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UI WLF 448 - Sex Ratio

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Sex RatioIntroductionMust be careful in examining sex ratios:Males more obvious plumage oftenIn breeding season, males more obvious behaviorallySexes may migrate separatelyTermsRatios commonly expressed:Males:females to make 100 total (50:50)Males / 100 femalesPercent of total which are male (or female)Less commonly as females/maleTermsErnst Mayr (1939) suggested a classification based on age:A. Primary sex ratio = at conceptionB. Secondary sex ratio = at birthC. Tertiary sex ratio = at later specified ageMammalian and Avian PatternsA. Basic tendency:Mammals - Sex ratio shifts toward more females in older age classesBirds - Opposite: shifts toward males predominating in older are groupsMammalian and Avian PatternsB. Why? What are reasons? Two types of expanations:1. Internal: Sex-linked lethal factor2. Environmentally induced mortality related to behavior and life history of sexesExamples: HumansWhy do women live longer than men? Holden, C. 1987. Science 238:158-160Women outlive men by a margin of 4 to 10 years throughout the industrialized world.Weaker Sex?Now that females are no longer being felled by childbirth, it has become clear that they enjoy an advantage in survival rates.James V. Neal (Univ. Washington): “We really are the weaker sex, biologically less fit than females at every step of the way.”By AgeAt conception: 115 males to 100 femalesAt birth: 105 to 100Male excess in spontaneous abortions, miscarriages, and stillbirths prior to birth and higher neonatal and infant mortalityAt 30: sex ratio is equalBy 65: 84% females & 70% males still aliveMale to female death ratio3.9 to 1 for homicide in Alameda County, Calif.Also higher for lung cancer, suicide, pulmonary disease, accidents, cirhosis and heart disease (2 to1)Y chromosomeMaleness seems to carry intrinsic risk.Study of Amish families with and without long arm of Y chromosomeIn families with: women died in mid-70’s and men 5 or 6 years earlierIn families without: women died at ave. age of 77 while 14 men died at 82HormonesMales and females have equivalent cholesterol levels until puberty.Males suffer an exponential rise in heart disease in their 40’s, but female rise does not start till 50’s after menopause.Animal and human studies show that estrogen protects against heart disease by lowering levels of low-density lipoproteins (LDLs, “bad” cholesterol) and keeping high-density lipoproteins (HDLs, “good” cholesterol) up.HormonesAndrogen, lowers HDL and raises LDL.Hormones also affect the immune system.Animal and human studies show that females have greater immune responses than males:This is associated with higher female antibody responses and tumor resistance,but also higher female rates of autoimmune diseases (arthritis, lupus).Examples in BirdsTable of Sex Ratio ChangesExamples in BirdsSex ratio in young close to 50:50In adults it increases towards malesUsual rationale for higher mortality in females is stresses and hazardsof nestingExamples in BirdsPheasants in Wisconsin Wagner (1957) found:late summer mortality higher in hens when nesting pushed later by poor springGrouse in northern WisconsinDorney and Kabot (1960) sawyear to year sex ratio changes affected by spring weather (poor nesting weather killed the females)Stress Relating female losses to stress factorsStress is somewhat intangible and difficult to measureCan look at physiological conditionPhysiological ConditionPeterson - OldsquawsPhysiological ConditionMost birds at peak physiological condition just prior to breeding season.After laying first egg females condition begins to decline.Extent of decline depends on severity of breeding season,i.e. number of clutches , number of eggs.EnergyEnergy stored asglycogen,lipids,protein.During laying and incubation thesestores are depleted.Gallinaceous birds, hens start out with relatively little fatGallinaceousLarge clutch or renesting leads to decline in conditionIn chickens, layers at end of season are skin and bones, breasts are hatchet shapedDeplete protein reservesMoult follows placing more stress on themCanada GeeseHanson and Raveling sawNo food during egg laying and most of incubationNeed large fat reserves for eggs and own energy requirementsEvery day that spring is late, bird burns up enough energy to lay one eggSo ave. clutch size declines one eggPredationFemales nesting on ground or low in vegetation and undertaking all of the incubation (typically) may suffer more predation than males.Keith (1961) studying waterfowl during nesting found:2% loss of drakes8% loss of hens (mostly due to predation while nesting)Intraspecific competitionPerhaps females lose out in competition for food.S. Dakota, after a hard winter 75% of phesants found dead were females though they only made up 57% of populationOn Protection Island sex ratio changed from 50:50 after introduction to 60:40 at high pheasant densitiesExamples from MammalsHigher mortality rates in males may be due to greater activity:a. Larger home rangesb. Reproductive behavior - ungulates rut in late fall and go into winter in poor conditionc. Physiology - testosterone in bulls inhibits fat depositionRichardson’s Ground SquirrelsSee table in class notes p. 54Juveniles 50:50 sex ratio11 months 30:70juvenile males hibernate 1 mo. later and emerge 2 wks. early - periods of no food andhigh predation12 months 11:89Males driven out by females and dominant males - to die in poor habitatUngulatesImbalance toward females accentuated when food shortages occur on overutilized range.Elk - Cowan 1950, Flook 1970Reindeer - Klein 1968Snowshoe HareNo change in sex ratios apparentSex Ratios in FishVary considerably but close to 50:50 in most. - Nickolsky 1963In freshwater fishes most studies show more males in young of the year while very strong preponderance of females in older fish.Sex Ratios in FishMales produce many more sperm than females eggsIf too biased, fertilization declinesTrout on spawning groundsSignificance of Sex RatioDependent on mating systemComponent of efficient herd management in mammalsPatterns in MammalsYounger females produce more male offspringFirst offspring at any age is more likely to be maleNatality Patterns1. Youngest reproductive age classes have lower birth rate than adults2. In less favorable environmental conditions, reproductive rates decline most in younger age classes.Natality Patterns3. In fish, reproductive success is extremely


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