|Optimal behaviour of foraging and broodcare
Optimales Verhalten bei Fressen und Brutpflege
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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.|
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.
|n mothers tested||Interbirth interval |
Mean ± SD days
|A0||81||27.2 ± 2.8|
|A1||62||28.3 ± 4.6|
|B||59||35.0 ± 8.1|
|C||27||40.4 ± 13.2|
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.
Wickler, Wolfgang & Seibt, Uta 1983: Optimal maternal care. Zeitschrift für Tierpsychologie 63, 201-205
Wickler, Wolfgang 1986: On intra-uterine mother-offspring conflict and a possible case in the pig. Ethology 72, 250-253