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Non-aggressive/non-lethal competition

Not all forms of sibling rivalry in animals involve direct aggression or death of a sibling. Sibling rivalry in animals may result in competition for food resources. This is not an extremely aggressive form of rivalry, however, there is reduced fitness in each sibling.

Spotted hyenas interacting agressively

Blue-footed booby nesting with multiple eggs. When there are multiple offspring in a single brood, the potential for sibling rivalry arrises.

Fetal programming in sheep Fetal programming (see also: Barker’s Hypothesis) refers to the persisting effects in adult life caused by the fetal environment. In sheep, competition for resources within the uterus may lead to impaired reproductive abilities and different body composition. Sheep that were born 600 grams lighter than their twins ended up having an impaired reproductive ability - the lighter the sheep weighed in comparison to their twin, the more impaired they were. Although the sheep are not competing physically see the sheep competing, we note a difference in their birth weights and thus overall fitness.

Sibling rivalry among suckling piglets Domestic piglets (Sus scrofa) have been shown to exhibit different forms of non-lethal competition such as uterine competition. The relative development of the embryo in the uterus can effect their chance of survival. This is because pig embryos follow different development paths given that during estrus, sows will ovulate the majority of their follicles during a short period of time and then a few during a long period of time. This pattern causes a difference in development so although all embryos are equally viable, the less developed embryos are less likely to survive. Competition exists over space in the uterus of the sow. The central portion of the uterus is the most crowded and the site of the great competition. This competition prevents some embryos from fully growing, often resulting in a low birthweight which may put piglets at a disadvantage during other forms of competition for resources once out of the womb.

Neonatal Competition Neonatal competition also exists among piglets as they directly compete amongst their siblings for their mother’s teats only hours after their birth. Competition appears to be the main reason for neonatal deaths due to starvation (43% of Piglet deaths). Under normal conditions (i.e. stable environment, average litter size), larger piglets appear to have an advantage in survival partially due to their ability to win more fights against smaller piglets over access to teats. It is believed that teeth in piglets evolved as a product of an evolutionary arms race caused by sibling competition thus resulting in armed sibling rivalry . Although not much of an advantage to a piglet under normal conditions, their teeth become more important when the litter-size is bigger than normal, causing increased competition. Thus, the individual piglet can better cope with the competition it is presented with.