Siblicide: Hyenas

I suppose you could say that the reason one sibling kills the other is due to predation or a lack of food. This hypothesis fits with postulates 2 and 3 because regardless of whether the mother bears one or two children, the input is constant - she cares for them to her greatest capacity. In terms of three, whenever they are travelling as a pack, food resources and shelter may be lacking, so there is a greater likelihood that one will be targeted as prey and the other will survive (survival of the fittest). This leads on to the fourth point, where the mother may separate the aggressive cubs and preferentially feed one over the other. The mother has the natural instinct to survive and will nurse the one that is more likely to survive and protect her. For the first point, I don't understand the working , but I'm sure it also fits in the same theme. An animal that also does this is the whooping crane. The mother will often lay two eggs, but only one of the hatchling survives to fledging age due to a lack of food or predation. The mother lays two eggs as a preventative way to survival.

Found some information that may clear things up.

Much research has been devoted to the phenomenon of siblicide in spotted hyena cubs.  The primary literature frequently cites that when dominance is being established soon after childbirth (first two weeks), 25% of cubs are killed by their siblings (Engh et al., 2000).  Although siblicide has rarely been seen in the wild since cubs are underground in the natal den for the first two-three months, these statistics are inferred from the number of single cubs raised in the wild (where food is limited) as compared to to the number of single cubs raised in captivity (where food is plentiful).  The number of singletons in the wild is much greater than the number of singletons raised in captivity since it is believed that neo-natal aggression is much more fierce in the wild where food is limited (for mothers and subsequently cubs) (Smale et al., 1999).  Same-sex litters, moreover, are also seen less in the wild (according to some authors) than in captivity since competition is more fierce when two cubs of the same sex are evenly matched in the fight for more limited resources (James and Hofer, 1999).

    Recent studies, however, have begun to cast doubts on the frequency of siblicide reported by some authors.  Supporters of the obligate model of siblicide suggest that siblicide among same-sex twin litters is a frequent occurrence (Smale, 1999).  Smale, however, reports that in her observations the ratio of same-sex twin litters to mixed-sex twin litters did not significantly differ from what would be predicted by chance.  Moreover, she suggests that the high proportions of singleton litters in wild spotted hyenas may result if the females were to resorb fetuses so as to concentrate their rearing efforts on a single cub instead of multiple cubs, a phenomenon which is know to occur in other species of mammals (Smale, 1999).  Such behavior may be an adaptive response to periods of low-prey abundance and would certainly limit the neo-natal aggression between siblings.

    The rates of neo-natal aggression differ depending on the status of the mother.  If the mother is of low rank in the clan, then she is likely to spend more time hunting than dominant females since the dominant members can take over carcasses of kills made by subordinates (Hofer and East, 1993).  She is also more likely to be away from her cubs natal den while she hunts.  Thus, the cubs of lower ranking females do not nurse as frequently as the cubs of higher ranking females (Hofer and East, 1993).  As a result, neo-natal aggression between cubs of lower ranking females occurs more often since their resources, namely their mother's milk, is more limited.

    Although aggressive behavior constitutes a large proportion of sibling interactions up to the second week of life, the dominant types of interactions between siblings following this period consists of play (Drea et al., 1996).  It has been suggested that after dominance has been established by the second week, play behavior serves to modulate aggressive behavior.  Modulating aggressive behavior is important because the cubs are transported to the communal den by their third month of life.  The communal den consists of all the older cubs in the clan and is not a good place for young cubs to engage in aggressive behavior since older cubs would surely respond viciously to such behavior (Drea et al., 1996).