1. Mating Systems
&
Parental Care

2. Mating Systems & Parental Care Chapter 18
Who Invests in Offspring?
Parent-Offspring Recognition
Not covered in lecture
Parent-Offspring Conflict
Sibling rivalry
Chapter 18

3. Factors Affecting Type of Mating System
Sexual Selection
Differential allocation of resources into gamete production and parental care
Sperm is cheap, eggs expensive
Chapter 18

4. Factors Affecting Type of Mating System
1. Males should be competitive among themselves for opportunities to mate (Intrasexual competition)
2. Females should be choosy: any mating may involve a big investment on her part, so she should be selective about it (Intersexual competition)
Chapter 18

5. Type of Mating System dependent on: Amount of parental care required
Sexual selection underlies the evolution of male competition and female choice.
Type of Mating System dependent on:
Amount of parental care required
Ecological factors
Chapter 18

6. Mating Systems
Most classifications of mating systems based on extent of bonding/association between male and female during mating.
Problem:
judgment is needed to determined what constitutes an association
Subjective
Emlen & Oring (1977)
Chapter 18

7. Mating Systems Emlen & Oring (1977):
Classification based on ability of one sex to monopolize or accumulate mates
Eliminates need for subjective judgment
Emphasizes the ecological and behavioural potential for monopolization
Chapter 18

8. Resource-based Mating Systems
Emlen and Oring (1977)
the ecology of an organism may not permit males to have more than one partner.
Females widely distributed and males cannot monopolize them.
Females may mate with another male so monogamy may serve to guard the female.
If males help rear young, fitness increases through increased young survival.
The evolution mating systems is driven by the distribution of resources in the environment for both the male and the female.
Chapter 18

9. Mating Systems Monogamy - 1 male, 1 female mate guarding
mate assistance
female enforced
Polygyny - 1 male, many females
resource defense
female defense
scramble competition
explosive breeding assemblage
lek
Polyandry - 1 female, many males
male defense
Promiscuity
Chapter 18

10. Mating Systems Males can produce lots of sperm almost continuously
Monogamy is not best for a male
Females often need help in raising offspring, so monogamy is good for her
Sexual selection theory and coupled with low parental investment of males suggest that polygynous mating systems should be most common.
Why are males?
Chapter 18

11. Mating Systems Monogamy (Association of) 1 male & 1 female
Neither sex is able to monopolize more than one member of the opposite sex
Mate for season only, or for life
Genetic constraints vs. social/ environmental factors (e.g., seabirds socially monogamous)
Chapter 18

12. Mating Systems Polygamy
Matings with multiple partners; non-monogamous systems
Polygyny: “many” females
Males able to monopolize >1 female
Polyandry: “many” males
Females able to monopolize >1 male
Chapter 18

13. Mating Systems Promiscuity Multiple matings by at least one sexes
Absence of prolonged associations
Chapter 18

14. Spatial distribution of resources influences type of mating system
Fig. 18.1
Dots = resources, circles = defended areas
Chapter 18

15. Polygamy - Types of Polygyny:
Resource-defence polygyny:
Males defend areas containing food or nesting sites that females need for reproduction
Territories differ quality (clumped distribution)
May reach a point where a female can do better by mating with an already-mated male on a good territory than an unmated male on a poor territory = polygyny threshold
facultative (optional) polygyny (within a species): dependent on locale (clumped, defensible resources vs. spread-out, non-defensible)
Chapter 18

16. Polygyny Threshold (a) (b)
Fig. 18-3
(a) (b)
The quality of territory at (b) is greater than at (a). A female joining an already-mated male at (b) will have the same RS (c) as a female in a monogamous pair at (a).
Chapter 18

17. Polygamy – Types of Polygyny
Resource-defence polygyny (cont’d):
Facultative (optional) polygyny :
Dependent on locale (clumped, defensible resources: polygyny; vs. spread-out, non-defensible: monogamy)
Thus, same species may behave differently in different environments
Chapter 18

18. Polygamy – Types of Polygyny
Female-defence polygyny:
Females aggregate for reasons unrelated to mating
E.g., female elephant seals haul out onto land to give birth, sites limited
Males compete to monopolize the grouped females, defend harem from other males
Chapter 18

19. Polygamy – Types of Polygyny
Male dominance polygyny
Neither resources nor females can be monopolized
Males gather and display, females choose based on quality of display
Usually on leks (mating areas where males congregate and defend small territories while displaying for females)
E.g., bowerbirds; sage grouse
Chapter 18

20. Polygamy – Types of Polygyny
Male dominance polygyny (cont’d)
Mating success skewed in favour of a small number of males
Dominant males may have:
Preferred territory locations within lek
Most attractive display (e.g., bower)
Most elaborate ornaments, etc.
“Copying” behaviour seen in females
Chapter 18

21. Polygamy – Types of Polygyny
Scramble Polygyny
Males actively search for mates without overt competition
Large groups of females congregate, males concentrate only on inseminating females or fertilizing eggs, ignore other males (e.g., wood frogs, horseshoe crabs); OR
Widely disbursed females are actively pursued by males (e.g., thirteen-lined ground squirrels)
Chapter 18

22. Polygamy – Types of Polyandry
Resource-defence polyandry
Females control access to males indirectly by monopolizing critical resources
e.g., spotted sandpiper – females compete for control of breeding territories
Chapter 18

23. Polygamy – Types of Polyandry
Female-access polyandry
Females do not defend resources essential to males, but they interact among themselves to limit access to males
e.g., American jacanas – some
females defend “superterritories”
which encompass the territories
of several males, prevent access
to other females
Chapter 18

24. Polyandry
Rare –
Females more brightly coloured, etc. (sexually-selected traits)
Females show philopatry: (returning to natal territory after maturity)
True of males in polygynous species
Chapter 18

25. Ecology & Mating Systems
Mating systems related to resource distribution
Two sympatric (i.e., coexist in same area) species of Agelaius (Orians, 1961)
An example of the way mating systems are related to resource distribution (distribution can be in space or in time)
Red-winged blackbirds
Tricoloured blackbirds
Chapter 18

26. Ecology & Mating Systems
Red-winged Black Birds:
Usually polygynous
Male defend territory with 2 or 3 females
Male arrives 3-4 wk before females (often natal territory), defends territory until young are fledged (several months)
Rarely help raise young – females build nests and incubate
Chapter 18

27. Ecology & Mating Systems
Tricoloured Black Birds:
Monogamous
Nomadic colonies of ,000 birds
Establish territories, find mates, build nests, lay eggs (biparental care) – ALL WITHIN 1 WEEK!
Both sexes make large investment, but in a smaller timeframe
Chapter 18

28. Ecology & Mating Systems
Why so different?
Depends on Energy source (food availability)
RWBB: Stable diet of seeds and insects, available for several months
TCBB: Mass feeding flights, attack ephemeral, concentrated food sources, e.g., ripe rice/grain fields
Designed to take advantage of an ephemeral, but rich and concentrated food source
Again, can see interaction between environment and life history traits.
Chapter 18

29. Mouse MHC Probe
Gibbs, et al Realized reproductive success of polygynous Red-winged Blackbirds revealed by DNA markers. Science 250:
Chapter 18

30. Alternative Mating Tactics
Members of same sex can use different tactics to obtain matings
E.g., some males fight, others sneak copulations by being inconspicuous or mimicking females
“Sneaky”, satellite, or parasitic males
E.g., bullfrogs: Dominant male calls to attract females to his territory; satellite male hides at edge, waits for females to approach
We’ve seen differneces between species, and diff’s between sexes, but even within a sex, members of same sex can use diff strategies…
Chapter 18

31. Explosive Breeding Assemblage
Females become receptive for only a brief time
They go to a breeding area when they are ready
Males gather at the breeding location and compete for the female.
Chapter 18

32. Alternative Tactics Morphology (Fig. 18.8)
coho salmon: jacks vs. hooknoses
Hooknoses: built to fight
Jacks: smaller, mimic females, sneak fertilizations
(Fig. 18.8)
Chapter 18

33. Parental Care Extent of parental care varies greatly across species
Ways to invest:
DNA (release sperm/eggs, move on)
Prenuptial feeding (e.g., crickets) – spermatophore nourishes female, more and better quality offspring
Direct care of offspring (incubating, feeding, brooding) – birds/mammals/some fish…
spermatophore
Chapter 18

34. See Text pg. 151 (Chapter 10)
Chapter 18

35. Which Sex Invests?
Taxonomic differences also exist in patterns of parental investment:
birds: monogamy / biparental care
mammals: polygyny / maternal care
fish: promiscuity/polygyny / no parental care or male care
Why these differences?
Chapter 18
Photo by: Alan and Sandy Carey

36. Determinants of Parental Investment in Vertebrates
Mode of fertilization
Internal fertilization – primarily female care
E.g., birds, mammals
External fertilization – primarily biparental or male care
E.g., fish
Chapter 18

37. Determinants of Parental Investment in Vertebrates
Certainty of Paternity Hypothesis
Reliability of paternity assumed to be greater when eggs are fertilized externally (e.g., fish) rather than internally (e.g., mammals, birds)
Chapter 18

38. Determinants of Parental Investment in Vertebrates
Association Hypothesis
Proximity of adults to offspring
Internal fertilization – females closer
External fertilization – both sexes close, but male/biparental care more common when offspring associated with a male’s territory
E.g., males defend territories, little extra energy to protect eggs already on territory (e.g., fish)
Chapter 18

39. Determinants of Parental Investment in Vertebrates
Phylogenetic Constraints:
Birds
Eggs develop externally
Incubation, feeding, guarding by both sexes
Mammals
Internal gestation
Lactation
Males can provide little care
Chapter 18

40. Determinants of Parental Investment in Vertebrates
Phylogenetic constraints
Fish
External fertilization
Both sexes free to desert, or
Guard nest site (territory)
Chapter 18

41. Determinants of Parental Investment in Vertebrates
Reproductive Effort: Total energy expended in reproducing, which includes mating effort & parental care
May direct all reproductive effort into mating (e.g., codfish - lay eggs, fertilize them, swim away), OR
May expend much energy in parental care (e.g., primates – 25% of offspring’s lifespan)
Chapter 18

42. Reproductive Effort Mating System Polygamous/ Promiscuous MonogamousME PE ME PE
Male
MALE PE ME PE ME
Female
FEMALE
Mating System
Polygamous/
Promiscuous
Monogamous
Chapter 18

43. Determinants of Parental Investment in Vertebrates
How Do Ecological Factors Affect Parental Investment?
K- & r-selection theory
“Bet-hedging” theory
In ecology, r/K selection theory relates to the selection of traits (in organisms) that allow success in particular environments. The theory originates from work on island biogeography by the ecologists Robert MacArthur and E. O. Wilson (MacArthur & Wilson, 1967).
In r/K selection theory, selective pressures are hypothesised to drive evolution in one of two stereotyped directions: r- or K-selection. These terms, r and K, are derived from standard ecological algebra, as illustrated in the simple Verhulst equation of population dynamics:
Where r is the growth rate of the population (N), and K is its carrying capacity.
Typically, r-selected species produce many offspring, each of which is unlikely to survive to adulthood, while K-selected species invest more heavily in fewer offspring, each of which has a better chance of surviving to adulthood.
Chapter 18

44. r - & K- Selection Theory
Stable environment
Larger body size
Slower development
Longer lifespan
Have young at intervals (iteroparity);
K-selected (at/near carrying capacity of environment, K)
few young that receive much care
Fluctuating environment
Small body size
Rapid development
Have young all at once (semelparity),
r-selected (at/near reproductive rate of population, r)
many young that receive little/no care
In unstable or upredictable environments r-selection predominates, as the ability to reproduce quickly is crucial, and there is little advantage in adaptations that permit successful competition with other organisms (since the environment is likely to change again). Traits that are thought to be characteristic of r-selection include: high fecundity; small size; short generation time; and the ability to disperse offspring widely. Organisms whose life history is subject to r-selection are often referred to as "r-strategists" or "r-selected". Organisms with r-selected traits include bacteria, diatoms, many insects and weeds.
In stable or predictable environments K-selection predominates, as the ability to compete successfully for limited resources is crucial, and populations of K-selected organisms are typically very constant and close to the maximum that the environment can bear. Traits that are thought to be characteristic of K-selection include: large size; long life span; and the production of fewer offspring that are well cared for. Organisms whose life history is subject to K-selection are often referred to as "K-strategists" or "K-selected". Organisms with K-selected traits include large animals such as elephants, humans and whales.
It should be noted that, although some organisms are primarily r- or K-strategists, the majority of organisms fall between these two ecological extremes, and may display traits considered characteristic of both ends of the r-K spectrum. For instance, trees have traits such as longevity and strong competitiveness that characterise them as K-strategists. However, in reproduction, trees typically produce thousands of offspring and disperse them widely, traits characteristic of r-strategists.
Chapter 18

45. “Bet-hedging” Theory
In environments where survival of offspring is low and unpredictable, parents may “hedge their bets” by putting in only a small reproductive effort each season
spread their reproductive effort out over time rather than all at once
e.g., California Gull: Lives 15+ yrs. As adults age, appear to increase effort (lay more eggs, feed chicks more food, defend more vigorously) due to their own decreasing reproductive potential.
Chapter 18

46. Parent-Offspring Recognition
Misdirected parental care can be costly (waste of energy, reduced fitness)
Fostering in seals
Recognition appears to evolve in species in which mixing of unrelated offspring is likely
Chapter 18

47. colonial ground-nesting gulls & terns:
precocial chicks mobile at ~5 days,
adults seem to develop recognition for own chicks at that time
kittiwakes
cliff-nesters;
chicks fledge at 3 weeks
Parents do not seem to recognize offspring
Chapter 18

48. Parent-Offspring Conflict
Offspring demand more investment from the parent (usually mother) than the parent is willing to give
Trivers (1974): Natural selection acts differently on mother and offspring
Based on coefficient of relatedness and parental investment
Chapter 18

49. Parent-Offspring Conflict
Mother - should invest a certain amount of energy into first offspring, “wean” him, then invest in producing second offspring, when the cost of raising #1 starts to exceed the benefits of having #2…
Offspring - should “demand” investment from mother until such point that the mother’s fitness starts to decline (&, thus, so does his inclusive fitness, as he shares half his genes with her).
when cost of raising #1 becomes 2x the benefit to mother, offspring's inclusive fitness 
Chapter 18

50. Parental Reproductive Success
How can parents increase their reproductive success and fitness?
Manipulation of dependent young
Chapter 18

51. Parental Manipulation
Parents may selectively provide care to certain offspring at the expense of others
Brood adjustment
Coots - selectively adjusts brood size by killing chicks (European) or feeding specific chicks (American)
Chapter 18

52. Parental Manipulation
Hatching asynchrony - milder form of brood adjustment: first-to-hatch have advantage, get more food, larger, can push others out of nest (i.e., siblicide)
It is in parents’ interests to create inequalities in size of chicks: WHY?
biggest, strongest offspring survive and are more likely to reach reproductive age
Chapter 18

53. Sibling Rivalry
Pigs: “runt” gets least preferred teat (little milk); piglets have sharp teeth and fight for access to best nipples
young predatory birds (ex: boobies, eagles, egrets) kill younger siblings soon after hatching,
may be because the second egg serves only as parental insurance in case the first fails to hatch.
Chapter 18

54. Case Study: Spotted hyenas
Chapter 18