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56 Cards in this Set
- Front
- Back
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Anisogamy
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• Sexual dimorphism, sexually discrete
– Reproduce only through sexual reproduction • Females produce few, large, sessile, and energetically expensive eggs • Males produce many, small, motile, and energetically cheap sperm • Difference in gamete size = Anisogamy |
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Anisogamy results in profound
differences in reproductive behavior of two sexes |
Small number of expensive female
gametes is a limited resource for which males compete • Male reproductive success = how many different females he can inseminate • Female reproductive success = how many young she can produce |
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differences between sexes
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Fertilization occurs in the oviduct
– Mother knows she’s the mother • Paternity not as certainFemales gestate and lactate = limits the investment males can make in offspring |
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Bateman Gradient
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Most females mate
• Differential mating success in males – Variance in copulatory success higher in males than females |
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Sexual strategies
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Males who copulated most sired most
offspring = Bateman Gradient • Females only need to mate once to produce maximum number of offspring |
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Sex Ratio
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Sex ratios of populations
of most species 50:50 – Often deviates in adult segment of population Operational Sex Ratio: ratio of only reproductive members of population • OSR influences mating system as members of abundant sex compete for access to members of scarcer one |
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Trivers-Willard Hypothesis
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Maternal condition influences sex ratio
• Mothers in best physical condition produce healthier offspring that are better able to compete for resources (including mates) Males more likely to compete for mates reproductive success is variable • Must be better “out of the gate” to succeed = needs more maternal investment |
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T-W example
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Male Antarctic fur seals are heavier at
birth, grow faster, and weigh more than sisters (at 60 days) Reproductive success of sons high if mothers are in good condition – Low (or zero) if mothers are in poor shape |
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T-W cont.
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Daughters likely breed regardless of
mother’s condition • Females in good shape produce sons – Daughters if in poor shape |
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Support for T-W
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Red Deer: Intense male
competition, successful sons achieve over twice the reproductive success as daughters – Dominant females produce more sons – Figure 22.2 |
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Fathers also benefit from Sex Ratio
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• Dominant red deer males are more fertile
– Have larger, more elaborate antlers preferred by females (inheritable) – More fertile males have faster-swimming sperm and fewer abnormal sperm than less fertile males • Females in good condition inseminated with sperm from fertile males produce more sons |
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Sex ratios in Matriarchal mammals
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Daughters remain around mother, sons
disperse • Daughters compete with mothers for food • Fewer daughters = less local resource competition |
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Example…
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In baboons, socially dominant females
produce more daughters, subordinate females produce sons – Daughters benefit from mothers social rank |
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How are birth sex ratios
manipulated? |
Timing of insemination within estrus cycle
– More of one sex conceived earlier than later (white-tailed deer, Norway rats, and humans) • Sex ratios at conception male-biased • Intrauterine mortality rates higher for male embryos – Probably highest among mothers in poor condition |
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Sexual Selection
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Success measured by number of offspring
and by quality or probable reproductive success of those offspring • Quality of mates important |
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Sexual Selection is …
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Sexual Selection: process that produces
anatomical and behavioral traits that affect an individual’s ability to acquire mates (Darwin 1871) – May produce traits that reduce individual’s survival |
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Intersexual vs Intrasexual Selection
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Intersexual: members of one sex choose
mates of other sex • Intrasexual: individuals of one sex compete among themselves for access to other sex • Both types produce and reinforce sexual dimorphism |
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Intersexual Selection
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One sex advertises their fitness using
secondary sex traits (usually males) • Other sex chooses among them • Can lead to exaggerated traits • If trait is heritable, females select for it, offspring have it and daughters select for it • Trait becomes further developed in males and more highly selected for in females |
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Runaway selection
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Traits produced by runaway selection are
arbitrary and not linked to male’s fitness • May become so exaggerated that survival of males is reduced • Counters election in favor of less ornamented males should then occur and selection will return to steady state |
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Good genes models
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Trait favored by females indicates male
fitness • Handicap hypothesis: – sexual selection can produce traits that are detrimental to survival – Costly to produce and linked to superior quality in males – Therefore advertisement must be hon |
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Symmetry Models
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Symmetry of paired traits may indicate
fitness • Fluctuating asymmetry: random deviations from bilateral symmetry – Ex. Measurements in right and left antler are different |
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Symmetry
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Reveals individuals ability to maintain
developmental homeostasis when faced with environmental variation and stress Greater asymmetry associated with low food quality, quantity, habitat disturbance, pollution, disease, and genetic factors (inbreeding, hybridization, and mutation) • Symmetrical individuals are more likely to be dominant in horned and antlered species |
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Fluctuating Asymmetry
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Most pronounced in sexually selected
traits – Ex. Canines used in male-male competition in primates • Asymmetry increased in canines (but not premolars) of lowland gorillas in 20th century (period of intense habitat degradation) |
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Intrasexual Selection
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Competition within one sex
• May occur before mating (leks) – One sex competes and winners do most of the mating • Sperm Competition: females mate with several males during single estrus – Sperm competes and one male’s sperm fertilizing disproportionate number of eggs |
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Sperm competition
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Selection process that leads to two
opposing types of adaptations – Reduce chances of second male’s sperm being used (first-male advantage) • Mate guarding, copulatory plugs – Reduce chances of first male’s sperm being used (second-male advantage) • Dilution of first male’s sperm by frequent ejaculation of large amounts of sperm |
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sperm competetion example
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Bighorn sheep: females
mate with multiple males – Dominant rams guard estrous females from forced copulations by sneaker males – If subordinate does succeed, dominant ram will immediately copulate with the female, effectively diluting sneaker’s sperm |
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Sperm competition (cont’d)
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Amount and quality of sperm typically
related to mating system • May explain high number of deformed sperm in mammals (high as 40% in humans) – Kamikaze sperm stay behind to inhibit passage of sperm from second male |
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In primates… amount and
quality of sperm relates to mating system |
Where winners of male-male competition
(gorillas) have free access to females, testes are smaller and produce lower quality sperm • In a free-for-all system (chimpanzees) males with best sperm will fertilize eggs = larger testes, higher quality ejaculates (more and more motile sperm) |
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primate example
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Wooly spider monkeys: aggregations of
males take turns copulating with females (every 20 minutes) • All competition is among sperm inside of female fallopian tubes • Males have extraordinarily large testes, little agonistic behavior |
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Testes and fidelity
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General trend in mammals:
– One male for one or more females = smaller testes – Multiple males per female = larger testes • Testes size also influenced by length of female vagina (longer vaginas need more ejaculate) – Ex. Vagina of female gorilla = 10 cm long Vagina of female chimpanzee = 17 cm long (even longer by genital swelling during estrus |
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Postcopulation Competition
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Bruce Effect: early in pregnancy, odors of
strange males cause spontaneous abortion – So far only described in lab situations with mice and voles • Infanticide: killing other males young brings females back into estrus |
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Parental Investment
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One parent or both provide some level of
care for offspring • Parental investment: any behavior pattern that increases the offspring’s chances of survival at the cost of the parent’s ability to rear future offspring |
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Anisogomy revisited
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Eggs require greater investment than
sperm = different reproductive strategies – Males more subject to sexual selection • Cost of parental investment rises more quickly for females than males – Optimum number of offspring lower for females – More choosy about mates |
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So, you’re my daddy
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Individual sperm is cheap, but much is
produced and the costs of delivery (pre copulatory displays) are high • Also, the number of ejaculates per unit of time is limited |
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Male investment
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Sperm competition leads to competition to
produce increasing amounts of sperm • Territorial defense can also result in some level of choosiness by males • Degree of investment translates to level of parental care |
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Northern elephant
seals |
Mate on land in dense
colonies – Males establish dominance hierarchies – High ranking males breed (top 5 may do 50% of breeding and ~30% of males don’t copulate at all)Young born 1 year later… males have no way of knowing paternity • Absolutely no male investment in offspring beyond insemination – Males often trample young to death in race to copulate |
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Silver-backed
jackals share equally in care of young |
Both parents defend
territory and hunt cooperatively – When father disappears, female and offspring typically die |
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Tamarins and Marmosets
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Females produce
twins • Males and young from previous years carry one of the babies – Females carrying both newborn suffer 17% reduction in leaping ability |
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Altricial vs precocial young
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Lower male investment in species with
precocial young – Higher in species with altricial young • Monogomy higher in species with altricial young – Parental care becomes more even with males helping in defense, mate guarding, providing food, babysitting, providing shelter, grooming, etc |
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What causes males to become
good parent? |
Hormonal changes (increased prolactin,
decreased testosterone) – Delivered through female’s urine – Also decrease infanticidal aggression • Receptors in forebrain for vasopressin triggered by interactions with young – Also reinforces pair bonding |
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Vasopressin
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“released into the brain during sexual activity, initiates and sustains
patterns of activity that support the pair-bond between the sexual partners; in particular, vasopressin seems to induce the male to become aggressive towards other males” |
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Parental Care and Ecological
Factors |
Extent of care varies from a few days to
several years • Reproductive effort: energy expended and risk taken for breeding that reduces repro. success in future – Reflect variety of decisions by parents: when to breed, where to breed, when to abandon, etc. |
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Environmental factors
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Species in stable environments have
larger body size, develop more slowly, live longer, and bear young at repeated intervals (iteroparity) rather than all at once (semelparity) • K-selected: populations typically at or near carrying capacity – Intraspecific competition intense, emphasis on producing few, high-quality offspring |
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Environmental conditions (cont’d)
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Species in fluctuating environments have
high reproductive rates, rapid development, and small body size and provide little parental care • Populations controlled by physical factors and mortality is high (r-selected) |
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Other factors
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Food abundance: scarce and limited food
favors high parental care – Lots of effort in finding and killing prey – Offspring need time to learn skills |
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Longer parental care, lower
reproductive output |
Large-brained mammals with complex
social lives – Need time to learn |
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Polygyny
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one male, multiple females
– Greater reproductive variation in males than females – Sexual selection causes sexual dimorphism |
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Resource defense polygyny
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males defend areas
containing resources critical for reproduction, females choose based on quality of territory |
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Female defense polygyny
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females group together for protection, not reproductive purposes
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Male dominance polygyny:
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Male dominance polygyny: when males can’t control
resources or mates, males establish leks and advertise • Single males perform most copulatory events |
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Hammer-headed fruit bat
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Males “clank” to attract females
• Once chosen, copulate immediately • Resume calling right after – 6% of males=79% of mating |
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Scramble polygyny:
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in absence of territories or
dominance, where females are highly dispersed males become highly mobile during breeding season and copulate with as many females over as wide an area as possible |
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Monogamy
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occurs when densities are low
– less than 5% of mammals (most cases in Primates, Carnivores, and Rodents – Facultative: low densities – Obligate: investment of male required for successful rearing of offspring |
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Polyandry
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one female, multiple males
– female investment in gametes exceeds males so polyandry is very rare – African wild dogs occasionally exhibit |
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Neuroendocrine control of mating
systems |
Hormones oxytocin and vasopressin
regulate behavior patterns associated with monogamy – Formation of longterm pair bonds, defense of nest/mates, levels of parental care |
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Endocrine control
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Number of receptors for these hormones
differs in number between monogamous and polygynous individuals • Vasopressin provides aggression for defense |
