Hamilton’s rule is a key principle for understanding how different types of social behaviour, from acts of altruism to acts of spite, have evolved in all living things from viruses to humans. Traditionally, Hamilton's rule suggests that altruistic behaviours (helping others) are favoured by natural selection when they benefit relatives. More recent studies have argued that cooperation could also help with unpredictable environments, by reducing the ups and downs in reproductive success; a concept known as "altruistic bet-hedging’’.
This study investigates this idea more fully, asking the questions; how does social behaviour influence reproduction in changing environments and are helping behaviours more likely to evolve if they reduce the unpredictability of a relative’s ability to reproduce?
The researchers from the University of Oxford along with colleagues at Lund University and the University of Oulu used a combination of theoretical modelling and data on 15 species of cooperatively breeding birds. This data allowed the researchers to examine the reproductive success of breeding pairs of birds with or without a helper in the nest, over several breeding seasons where rainfall and temperature varied.
The mathematical model showed that reducing variability in reproductive success does not always favour the evolution of helping behaviours. The bird data revealed that helping did not consistently reduce the variability in reproductive success but instead the overall benefits of helping on an individual’s average reproductive success were generally more important than its effects on variability. This suggests that helping is beneficial most of the time, and that reducing the unpredictability of reproductive success in unpredictable environments has not been a major factor in the evolution of helping behaviour.
Lead author Dr Miguel dos Santos says
''Our theory predicts that behaviours that either decrease or increase the variance in reproductive success of relatives should be common in nature, and this is what we found in our bird data.''
Professor Stuart West adds
''Although helping may reduce variation in reproductive success, it does not necessarily tell us about whether the behaviour will be favoured by natural selection''
By increasing reproductive success in most environments, helping behaviours in these birds might actually contribute to their ability to cope with environmental change. Regardless of helping’s effects on the environmentally-induced reproductive variability, it might both alleviate the harmful effects of harsh environments, and allow these populations to grow even more in good environments.
The findings are currently limited to the small sample of bird species, further work will aim to increase the sample size across more species of birds as well as other cooperatively breeding animals such as fish or meerkats. The study also focused on the benefits of altruism, the next challenge for our researchers will be finding a way to collect data on the costs of altruism.
To read more about this research, published in PNAS, visit: www.pnas.org/doi/10.1073/pnas.2402974121.
