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Lactation is the most energy-intense period in the life of a female mammal. This causes a severe conflict between mother and offspring over the length of lactation, but also between
siblings over the amount of milk each pup gets from its mother. Differently from birds where parents can decide about the amount of food given to each individual chick, mammal
mothers can not regulate the flow of milk to individual pups. Thus competitive interactions between siblings are expected and competition is likely to increase with litter size, especially
when the number of offspring exceeds the number of teats.
We study sibling competition in litters of the domestic guinea pig (Cavia porcellus). Since guinea pigs have two teats, we create non-competition litters with two pups and
competition litters with four pups by cross-fostering. We expect competition to be harder and interactions to be more aggressive in larger litters. Furthermore, we aim to answer the
question whether the individual need is responsible for competitive interactions. We thereby distinguish between a long-term need which is represented by the weight of the pup,
and a momentary need.
In a first experiment half of the pups of each litter were separated for two hours from their mothers, subsequently the behaviour of pups and mother was
observed for 20 minutes. We expected hungry pups to spend more time with suckling and to tussle more than their non-hungry siblings, but our first results suggest that access
to teats depends on a determined rank order of the siblings and not on individual momentary need. In the following experiments we will therefore concentrate more on the rank
order to find out whether a pup's rank is correlated with its body weight. Additionally, we measure individual levels of cortisol to detect possible correlations between individual
behaviour and stress levels.
Apart from competitive interactions between siblings we will continue to investigate interactions between mothers and pups to reveal mechanisms that induce mothers to nurse
more intensively or longer. In contrast to former studies where interactions between mothers and their litters as a whole were investigated, we will concentrate now on behavioural
strategies of individual pups. We plan to investigate e.g. which pups of a litter are weaned first, and by which means (body condition, begging behaviour) pups can express
their individual need of milk to their mother.
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