User:Ihyuan/sandbox

Galapagos fur seals have the lowest reproductive rate reported in seals, and it takes an unusually long time to raise seal pups to independence. Females bear only one pup at a time, and she remains with her newborn for a week before leaving to feed. She then periodically returns to the pup and stays to suckle it for a few days before leaving on another hunting trip. Females recognize their own pups by smell and sound, and pups also learn to identify their mothers by the females’ “Pup Attraction Calls”. Mother-pup recognition is crucial because females exclusively nurse their own pups, often violently rejecting strange pups that approach. Orphaned seal pups usually try to sneak up on sleeping or calling females to suckle, but stealing milk is not enough to sustain the pups, and they usually die within a month. Galapagos fur seal pups rely on their mother’s milk for the first eighteen months, and weaning may be delayed for up to two or three years if conditions are poor. The result is that every year up to 23% of pups are born when an older sibling is still suckling (b). 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. (b) 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.(2) Survival of the younger sibling greatly depends on the availability of resources. In years when there is abundant food, the mortality rate of second pups is as low as 5%, which is equivalent to the mortality rate of pups without siblings. In years when food is scarce, 80% of pups with suckling older siblings die within a month. Therefore Galapagos fur seal population is drastically affected by El Nino, a period accompanied by high water temperatures and a deepening thermocline. (4) Food becomes scarce during El Nino, and thus older seals exhibit an intense aversion to weaning, causing the mother seal to neglect the younger sibling. The younger sibling thus serves as an insurance in case the first sibling dies, and also provides extra reproductive value in case conditions prove better than expected. Such a bet-hedging strategy is 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.