In contrast to young from most other carnivores, spotted hyena cubs are born with their eyes open and teeth fully erupted. When a mother gives birth to twins, the two siblings engage in aggressive interactions to control access to maternal milk immediately after birth. These fights during the first few weeks after birth lead to the establishment of a dominance hierarchy between the two twins. After this initial period, the rank order among the twins does not normally change any more.
In the Serengeti, the competition among twins is fierce. This is mainly the result of fluctuations in access to food. Indeed, prey abundance in the territories of hyena clans can get very low in the Serengeti during extended periods because their main prey (wildebeest, zebra and gazelles) migrate over large distances. During these periods, hyena mothers leave their dependent cubs at the communal den and regularly travel (‘commute’) to the migratory herds of prey to feed. When the herds are far away, mothers may return to the den only after several days and fail to provide sufficient milk to nourish both cubs of a twin litter. This can lead to the death through starvation of the subordinate cub (siblicide) if the dominant cub monopolises the access to the milk.
The Ngorongoro Crater is a safe haven for hyena cubs. Indeed, in contrast to the Serengeti, wildebeest and other herbivores do not migrate in the Ngorongoro Crater, and there is always prey nearby. Mothers therefore can nurse their cubs at regular, short intervals. As a result, aggression between twins usually is very low and we have never observed cubs starving due to sibling rivalry.
How does one twin sibling become dominant?
The first-born twin may have an advantage in spotted hyenas. Observations of births of spotted hyena cubs in captivity suggest that twins are born at an interval of about one hour. It is likely that the first-born offspring has a competitive advantage over the second one, particularly if it had time to suck milk from the mother before its sibling emerged.
Cubs progressively learn to be the loser (or the winner). The competitive asymmetry associated with the birth order of cubs may be reinforced by a ‘training effect’. This means that the experience of winning or losing a contest may affect a cub’s probability of winning or losing future contests. Our colleagues studying spotted hyenas in the Serengeti demonstrated that young subordinate twin siblings progressively learn to submit to their dominant sibling during nursing periods. Expressing their submission enables subordinates to avoid receiving intense aggression from their dominant sibling. As the two cubs grow older, aggression by the dominant cub decreases, the subordinate cub learns to respect the dominance conventions, and the hierarchy becomes more stable.
Hungry subordinate twins may become bold and rebellious though, when the threat of starvation is high. They start challenging the authority of their dominant sibling and are less likely to submit to it than when they are not hungry. Subordinate siblings thereby can enhance their own access to milk to some extent.
Sisters may be better competitors than brothers. When a twin litter consists of a female and a male, females more often are the dominant sibling than males. Subordinate females also tend to be better at reversing the hierarchy and overthrowing their brother than are subordinate brothers. This asymmetry of competitiveness in favour of females may be particularly relevant in matriarchal societies such as that of spotted hyenas.
Benhaiem S, Hofer H, Kramer-Schadt S, Brunner E, East ML, (2012) Sibling rivalry: training effects, emergence of dominance and incomplete control. Proceedings of the Royal Society B: Biological Sciences 279(1743): 3327-3735.
Wachter B, Höner OP, East ML, Golla W, Hofer H (2002) Low aggression levels and unbiased sex ratios in a prey-rich environment: no evidence of siblicide in Ngorongoro spotted hyenas (Crocuta crocuta). Behavioral Ecology and Sociobiology 52(4): 348-356.