Reverse Sexual Dimorphism in Raptors Size-Related Advantages for Reproduction in a Slightly Dimorphic Raptor: Opposite Trends between the Sexes

Reverse Sexual Dimorphism

Hypotheses for RSD Four Main Categories Ecological Behavioral Sex-role differentiation Sexual Selection

Ecological RSD promoted by size-mediated niche partitioning between the sexes Lowers intersexual competition for limited resources

Behavioral Larger female size favors pair formation and maintenance, allows female to force male to provision food

Sex-Role Differentiation Larger female size may increase fecundity, larger eggs, more effective incubation, and higher fasting endurance (Big Mother Hypothesis) Smaller male size leads to higher agility, lower flight costs, increased hunting efficiency (Small Efficient Male Hypothesis)

Sexual Selection Intersexual Selection: females prefer smaller males because of the reproductive advantages due small size Intrasexual Selection: large female size an advantage in competition for high-quality males (Acrobatic male courtship displays)

Black Kite (Milvus migrans)

Morphology and Behavior

Black Kite Distribution Orange: Summer Green: Year Round Blue: Winter

Study Design Novelty of Study Initial Field Procedures Body Mass Tarsus Length Sex (via blood sample) Nest Analysis Clutch size Hatching success Number Fledged after 40-45d

Study Design When were birds captured? Is this important? RSD in Black Kites How much larger were females? By what metric?

Study Design Six Hypotheses Tested Reproductive Effort Hypothesis Starvation Hypothesis Female Supplementary Feeding Hypothesis Small Efficient Male Hypothesis Intrasexual Selection Hypothesis Intersexual Selection Hypothesis

Which Hypotheses were Supported?

Study Results Six Hypotheses Tested Reproductive Effort Hypothesis: lack of correlation between laying date, clutch size, and hatching success related to female size/condition Starvation Hypothesis: lack of correlation between laying date, clutch size, and hatching success related to female size/condition Female Supplementary Feeding Hypothesis: partial/inconclusive support Small Efficient Male Hypothesis: higher probability of success Intrasexual Selection Hypothesis: higher probability of success Intersexual Selection Hypothesis:

Discussion Reproductive Advantages for Females Reproductive Advantages for Males RSD in Slightly Dimorphic Raptors Critique of Study Design