FIGURE 8.2 Fitness advantages of sociality when siblings and offspring are equally valuable. Arrows show transitions from one time period to another, and enclosure within dashed rectangles indicates the same nest. (a) Leaving vs. Fortress Defense. If an individual (self) leaves her natal site, she survives dispersal with probability sd, whereas her mother survives in the safe natal site. Both then produce offspring. Alternatively, if self stays in the safe site with her mother, she avoids a bout of dispersal mortality and doubles her mother’s output. (b) Leaving vs. Life Insurance. Here we assume that dispersal carries no cost but that a parent survives the period of parental care with probability sc. If she dies during this period, her offspring also die. When self stays with her mother, there are three ways for the offspring to survive: if both adults survive, if only the mother survives, and if only self survives.