Overall, we find no evidence that paternal care has evolved in response to benefits of supporting females to rear particularly costly large offspring or litters.
analyses to test for evidence of evolutionary associations between male care and life-history traits, we explore if biparental care is likely to have evolved because of the importance of male care to offspring survival, or if evolutionary increases in offspring production are likely to result from the evolution of biparental care. Here, we investigate the evolution of paternal care in relation to offspring production. Read moreīiparental care of offspring occurs in diverse mammalian genera and is particularly common among species with socially monogamous mating systems.Despite numerouswell-documented examples, however, the evolutionary causes and consequences of paternal care in mammals are not well understood. The rate of growth then adheres to the time curve of capacity for production of milk. It is shown that a growth curve can be obtained for an ideal litter of I, under the condition of milk supply that on each day the mother is able to provide a constant fractional increase of milk for each additional young mouse in the litter. During the suckling period the relation DeltaW/W = K (DeltaN/N) is maintained for young mice, but with modifications in the case of small and large suckling litters due to (1) the time course of milk yield, and (2) the effect of litter size upon this. This weight of an ideal litter of 1 should be employed to disentangle the effects of heterosis and fertility factors from those having to do with individual weight at birth.
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It is pointed out that for genetic purposes the ideal weight of a litter of 1 is obtainable from a series of measurements of N and W, free from disturbances affecting the apparent value of this quantity as observed in single births. Although difficult, from the standpoint of classifying the individual young, it would appear to be distinctly worth while to make such an experiment, and we have planned it for the future. It is to be presumed that in the case of the growth of a litter containing two categories of individuals so far as concerns intrinsic drawing powers with respect to the nourishment provided by the mother, it would be possible to investigate the way in which K is open to modification.
We have been dealing with the growth of litters in inbred strains. A parallel for the case as it appears in the consideration of relative growth rates of organs in a single individual, and in which the varying magnitudes of the heterogonic growth constant K are presumed to reflect diverse drawing powers of the respective tissues, would be given by intrauterine growth of a litter containing individuals with diverse capacities for growth, -that is, individuals differing genetically with respect to the factors determining the magnitudes of w(1). With developing embryos, the intrinsic drawing powers of the tissues concerned in embryos and mothers are in each case of the same general character, at least among mammals the constancy of K reflects this. In the case of power function relationships between masses of components of a single individual, the respective "drawing powers" of the several organs are diverse, and diverse magnitudes of K are encountered. The "heterogonic" relationship which the data exhibit between N and W shows that the constant K, defined by log W = K log N + const., is independent of the species, and has an essentially constant value (0.85+/-) in all multiparous mammals it is therefore regarded as a partition coefficient. between mother and young, and on an equal basis among the several embryos of a litter.
The connection between number in litter (N) and weight of litter (W) has been interpreted as due to the partition of nourishment. The weight at birth represents, it is to be presumed (at least in mice, and for certain other cases), the weight at a particular developmental stage. We have been concerned with the connection between size of litter and weight of litter at birth, especially in mice.
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