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Principle Involved: Age and sex ratios are important influences on the birth rate. Managers can manipulate these ratios and have profound effects. |
Primary sex ratio = at conception
Secondary = at birth
Tertiary = at later specified age
Operational Sex Ratio = only reproductively active members used to calculate ratio
Expression
usually males: females to make 100 total (e.g., 50:50)
less commonly expressed as females : males
large mammals: males per 100 females
% of total that are males or females
How to compute sex ratios
Fisher (1958) introduced a "symmetrical argument" that suggested the sex ratio should tend to be 1:1
Deviations from 1:1
Protogynous fishes: undergo a sex change
Example: sea bass (Anthias squamipinnis) - triggered by death or departure of dominant male
Bateman model: based on maternal condition
Reproductive success of offspring is related to maternal condition
Male reproductive success is highly variable
Female reproductive success is not as variable in relation to maternal condition
Therefore, females in good condition produce more males. Females in poor condition produce more females
Evidence for the Bateman effect found in mink, deer, seals, sheep, and pigs
mammals: sex ratio shifts toward females in later age classes
birds: sex ratio shifts toward more males in later age classes
freshwater fishes: more males in young-of-the-year and a shift toward females in older fish.
Significant sex-ratio changes in some birds and mammals
Hypothetical changes in a cohort of deer subjected to a constant annual removal of adult males
Fishes
a. Depends on mating system
b. Component of herd management in mammals
c. Changes in sex ratio can be used to estimate population size
Change-in-Ratio estimators
Zheng and Mathisen (1998) greatly improved catch per unit effort methods for estimating pink salmon abundance by incorporating sex ratios
1. Stable-age distribution
1. Definition: Proportion of population in each age class remains the same through time
2. Occurrence
can occur in stationary populations (B - D = 0)
can occur in populations increasing or decreasing at a constant rate, i.e., constant birth and death rates in each age class.
2. Changing age distributions
a. Age distributions will strongly influence population processes, especially birth and death rates.
b. Summary
Changes in birth rate will produce changes in age structure.
Changes in mortality rate will usually produce changes in age structure, except if death rate changes affect all age classes by some constant factor.
Higher birth rate results in a shift toward younger age structure.
Increase in mortality rate shifts population toward a younger population.
3. Interpretation of age ratios
Must be cautious when using age-ratio data to make inferences on population growth or projected population changes (see Caughley 1974).
Effects of bull age on conception dates and pregnancy rates of cow elk (Hoyes et al. 1996)
1. Nature of birth and death rates
a. Natality patterns
Ungulates (and birds?): youngest reproductive age classes have lower birth rates than adults.
Ungulates (and birds?): in less favorable environmental conditions, reproductive rates decline most in younger age classes.
Fishes: reproductive success is extremely variable.
b. Examples
age-specific pregnancy rates for some North American ungulates.
age-specific fawn production by New York white-tailed deer.
Alexander, M. M. 1958. The place of aging in wildlife management. Amer. SCI. 46(2):123-137.
Caughley, G. 1974. Interpretation of age ratios. J. Wildlife Manage. 38(3):557-562.
