|Principle Involved: Age and sex ratios are important influences on the birth rate. Managers can manipulate these ratios and have profound effects.|
a. Mayr (1939):
Primary sex ratio = at conception
Secondary = at birth
Tertiary = at later specified age
b. Operational Sex Ratio = only reproductively active members used to calculate ratio
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
d. How to compute sex ratios
2. Mammalian and avian patterns
a. Basic tendency
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.
Internal: sex-linked lethal factor
External: environmentally induce mortality related to behavior and/or life history of the sex
Equality of the investment by parents in offspring of each sex
3. Examples: humans, birds, mammals, fish
Significant sex-ratio changes in some birds and mammals
Sex ratio changes in Richardson's ground squirrel
Brown rats in laboratory
Hypothetical changes in a cohort of deer subjected to a constant annual removal of adult males.
4. Significance and applications
a. Depends on mating system
b. Component of herd management in mammals
c. Kelker method: using changes in sex ratio to estimate population size.
1. Stable-age distribution
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.
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
a. Age-sex distribution pyramids
b. Must be cautious when using age-ratio data to make inferences on population growth or projected population changes (see Caughley 1974).
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.
2. Effects on age structure
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.
Updated 05 August 1996