Conflict over Care: Mating Systems Flashcards
Male and female reproductive potential
Broadly speaking, males have higher reproductive potential than females so their reproductive success is limited by access to mates, while female success is limited by access to resources. This leads to a general model for the evolution of mating systems: the distribution of resources determines female dispersion, which in turn determines male dispersion. There is plenty of correlational and experimental support for this model (see notes from APS209 Mating Systems lecture) from diverse taxa, especially the link between female and male dispersion.
This link determines the Operational Sex Ratio (OSR – the ratio of sexually receptive males to sexually receptive females) and hence the mating system. This simple model becomes more complicated in systems where males provide parental care (which is the norm in birds, for example). Now, the male becomes an important resource for female. A comparative study by Clutton-Brock & Vincent (1991) illustrated the impact that male care has on male and female reproductive potential, the intensity of competition and sex roles. In this lecture, we will consider the implications of paternal care for the emergent mating system and the factors that influence the extent of conflict between parents over care.
- Monogamy
Monogamy is the typical mating system among birds. It takes various forms:
(a) Obligate monogamy
Both sexes are essential for successful reproduction. This seems to occur under two particular circumstances:
(i) Parents must forage a long way from the nest, e.g. seabirds
(ii) Parents have specialized caring roles, e.g. parrots and raptors often provision incubating females; this is taken to an extreme in hornbills where survival of both females and offspring is totally dependent on the care of males for at least part of the breeding cycle.
In obligate monogamy, the interests of the two sexes are closely aligned; pair-bonds often last for many years and extra-pair paternity occurs at a low rate. As a result, we expect there to be relatively little conflict between the sexes.
(b) Facultative monogamy: failed polygamists
In many monogamous bird species, biparental care may not be needed throughout the breeding cycle, so although biparental care may increase reproductive success relative to uniparental care, it is not essential, so monogamy is facultative and either sex might do better by desertion (females preferring polyandry, males polygyny).
e.g. 39% of European passerines are occasionally polygynous (Moller 1989)
Facultative monogamy usually results from limited opportunity for polygamy, due to competition among males and/or among females. When one sex does desert, it is usually male because they have greater opportunity for desertion (internal fertilization), and more to gain (higher reproductive rate). However, this is not always the case – in some species (e.g. Tengmalm’s owl), females desert to find a new mate and increase their reproductive success, leaving males to care for the first brood. The conflict between partners is often substantial in such cases (see next lecture).
Polygyny
If resources or females are patchily distributed, males may be able to defend multiple partners and become polygynous (e.g. Montezuma oropendola). But, why should females accept polygyny if it means they have to share a male’s care with other females? There may be:
(a) No cost of polygyny to females
If males provide no care, females should simply settle in relation to other key resources that determine their fitness.
(b) Cost of polygyny to females
Polygynous females may do worse than monogamous females, but accept this cost if:
(i) There is no choice. In female-biased populations (e.g. marsh wrens), or when females are deceived into mating polygynously (e.g. polyterritorial pied flycatchers – Alatalo et al. 1981). (ii) The benefits outweigh the costs. When faced by a choice between breeding with an unpaired male (i.e. becoming monogamous) or an already paired male (i.e. becoming polygynous), the cost of polygyny may be outweighed by the benefit of access to good resources, and/or the benefit of conferring the male's 'good genes' on offspring. In other words, paired males may be better than unpaired males, or may defend better resources. This idea is formalized in the 'Polygyny Threshold Model'. There is not much evidence for this model (despite its intuitive appeal, but there is limited evidence that polygynous females may not suffer a cost relative to monogamous females. e.g. great reed warbler (Bensch 1996).
Polyandry
This is an unusual mating system where two or more males mate with one female. Females are likely to benefit by gaining access to the paternal care of multiple males. But, if males have to share paternity with other males, why do they accept polyandry?
(a) Cooperative polyandry
Under harsh conditions or when the chance of reproduction for males is low, the benefit of increased productivity from a shared brood may outweigh the costs of shared paternity. e.g. Galapagos hawks - groups have one female and 1-8 males, who share copulations, paternity and parental care.
(b) Sequential polyandry
More often, females lay a series of clutches for multiple males, each of whom cares for that brood. These species are characterised by sex role reversal, e.g. painted snipe, phalaropes and jacanas. In these cases, males seem to be caught in a ‘cruel bind’, where caring for the clutch is their best option.
Sexual conflict over mating systems
The overwhelming message is that far from being a harmonious, cooperative activity, there is often sexual conflict over reproduction and the preferred mating system for males and females. Males would generally prefer polygyny because access to multiple females would increase their RS. In contrast, females’ RS is most likely to be increased by gaining access to the parental care of multiple males in polyandrous mating systems. However, the extent of this conflict is extremely variable across and within different mating systems.
The intensity of sexual conflict over reproduction, and hence the emergence of different mating systems can be viewed as the outcome of a hierarchical process.
Life history constraints – whether both parents are able to provide care or not (e.g. lactation), and how much care offspring need (e.g. altricial [dependent] v precocial [relatively independent]).
Ecological factors – the distribution and abundance of resources will influence the dispersion of mates and the potential for polygamy (e.g. abundant food may facilitate polygyny or sequential polyandry, sparse food may facilitate obligate monogamy or cooperative polyandry)
Social conflict – individuals will try to maximize their own reproductive success at their partner’s expense, often through coercion or deception (e.g. facultative monogamy).
