particularly useful in Galapagos fur seals, since there is a great deal of maternal investment in raising a seal pup to independence in an environment that has great fluctuations in food. The high level of resource uncertainty, late weaning, and potential overlap time of suckling young all lead to violent sibling rivalry and provide a good environment for studying parent-offspring conflict. From an offspring's point of view, it would be most beneficial to continue suckling and receive more than its fair share of milk, but to the mother seal, it would pay to wean the older, more independent offspring in order to invest in the next pup. Thus, studies show that 75% of mothers intervened, often aggressively, when the older sibling harassed the newborn pups. Mothers would bite or lift the older offspring roughly by its skin, which sometimes caused open wounds. Maternal aggression towards the older sibling diminishes with time after the second sibling's birth. Even without direct aggression, older siblings may still indirectly harm their younger siblings by outcompeting them for milk. The older offspring usually suckles first and allows their younger sibling access to the mother only after it is satiated, resulting in very little milk left over for the younger pup. Thus, the younger siblings often die from starvation. During periods when there is very little prey, interbrood conflict increases. Galápagos fur seal population is drastically affected by El Niño, a period accompanied by high water temperatures and a deepening thermocline. Food becomes scarce during El Niño, and thus older seals exhibit an intense aversion to weaning, causing the mother seal to neglect the younger sibling. Male competition Males will compete for the females when they become large enough by establishing territories on the beaches. The beaches are valuable pupping substrate for females. They have territories that average 200 m2. This is large compared to amount of territory occupied by most otariid males. Feeding and predation The Galápagos fur seal feeds primarily on fish, squid and shellfish. They feed relatively close to shore and near the surface, but have been seen at depths of . They primarily feed at night because their prey is much easier to catch then. During normal years, food is relatively plentiful. However, during an El Niño year, there can be fierce competition for food, and many young pups die during these years. The adult seals feed themselves before their young and