Evolution is the gradual progression of life towards complexity, and group living is a breakthrough in this regard. It is a mystery how the organisms evolved to come together in groups. Understanding cooperative breeding is vital towards the development of social behavior in animals.Some species have several constraints on independent breeding(maternal mortality, nest destruction,lack of breeding territory under saturation);under such circumstances female company of the same generation could provide insurance against nest failure and the death of the mother.
Ecological constraints on dispersal and high fitness benefits of staying at home might have lead to the evolution of cooperative breeding societies (5).Cooperative breeding may be defined as the phenomenon when the related members of the same species cooperate with the the primary breeding pair to raise their offspring , where it appears that the non-breeding individuals act altruistically by delaying their own reproduction and behave as alloparents in the reproductive attempts of others.
Both the breeder and the subordinate have to pay certain costs( forgoing their individual fitness) to enjoy the benefits of group living.Helpers or subordinates have to bear the cost of group living in terms of- reduced residual body mass, reduced future fecundity and reduced survival. Subordinates do not always help, but they can decide to help despite the fact that it may take a toll on the individual’s fitness.
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Eg. Male great tit suffers increased parasite burden as the brood size increases while the female does not. In Seychelles warblers, the subordinates are more prone to parasitic infections.In certain cases helping may be neutral and not a selection criteria as some of the animals ( mostly birds) are conditioned to feed the begging young one,here selection is a reflection of provisioning behavior in reproductive animals. The benefits of group living for the subordinates consist of indirect fitness benefits -ability to pass on related genes to the next generation
A positive correlation is observed between the relatedness of the subordinate female to the brood and the investment made by the subordinate female in terms of provisioning. However , no such correlation was observed between a subordinating male and brood in Seychelles warblers(6).If we remove the subordinates from a territory , the breeding pair produces less offspring , thus reducing the reproductive success of the entire population.
The dominant birds enjoys the perks of group living as-higher reproductive success, reduced risk of egg predation ( by fodies),the primary female now spends comparatively less time in provisioning and incubating .Hence the life span increases. It may appear that the dominant bird hardly bears any costs of group living , but in certain species like the banded mangoose, the costs are quite evident ; where the dominant female evicts the subordinate female from the group thereby inducing subordinate abortion. It was found that pups born to the primary breeder who evicted the subordinate while pregnant were lighter than the ones born to the dominant breeder who had uninterrupted gestation.
Long term implications come into play when a former helper may acquire a breeder status ina group and is helped by the former breede i.e when breeder increases the reproductive success of the helperr. Long term benefits include improved chances of reproduction, enhanced reproductive success, and/or survival(1). In many cooperative breeding groups, a proportion of helpers can inherit their home territory ,for example, in green woodhoopoes and female dwarf mongooses, 58% and 43% of breeding positions, respectively, are obtained by inheritance.The chief long term implication of group living being the ability to leave behind a large number of decedents.
Experiment to measure the long term fitness implications like – no of offsprings in each population after time t ,the average lifespan of the breeder and helper and residual body mass of the original helper and breeder after time t.Four populations of seychelles warblers were observed for time t – Population ‘a’ consisted of only one breeding pair was allowed per territory; population ‘b’ consisted of a breeding pair with a helper female per territory; population ‘c’ consisted of breeding pair with a subordinate pair and a natural predator(fody);population ‘d’ consisted of an already established nest in a territory with its original inhabitants.
The four populations were tested for the above mentioned parameters .It is predicted that in each of the four populations that the number of offspring, the average life span increases as the number of helpers increase in the population, however the residual body mass is decreased for helpers. Some anomalies may be seen in Population ‘d’ , due to the interplay of other ecological factors , as it consisted of already established colony in its natural habitat.
It is absolutely fascinating to try to comprehend the interplay between the costs and benefits of group living, decision made by a species population subconsciously taking into account the direct and indirect benefits one may gain from such a social structure. It may also provide an insight to the evolution of eusociality in humans , where the different family members help the parents to raise the young one and reflect a life history of cooperative breeding.
References
- The Evolution of Cooperative Breeding in Vertebrates: Hypotheses and Empirical Tests
- The Evolution of Cooperative Breeding
- Understanding the Evolution of Cooperative Breeding: Insights from Comparative Analysis.
- Cooperative breeding behavior in birds is not consistent with the reproductive assurance hypothesis.
