Think about the difference between birds and mammals. You’re a mammal, so you might be tempted to use yourself as an example mammal, but humans are actually kind of bad examples of mammals, so perhaps we’ll use a well studied hoofed animal instead. Elk. Think about the difference between a typical bird, say, your favorite common songbird, and Elk.
And in particular think about the difference with respect to having offspring.
Both Elk and songbirds have the same basic method of inseminating eggs, which is one of the steps in reproduction (but not by any means the first!), as both use internal fertilization. But songbirds and Elk are different in how the offspring is nurtured in two important ways. Songbirds grow the offspring internally for only a very short while, and then pop out an egg, which is then cared for over a significant period of time until it hatches. “Hatching” in Elk takes the form of birth, of an offspring that is probably more developed in most ways than the songbird at hatching, and the Elk offspring was nurtured inside the female for that entire time.
That alone is a huge difference because in Elk, while the male parent can contribute sperm to the process of producing offspring, he can’t contribute much else after that. All the nurturing has to come from mom. In songbirds, by contrast, both the male and female can contribute equally to the very important job of incubating and protecting the egg. The female has put a lot more into the initial formation of the egg, which is quite large compared to her body size, but after that it can be cared for by both parents.
After hatching, the same contrast pertains. In the Elk, the offspring nurses from its mother. Birds do not lactate, so in songbirds, there is an opportunity for males to invest in feeding the offspring. If you asked male Elk what they would do under these circumstances, they’d probably say “Opportunity? What’s that? I’m busy growing my antlers, leave me alone!” But the investment in care of offspring by both parents significantly increases bird survival. Since both eggs and chicks are nest-bound and highly vulnerable, while one parent is off in search of food, the nest can be robbed (probably by some owl). This means that males who are predisposed by their genes to not invest in the chick have far, far fewer chicks than those who are. The most like strategy of investment by bird parents to evolve is probably both parents caring for the offspring, probably taking turns at the nest, and bringing food back to the chick.
Prior to insemination, things are different as well. Elk and bird females both have more to lose than their respective species’ male by making bad mating decisions, so both are likely to be a bit choosy in their mates if possible (not to say that the males won’t be a little choosy as well). But since males are useless after insemination, female Elk might do better choosing on the basis of some perceived or estimated genetic quality, while female songbirds might be looking for males that are less likely to abandon the young or eggs in the nest.
Thus, the two types of animals have very different courtship patterns.
In Elk and other deer, to varying degrees and depending on species, males grow antlers that seem to demonstrate their qualities as potential mates. It has been noted that the amount of calcium that must be obtained from the environment through feeding to produce a nice rack of antlers is very close to the amount of calcium needed to produce a baby deer. So, perhaps the female Elk sees a male with a nice rack as an adequate (or better) source of genes for her future daughter; If the male can obtain sufficient calcium, a rare-ish element in the diet, so might her daughter.
It has been shown that courtship is negatively correlated with abandonment. In various studies (which we’ll look into in more detail at a later time) birds that court longer and “better” seem to be less likely to walk away … well, fly away … during that long period of intense work of caring for offspring. This may be part of the reason we see birds involved in long and elaborate courtship. Even in species where the total amount of time required to raise a nest full of little ones is very short, a significant percentage of time will be spent on this pre-mating period.
The Elk-songbird comparison clearly exposes a difference between mammals and birds, and it exposes differences in the symmetries between male and female parental investment. Before insemination, both species spend time in a courtship and choice phase, but in Elk males invest heavily in their own bodies, both in displaying expensive antlers and in tournament fighting among males, which apparently impresses the ladies. In many songbirds, courtship may be more symmetrical with both sexes displaying potential quality (another topic will go into more at a later time) with the intent of showing the ability to commit, as it were, rather than just “good genes.”
After insemination, in Elk, it is best that the male just leave. All he’s going to do is to eat some of the available food and make a general nuisance of himself. In songbirds, however, the male that leaves as truly a cad, and his presence significantly enhances the chance of survival of offspring. From the time of hatching the eggs to the fledgling of the young (and a bit beyond) the male and female spend roughly equivalent amounts of time on offspring care.
In some birds, males and females look similar, in some quite different. In some birds (and we’re looking beyond songbirds here) the females spend most of their time on the nest while the male brings food. Some birds mate monogamously for a season, others for life, some are not overtly monogamous at all, while others appear to be monogamous but when we test for paternity, it turns out that the appearance of monogamy is not entirely accurate. In both mammals and birds, the signals individuals put out for the opposite sex to evaluate (behaviors, physical features such as antlers) tend to be honest signals of individual quality; Nobody seems to be faking.
And all of these things and other aspects of bird physiology and behavior can be understood in relation to the nature of investment in offspring by parents. All wrapped up together we call this Parental Investment Theory and it explains a lot.