1. Infanticide

2. Ultimate Explanations for Infanticide
Langurs
Killing pups leaves the mother no choice but to mate with the new male
Elimination of the nursing youngster enables the female to resume cycling more quickly
If this is the adaptive reason, then the behavior should be seen in other species with a similar social system
Lions, mice, 50 other mammalian species
Females can be infanticidal too, particularly when the male provides the parental care (water bug, jacana waterbird)

3. Determining Paternity
In langurs and lions, infanticide occurs when a male invades a new group
In mixed population, colony breeding animals like mice, paternity must be determined in some other way
Circadian monitoring of # days since last ejaculation/mating experience

5. Mechanisms of Infanticide vs Parental Care
Hormones
Testosterone
Progesterone
Estrogen

6. Progesterone Receptor KO mice do not kill their pups
Mated males, these were their pups, genetically
Schneider et al 2003

7. Virgin PR KO mice are more parental
Schneider et al 2003

8. PR activation correlates with parenting
Blocks PR
Less PR activation, better parenting behavior in fathers
Activates PR
Schneider et al 2003

9. Hormone levels in California mouse

10. And in the experimental PR KO mice
Not a significant difference
Schneider et al 2003

11. Extra P doesn’t change parental urge
fathers

12. But those non-parental males were more violent on P
Not fathers
No change in testosterone levels
Light grey= attacked pups
Dark gray=ignored pups
Schneider 2009

13. Human ‘Infanticide’ and the cost of motherhood
Deaths occurred in the year following the birth of the child

14. Abortion is a form of infanticide
Embryos unlikely to survive are spontaneously aborted
Miscarriages
Sometimes completely undetected by the mother
Willingness to terminate a pregnancy that may produce healthy offspring
Ability to provide and ensure that child reaches adulthood healthy and viable

15. Other factors affect the decision
Age, marital status, health of mother, likelihood of future opportunity to marry