Originally, evolutionary decisions were thought to have been made to benefit the species as a whole, and not the individual itself. After a study of reproductive ecology by David Lack, it became apparent that there are distinguishing decisions made between individual and group interests concerning reproduction.
A study of the langurs has illustrated the individual reproductive decision making well. Langur males will commit infanticide on those infants who are not their own when taking over a brood. Once thought to be random acts of violence have now been concluded as a decision made by the male individual, not on behalf of the species, but on behalf of himself. Once the female’s unweaned offspring has been killed, the female will become sexually receptive sooner, thus the new male will have a higher chance at reproducing his own offspring. The violent manner in which this is executed also has to do with the short-lived reproductive life of the male. Males are only in control of their brood for a few years, thus if he can get his females to be sexually receptive sooner, the more offspring he will have and the higher chance his offspring have at surviving to reproductive age.
The female langur also makes a reproductive choice concerning her as an individual rather than for the good of the species. The female could fight off the male who is trying to kill her infant, but with sexual dimorphism and the male’s relentlessness stacked against her, she is not left many options. The female could wait to solicit a kinder male, but her offspring would be at a disadvantage against the more aggressive competition. Again, the decision is made in favor of the individual’s offspring surviving to reproductive age, and not the survival of the species as a whole.
Brian Bertram studied the reproductive behavior in lions and produced two observations. While the male lions behave as the male langurs, committing infanticide when they take over a pride, thus a reproductive decision based on the individual, the lionesses have made reproductive decisions that have enabled the cubs to more successfully mature. The lionesses will all have a synchronous oestrus, thus the adaptive advantage is having more cubs survive. Lionesses have communal suckling, and raise young together, thus when all females are lactating the synchronously born are cared for more easily. Male cubs are also evicted from the pride and survive better if they have a companion, yet to have a companion the cubs need to be the same age. These reproductive advantages are for the interest of the group, rather than the specific individual, such as from the male lion’s decisions.
There is a distinguishing importance when defining whether the observation is actually in the interest of the group or the interest of the individual. How one observes the study will depend on how it is recorded and understood. For instance, the female langurs could be considered as ‘bad’ mothers, or as Hrdy as stated, ‘less intelligent’ when it comes to making critical decisions for the group’s well being, but when viewed as making reproductive decisions from the interest of herself as an individual, the female langur is making the best reproductive choice.