Thursday, 6th September - Mating systems and parental behaviour Flashcards
What are mating systems?
- The way animals gain access to mates
- the number of mates they interact with in a breeding season
- the duration of social bonds
- and the relative investment of each sex into parental care
Explain the difference between Polygamy and Monogamy and give an example animal of each.
- Polygamy: some members of one sex control access to >1 of the other sex.
- Monogamy: 1 male, 1 female
Driven by the distribution of resources in space and time.
– Distribution of females is governed by resources.
– Distribution of males is governed by that of females.
What are the 4 main types of mating systems?
1. Sexual promiscuity - Multiple partners, no real peer bonds formed.
2. Polygany – 1 male (Alpha) and >1 female (common, e.g. baboons, gorillas)
3. Polyandry – 1 female and >1 male (rare, e.g. jacana)
4. Monogamy – for life (in long-lived species) – for each season (in short-lived species; e.g. songbirds)
Why are some animals polygynous?
- Where the distribution of resources is patchy, there is more potential for some individuals to monopolise access and deny others.
- Polygyny is the best strategy for males due to anisogamy, but there is high cost for unsuccessful males.
When females or resources are defensible:
1. Resource defence: males control a critical resource needed by females (potentially before females arrive).
– Observed in a wide variety of animals: invertebrates, fish, mammals, birds.
2. Female defence: males control groups of females (harems), which are grouped around patches of resources.
– Common in lions, horses, deer.
Forms of polygyny
When females or resources are not defensible:
3. Lek polygyny
- Males do not provide parental care (just provide copulations).
- males congregate and display in breeding arenas where all breeding occurs.
- Males defend small territories with no vital resources for females.
- Females can choose freely among males – allows ‘window shopping’.
- Most copulations involve a very small proportion of displaying males
Polyandry: male-female role reversal
1. Classical polyandry:
one female mates with >1 male.
– Females compete over males and control territory/resources.
– Males care for offspring
2. Cooperative Polyandry:
>1 males associated with a single female during a breeding attempt/season. Evolutionary/ecological reasons for polyandry still unresolved!
Consequences of polygamy
• High polygamy means high variance in lifetime reproductive success of males, but not in females.
– high rewards for best males and few or none for others produces fierce competition over females.
– that then increases effects of sexual selection.
– visible in larger size, stronger colours or behaviour of best males and their offspring
• Degree of polygamy of a species partially predictable from extent of sexual dimorphism
– Males usually larger, stronger, more colourful than females.
– But is the opposite for polyandrous species!
– This suggests that sexual selection is a product of mating systems.
Monogamy
• Optimal strategy only in limited conditions:
– When neither sex can monopolise >1 of opposite sex
– parents achieve best individual lifetime reproductive success by cooperating to raise young.
- Occurs either within breeding season or for life.
- Can even vary within populations, depending on resource availability.
Are monogamous animals always faithful?
Extra-pair copulations are frequent in some “monogamous” species; especially passerines.
Extra-pair paternity increases offspring without having to care for them!
– Benefit for males: get another dad to care for your offspring; males are mostly limited by access to females and not sperm.
– Benefit for females: can get good genes from high-quality males (limited resource) + parental care (even if from lower-quality male).
– Cost for males: You might get stuck wasting your energy on someone else’s offspring.
– Cost for females: Very little = Females typically encourage EPP wherever possible.
Parental investment
Any behaviour towards existing offspring that increases their survival (at a cost to the parents!).
• Anisogamy is the key factor; it means selection acts differently on males vs. females.
– Female makes few costly offspring: choosy about father, tries to retain his help if possible but many can manage alone.
– Male makes abundant cheap sperm: competes to maximise matings, less choosy, will escape parental responsibilities if possible.
– Always exceptions! If male invests more in offspring he becomes choosy & female competes for his services. • Conflict between unrelated sexual partners inevitable. • Conflict among siblings over parental care probable.
Costs of parental investment
Costs are paid in three different ways:
1. Energetic costs – Reduced feeding opportunities & increased metabolic expense during care.
2. Survival costs – Starvation or increased susceptibility to predation.
3. Reproductive costs – Lost future opportunities for mating.
Variation in strategies of parental care
Probability of parents remaining together depends on relative investment of male and female
• Option 1: Maternal care
– male invests less than female, e.g. only sperm,
– and female can rear young alone,
– Then, male can desert (most mammals, some birds).
- Depends on probability of survival of young.
- Mallards males might stick around, but just to donate more sperm if offspring die!
Variation in strategies of parental care
• Option 2: Paternal care
– male invests much more, e.g. in defending territory
– and young can be fed by him, or can feed themselves
– Then, female can desert (e.g., seahorse, jacana, cichlids, some arthropods).
Variation in strategies of parental care
The evolution of sexy super-dads in a neotropical harvestman:
- Males tending eggs are more attractive.
- Females more likely to lay there eggs next to a male that is already tending eggs.
- Imposes energetic cost on males; cannot forage while caring for eggs.
• Option 3: Biparental care
– investment equal, e.g., he defends territory, she broods
– or shared care (feeding, defence) needed for success.
– Then, neither deserts: most birds… but also arthropods!
Allocation of parental investment
1. Life history – Long-lived species might invest less in current offspring; always another chance. – Short-lived species might invest everything as it’s the only chance! – Investment into offspring also increases with age in long-lived species
2. Confidence in paternity – Females can be pretty sure its theirs; guarantees 50% genes present in progeny. – Can be much harder for males to tell; this can be extremely costly!
