Do mothers care more than fathers? The answer is not so simple. It's something that has multiple levels to it. To understand this, let us begin from the start. When exactly in the evolutionary tree did parental care originate? I mean, tapeworm parents certainly don't look after their offsprings. The earliest phylogenetic examples are insects; hymenopterans such as ants, bees and wasps to state a few. However, it is the maternal care that is common among insects and not the bi-parental care as we know in humans. This bi-parental care is believed to have originated in the common ancestor of reptiles, birds and mammals around 200 million years ago. It's interesting that even in mammals, 95% provide maternal care. Only the remaining 5% which includes humans provide bi-parental care.
This raises a question that why is it that maternal care is so prevalent across these phylums? This is explained by the hypothesis revolving around anisogamy and gestation. Anisogamy is the phenomenon of differently sized gametes where the female gamete is the one which is larger. Since a mother invests more in a gamete, she cares more. The more concrete argument for it is gestation. Fertilisation in sexually reproducing species is either external or internal. External fertilisation gives a chance to both the parents to desert their offsprings for further mating ventures. However, in species where internal fertilisation occurs, gestation occurs inside the body. This leads to tremendous investment from the fetus bearing organism and hence the greater care. But we should know that internal gestation is not restricted to mothers. In organisms like seahorses, males bear the pregnancy. And yes, you guessed it right, the male seahorses care more.
There are some other evolutionary constraints as well that favour increased maternal care. Such as in mammals, the female produces milk along with internal gestation which only allow females to feed the offspring in infancy which also leads to body to body contact of the mother and the child.
Now that we have an idea about why maternal care is prevalent across the phylums, we can attempt at understanding what other factors might influence this disparity in care. Talking about bi-parental care, it is immensely dependent upon the social setting the organism lives in, and the reproductive fitness of a deserting father or mother. A society which is predominantly monogamous, such as in many species of birds, many human ethinities, there is no paternal uncertainty. What this means is, there is essentially nil chance that their offspring is not their own but of any other individual. In such settings, the difference in care is more or less similar. On the contrary, in a polygamous society, theis paternal uncertainty cannot be ruled out by the father. The mother will always be cent percent sure that the child is hers because the fertilisation was internal but the father has no way of confirming if it's his child or not. Hence, a father would not want to devote a hundred percent of his resources to the caring of an offspring if there's a chance it's not his. In such settings, male deserting the females is common.
Another aspect was the reproductive fitness of the parent. A male will always be fit for reproduction. Therefore his choice to stay and provide care or to leave depends on his chances to have viable offsprings once he leaves. But that means the male has to be fairly certain of his attractiveness in order to be chosen by other females after deserting. A male aware of his scarce chances to be sexually selected over and over again might choose to stay and focus his resources on the care. On the other hand, a female has to spend much greater amount of time to be fit for reproduction again. Not to mention the energy demanding process that reproduction is. A female may rather choose to not desert the offspring. Again, understanding by the example of seahorses, since the fertilisation is external, female quickly regains reproductive fitness and is the one to desert her offsprings.
In the social context of this discussion, both the maternal and paternal care might be affected in a bi-parental care. Both the sexes may choose to care less thinking it is the other partner's job as well to provide care. In humans too, it has been seen that mothers are more altruistic for their children than the fathers as a single parent. In the bi-parental case, both the parents think that the other shares some responsibility as well which leaves the offspring with the risk of considerable less care if both parents choose to hold back hoping for the other to step forward.
The last aspect that needs to be understood after knowing about the phylogenetic, ontogenic and social aspect of parental care is the neurobiological aspect. All our actions are mediated by these chemicals inside our nervous system. They would certainly play a role in such an important aspect of life.
The principal hormone as you all must know already is oxytocin. Its level increases during mating & parturition and is responsible for increased affinity, empathy and trust through its interaction with the dopaminergic and opioid systems. It's effect is more potent in females rather than males. In males, the care is initiated by a decrease in testosterone and increase in arginine vasopressin after birth. Human studies on role of AVP are still not sufficient for any concrete conclusion. However, it has been seen in rats, AVP is implicated in paternal behaviours such as grooming, crouching over and contacting their offsprings.
Monogamous prairie voles mate for life, care for and defend their offsprings. Genetically similar but behaviourally distinct polygamous meadow voles are promiscuous and rarely engage in parenting. It has been found that the vasopressin receptor associated with paternal care has a markedly different distribution in prairie voles vs meadow voles. This difference is believed to be the reason for the contrasting paternal behaviour. Infusions of AVP facilitate pair bonding in monogamous prairie voles but not in polygamous meadow voles. Animals models have the issue of external validity but the insight allows us the chance to understand the reason behind such parental actions.
After understanding all these different aspects, let us come back to the original question. Do mothers care more than fathers? Well, mostly yes, as far as the birds and mammals are concerned, the evolutionary, social and neurobiological constraints bind them to. For the pisces phylum(fishes), yeah it's the fathers usually. For humans, we have an extremely intricate social setting which influences a lot of decisions. In most of the societies, the mothers spend most of the time with their offsprings while the father has to be out to earn, mothers do care more but not as a rule. Some fathers just might care more.
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