Since the 1960s, whether an animal cares for its offspring (and, if so, how much) has been expressed in the cold, hard language of mathematics. Natural selection doesn’t care how a mother’s or father’s devotion tugs at the heartstrings, and parental care is clearly beneficial in producing stronger, more resilient offspring — improving the chances of those parents having grandchildren. But a key problem for researchers trying to mathematically represent the seemingly infinite variety of parenting behaviors in nature is that such care comes at a cost to parents.
Their research sheds light on the extent to which animals take on parental roles, and how the costs and benefits are weighed. Gains are reflected in improved survival of offspring and subsequent reproductive success. The cost comes from the time and energy spent caring for the offspring, limiting the chances of the parents turning away to reproduce more. From the interplay of these forces, natural selection strikes a compromise between caring for offspring and producing more.
Robert Trivers’ views on parent-child conflict are based on this type of research, and his observations of weaning monkeys illustrate well how costs and benefits affect mothers. When a mother breastfeeds, she benefits from helping her offspring develop, but it also has a cost to her, because by breastfeeding, she is not going to have another offspring.
A key point is that the costs and benefits here are not constant, they change over time. Providing milk to newborn baby monkeys (or any other mammal for that matter) is a necessity – the payoff is absolute. But as the hatchlings develop, and they become more and more able to fend for themselves, breastfeeding becomes less and less beneficial to the mother, until her investment in producing milk exceeds what she can get in return for it.
This doesn’t just apply to monkeys. In all mammals, theoretically, how a species breastfeeds is shaped by its costs and benefits, which vary by the species’ specific biological mechanisms. For example, mammals with abundant food during the breeding season can provide a lot of milk every day, but those species with fewer resources nurse for longer. Hooded seals pump 7kg of milk per day and nurse for only 4 days because of the unique reality they face: living on ice floes in extremely cold temperatures. In primate groups, orangutans will pull their daughters until they are 6 to 8 years old. In such an environment, the value of the mother-daughter relationship goes far beyond the pure nutritional relationship.
The same is true of any aspect of parental investment: whether or not to care for offspring, and to what extent, depend on the specific circumstances of the species. On a larger scale, for example, fish, amphibians and reptiles have sporadically evolved child care, mostly in species living in dangerous or unpredictable environments, often surrounded by predators. In this case, the benefits to the parents of guarding the eggs or hatchlings more than offset their costs.
The author of this book, neurobiologist Liam Drew.
Reproductive evolution of mammals
For mammals and birds, parental care of newborns and young is unquestioned. Maintaining a warm-blooded physiology means that the parents need to provide nutrition and often help maintain body temperature, or the hatchlings will die. Mothers in every mammal species (and most birds) care for their young. Remarkably, young mammals are energy-poor in pure energy form—they take in less energy than they expend—while mothers struggle to get more energy than they need to sustain themselves alone. But then, we find a huge difference between birds and mammals: in 90% of bird species, the male partner will help the female mother, and as mentioned earlier, in 95% of mammals , the father is doing nothing. The prevalence of parental care by bird fathers is often attributed to the need for two adults to meet the energy needs of young birds.
Birds often have to fly long distances to find a variety of food, and mother birds cannot collect energy in advance and store it to provide food to their young like mammalian mothers. Combined with the fact that it is easy for predators to find young birds, there are obvious benefits to having one parent guard and one forager.
Most of us human fathers like to think we are useful, and it’s embarrassing to see mammals that way. But then again natural selection doesn’t care what we think. The lack of paternal love in mammals raises two questions: First, why are fathers usually not involved? Second, what about the exceptional 5%?
First, internal fertilization is generally at odds with paternal investment. When a male stickleback looks at some of his eggs sprinkled with his sperm, he knows the hatchling he’s about to guard is his own. A male mammal who sees a female go away has no way of knowing what she’s going to do next, and he can’t be sure that the offspring she later produces will be his. And when a male is less sure of his paternal kinship, he’s less concerned.
In addition, the reproductive evolution of mammals has also reduced males to supporting roles. Males are of limited help when the female is pregnant or breastfeeding her young. Therefore, the income brought by the father’s investment is relatively small. Are two mates better for hatchling protection than one? What skills can a father teach his cubs that a mother cannot? In mammals, however, the benefits to the paternal investment are large because the costs to the male are also large. To put it crudely, suppose a woman sleeps with 10 different men a month, and a man sleeps with 10 different women in the same amount of time. The former doesn’t increase the number of offspring the woman might have, while the latter makes the man father 10 babies—a male’s fertility potential increases with bed count nicks, but a female’s doesn’t.
Making large eggs and making cheap, fast sperm are fundamentally different investments; this difference in reproductive rates between the sexes exists in almost all animals, but is especially pronounced in mammals. This mismatch profoundly affects the reproductive behavior of both sexes, and for male mammals, turning around to find new sexual encounters has considerable potential rewards and means staying put is costly.
We Homo sapiens have a hard time reflecting on the evolution of behavior because we tend to think of it as being driven by will. We think of the brain as an organ that enables animals to decide how to act. However, male platypus does not actively choose to abandon females after mating because they have more tails to bite outside; baboons do not think deeply about how to reproduce and nurse, and rats do not rationalize their breeding strategies. Species other than humans differ in behavior, but their core functions are fixed. In 95 percent of mammals, natural selection simply bred males who sought out more opportunities to reproduce, at the expense of males who stayed behind to help with their young.
The Evolution of Monogamy
So why did human males and the other 5 percent of mammalian fathers stick around? The first premise of paternal care seems to be that the male is in a monogamous relationship with the female. Only three species are known to have paternal care but an unstable parental bond: a mongoose, a marmoset and a lemur.
Mammals live in many modes, among which a variety of mating modes exist. Sex ratios in groups often vary: males may mate with multiple females; females may mate with one or more males; sometimes social rank may be important; males may do various things to gain access to females: Horn blows, head banging, teeth baring, scrotum exposure, foul-smelling secretions shaken, neck banging, howling, or punching. Sexual encounters may occur at very specific times of the season—female porcupines, for example, only 12 hours a year—or throughout the year. In all of these models, the theory postulates that the cost-benefit pattern for both sexes determines which option to pursue. And amid all the rowdy sexual strategies, there are a small number of monogamous mammals.
There are two dominant views explaining the evolution of monogamy in mammals. One is mostly geographical, and the other deals with the ugliest behaviors of mammals. Males of many mammalian species kill the offspring of other males so they can reproduce with females. The prevalence of infanticide is appalling. It is the leading cause of death in juveniles of some species. This occurs mainly in species where the death of offspring causes the female to go into heat again, enabling the killer to mate with her. So one of the theories about monogamy postulates that it’s a male strategy that evolved to guard against infanticide.
It is also possible that, in addition to preventing intra-species infanticide, paternal care has a positive effect on offspring survival and development, such that the benefits of monogamy involving fathers in caring for young outweigh the lost mating opportunities for fathers. Another view is that when females are particularly geographically dispersed or “intolerant” of other fertile females, monogamy evolves as a mate-guarding strategy for males. In this case, the male makes sure that no other males can reproduce with his mate.
To discern these theories, Dieter Lucas and Tim Clutton-Brock of the University of Cambridge surveyed as many as 2,545 species of nonhuman mammals and mapped their mating behavior and The level of paternal care. In this 2013 survey, Lucas and Brock deduced that 229 species, 9 percent of the total, had evolved conditions for monogamy. First, in 94 of the 229 monogamous species, the males were completely childless. For example, the males of the kudu (a small antelope in South Africa) are completely loyal, but they do nothing to protect, feed or teach their young. Importantly, the fact that there are more monogamous species overall than monogamous males with paternal care suggests that paternal care evolved later than monogamy. Another possibility is that paternal care evolved first—outside the monogamous relationship—and then led to an exclusive relationship between parents, in which case you would expect to find But not a monogamous species.
Their data suggested that pair formation followed by male care for offspring led Lucas and Brock to search for factors that favored the evolution of monogamy. They found that pair-bonding relationships developed in species in which females were distributed both widely and at great distances and lived solitary lives over large areas. When the females are distributed like this, males seem to have trouble protecting multiple mates, so their best reproductive strategy is to find a single female and form a pair. As one reviewer put it: “The female mammals set the rules, and the males match themselves to their distribution map.”
Monogamy in only one species (a lemur) appears to have evolved from social life. Whether humans represent a second example is an open question. Patterns of paternal care in mammals suggest that it is most common among primates and carnivores, but also sporadic in other orders. Wolves and African hunting dogs are often hardworking fathers who bring chewed meat to their children. Tiny monkeys are initially protected and cared for by their father, and only go to their mother when grooming and feeding. Lucas and Brock also found that monogamous couples whose fathers helped in some way produced more litters per year than monogamous couples whose fathers left their offspring or solitary females.