In scramble competition males go out to find females that are widely dispersed in order to mate with them. In the 13-lined ground squirrel males will actively search for females over a substantial area and will mate with any they encounter. They do show memory for areas that were occupied by females and will return to those places. Another substantial difference is that female deer will show choice in which males they take from the lek while in the ground squirrel finding a female is so difficult they will mate with as many females [as] they encounter."
#"Although the lek mating system and the scramble competition polygyny both have females widely dispersed, there are some obvious differences between the two. The lek mating system in some birds are territories dfended by males from their [male] conspecifics for access to mates [white breasted manakins]. These areas hve no resources (except for the birds themselves) and they are traditional mating arenas where females choose their mates. The lek mating system will only evolve if other mating systems do not pay off (thanks to the wide distribution of females). Scramble competiton polygyny on the other hand are [sic] when receptive females are also widely distributed and males compete and outrace rivals to receptive females. The male that 'wins' will practice this. Males that are perceptive will succeed, not the aggressive ones. This example is seen in 13 [line] ground squirrels where mate [males] compete for access to females since she is only in estrus for 4 hours, and females will mate with the first male that finds her."