Cost/benefit selection operates on the individual's maintenance as well as social behaviour. We studied optimality aspects in the contexts of foraging and broodcare (see below).

Between-litter competition in the gerbil Meriones unguiculatus
Kosten/Nutzen-Analyse der Konkurrenz zwischen Wurf-Geschwistern der Wüstenrennmäuse

We formulated theoretical predictions about which factors should influence a mother's decision to continue or not continue broodcare; we test these predictions with gerbil families.

Meriones unguiculatus is a monogamous iteroparous and polytokous species with post-partum oestrus. Under these special circumstances, polytoky allows for competition among simultaneously present litter mates. Together with iteropary there may be a conflict between motzher and an exceptionally small litter over continuation of maternal care. Permanent monogamy ensures full sibs within and between litters. Post-partum oestrus then opens the possibility for an intra-familial between-generation conflict, i. e. some kind of competition between successive litters. We studied separate families with both parents always present.

If her first brood fails, a mother will give birth to the subsequent litter after another 27 days. If engaged in brood care, however, the subsequent delivery is postponed. The Interbirth-interval between successive litters thus can be expected to differ according to the amount of maternal broodcare requested for the present litter.

This was tested under four experimental situations:

A0:	first litter completely removed
A1:	first litter reduced at birth to one single young (unless but one was born)
B:	all surviving young (regardless of litter size ranging between 1 and 9) stayed with the mother up to day 15 after birth (when they can eat by themselves and survive on their own)
C:	all surviving young (regardless of litter size which ranged between 1 and 9) stayed with the mother up to delivery of the following litter.

Within each group every mother was tested only once.

Results
The number of young born did not influence the mother's broodcare; she did not even abandon singletons.
The number of young presently cared for did not influence the number of young, nor their birth-weight, in the subsequent litter.
Maternal bodyweight changes during broodcare did not correlate with the number of young presently cared for.

The results do not suggest that caring effort for a normal sized litter would not allow the mother an additional investment in developing embryos. They rather suggest a between-litter competition such that present young postpone the development of their next-to-be-born full siblings. In analogy to the 'dormant seed' situation which in many plants is initiated by the mother, one could call this a 'dormant embryo' situation initiated by older offspring.

Literature
Wickler, Wolfgang & Seibt, Uta 1983: Optimal maternal care. Zeitschrift für Tierpsychologie 63, 201-205

Additional Literature
Wickler, Wolfgang 1986: On intra-uterine mother-offspring conflict and a possible case in the pig. Ethology 72, 250-253