A pair of Crested treeswift with its egg [Image credits: Aditya Pal / CC BY-SA 4.0]
Mathematical models show that males should be selected to care more for their offspring rather than desert them.
Building nests, incubating eggs, feeding the young ones, protecting them from predators, teaching the required skills for life—parenting is hard work! Nevertheless, many animals do it to ensure their progeny continues through their offspring. When it comes to parenting, not all animals follow the same playbook. Among most birds, both parents are involved in caring for the young ones, while most mammals rely on the females. Then there are many fish and amphibians, where the females are not too keen on any care—they often just lay their eggs and hope for the best. In some of these species, only the males provide care, while in others both parents abandon the eggs.
Why is parental care so diverse? What decides these patterns? Science has shown that parental care patterns depend on the number of offspring an animal has and their probability of surviving until they are adults. In a recent theoretical study, researchers at the Indian Institute of Science, Bengaluru, the University of British Columbia, Vancouver and Indian Institute of Science Education and Research, Mohali have found some new insights on how parental care may be expected to evolve. They show that when costs and benefits are considered and the only difference assumed between the sexes is the size of the gametes they produce, male-biased parental care should be more likely. This result is in contrast with the wide-spread prevalence of female-biased care in nature. The study, published in the journal Evolution was funded by the Department of Science and Technology (DST).
Robert Trivers, an American biologist, proposed a theory on how parental care depends on the gamete size differences between the sexes. He argued that since females produce a limited number of large eggs in their lifetime, as compared to unlimited tiny sperms produced by the males, they lose more if the offspring do not survive. Hence, females tend to invest more in parental care. Males, on the other hand, fiercely compete with each other to woo the females—a tremendous task in itself—and hence have a minimal role in raising their young ones.
"Trivers's parental investment hypothesis has been very influential, not just in the field of animal behaviour, but also in shaping the understanding of human behaviour and psychology. People have pointed out flaws in these arguments before. Still, it continues to be invoked," says Dr Priya Iyer, the lead author of the study.
Enthused by this hypothesis, several mathematicians tried to construct models to evaluate it. One such model was by John Maynard Smith, a British evolutionary biologist. He designed a framework based on game theory, where each player had a particular strategy that decided their actions. In his framework, mating males and females could choose to care for the young ones or desert them. Females who abandoned could lay more eggs, while abandoning males got to mate with other females. Solving this game would result in four patterns of parental care—male-biased, female-biased, biparental, or no care. However, a 2002 study found a fundamental flaw in this model—it was not self-consistent.
“Self-consistency is where each offspring has exactly one mother and one father. In a model, this means that the total number of offspring of males in a population has to be equal to that of females in the population”, points out Priya.
The researchers from IISc built on Maynard Smith’s model by developing many self-consistent versions that incorporate the consequences of the gamete size difference, and the trade-off of gamete production with parental care. “We wanted to explicitly incorporate the consequences of egg production being expensive and sperm production being cheap to re-examine the parental investment hypothesis,” explains Priya.
They developed two sets of models, one assuming that all receptive females become available to mate at the same time (synchronous model). The other model considered that receptive females become available to mate sequentially (asynchronous model). Thus, a male deserting one clutch has the possibility to mate with an unmated female in the asynchronous model, but not in the synchronous model. Each of these models had three game frameworks.
The first game of the synchronous model considered that deserting males could mate again. Since the number of females is finite, more deserting males within the population would mean fewer chances for males to mate. On the other hand, when only a few males desert, the males benefit more by mating with many females.
Deserting females get to lay a second clutch of eggs, increasing the number of their offspring. Hence, the two coevolving male and female strategies led to males and females to care equally for their offspring, resulting in biparental care in the first game.
The second game was similar to the first, except that, males who cared could also get to mate again, especially as sperm production is assumed to be cheap, and hence need not trade-off with male parental care. However, it can be assumed that deserting males have an advantage with these rematings. The third game assumed that females who abandon could lay more eggs in the first clutch itself and that only males who deserted could mate again. These two games gave rise to either similar results as the first model or male-biased care. It would be more beneficial for females to abandon if males cared for their offspring.
In nature, seldom does mating occur synchronously. When the researchers included asynchronous mating, the first game resulted in all four patterns of parental care. The second game showed either male-biased or biparental care, and the third game resulted in more male care than the first game. Though these results differ from the synchronous models, the assumptions that go into games 2 and 3 select for more male care.
The results of this study contradicts Trivers' hypothesis in that, incorporating the logical consequences of gamete size differences between sexes, male-biased parental care is the only biased pattern of care in the synchronous model. The asynchronous model also selects for more male care when these consequences are incorporated. This result is seen despite the differing costs of sperm and egg production and their trade-off with parental care. The researchers insist on the need for a better understanding of the implications of differential gamete size between sexes, coevolution of parental strategies and competition for mates to interpret the patterns of parental care observed in nature.
"We are working on two follow-up studies to this, where we also incorporate parentage and sexual selection considerations. In all of these studies, the intent is to question and extend some of the existing theories of evolution of sex roles, and attempt to explain the diversity of sex roles seen in nature," signs off Priya.
This article has been run past the researchers, whose work is covered, to ensure accuracy.