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In human race usually males are larger than females, and probably is this for all primates. But some animals there are the females bigger, but what determines whether the male or female should be bigger. Is this just random with no logical system as evolution can be or are there some marks which indicates that it is better that the female should be bigger?

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    $\begingroup$ I seem to recall that some primates show dimorphism in which females are larger than males. $\endgroup$ – canadianer Mar 13 '17 at 16:55
  • $\begingroup$ See Sexual dimorphism $\endgroup$ – mgkrebbs Mar 14 '17 at 5:31
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In short, there is not a rule of thumb whether a given species will have bigger males or females. In some cases females are larger (often insects, invertebrates or fish), in other cases males are larger (often mammals and birds) but even these generalizations have exceptions (Ralls (1976)).

When a males and females of a species have morphological differences other than differences in reproductive organs, this is called sexual dimorphism. These dimorphisms can be thought of as adaptations which have evolved due to a specific evolutionary pressure.

In males, larger sizes can evolve from sexual selection on males in competition with other males for mates. Simplified, the bigger males are better competitors. In females however, larger sizes can evolve from a number of different pressures, but rarely due to competition for mates. More common pressures resulting in larger females are rearing success, competition for resources other than mates.

Whether males or females of a species are bigger is certainly not random, but rather a precise outcome of numerous trade-offs and benefits of being bigger.

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