Parental care by males, part 2

This essay will consider male parental care in birds and mammals. Both birds and mammals evolved from reptiles, and some ancient reptiles did have parental care by at least one parent, but modern reptiles have no record of male parental care, so they will be ignored here. As is true for fishes and amphibians, the factors that govern the evolution of patterns of parental care are no doubt several and still subject to debate and future research.

Biparental care is the usual thing among birds: both parents tend the young in over ninety percent of bird species. Females often do the incubating of eggs, but her male may feed her while she does so and the males generally help feed the chicks. This is the case for American dippers, for instance; as one of my field techs said, during our intensive study of this species: there are no dead-beat dads! In fact, we even know of one hard-working dad who raised at least a few of his chicks by himself, after his mate disappeared. The emperor penguin male goes a step further: he incubates a single egg on his webbed feet while his mate goes off to sea and feed; then they both tend the chick.

In some taxonomic groups of birds, including hummingbirds and grouse, females generally do all the work while the males run off to find more females. But even in these groups, there are unusual species in which both parents provide parental care; the willow ptarmigan is a local example.

Still more unusual are avian species in which males both incubate and tend chicks by themselves. Here a few examples. Spotted sandpiper females often lay one clutch of eggs and leave it to the male to do the incubation and guarding while she proceeds to lay another clutch (with the same or a different male) that she incubates and tends; this is a pattern found in several shorebirds.

Spotted sandpiper nest–is this tended by the dad? Photo by Katherine Hocker

In two of the species of kiwi in New Zealand, the Australian emu, and several other species, males incubate and tend the chicks alone. The cassowaries of Australian and New Guinean rainforest also have hard-working males, who incubate the eggs for weeks and then tend the chicks for months. They are fierce defenders of their little families: One day in the Australian rainforest I encountered a cassowary family; we were all looking for fallen fruits. Imagine looking up from the forest floor and seeing a very large bird, almost as tall as you and with claws that could rip you open, glaring at you from just a few short yards away. You can bet I apologized for my presence most abjectly and discretely retreated rather quickly!

What about the mammals? Virtually by definition, females are the ones that feed the infants, and lactation is considered to be the single most expensive thing a female mammal ever does. Dependence of the infants on mother’s milk means that females are always involved in parental care, so uniparental care by males is not an option. Biparental care is not common, but males are reported to be closely involved with parental care in about five percent of all mammal species. The best known cases include carnivores and primates, but regular male care occurs in other groups too. Here are some examples:

Among the carnivores, the males of foxes and wolves regularly bring food to their young. Asian raccoon dog males participate in all forms of parental care except lactation, and also tend the female during the birth process. Male members of packs of African wild dogs bring food to lactating mothers and young pups.

Male baboons and macaques carry babies around, which may help protect the infants from predators or intruding strangers. However, this situation is more complex than that, because the male may obtain direct benefits too: a male with a baby in his arms suffers less aggression from other males and may also gain favor with the infant’s mother. And if there is a fight between males, the infants are in great danger. In some small New World primates called tamarins, including the cotton-top tamarin, males regularly carry and care for babies. Males of the endangered pied tamarin reportedly do most of the parental care except for lactation.

Wild horses and zebras live in groups, often a male’s harem of females plus foals. Males defend their foals and females from predators.

It’s a rare herbivore that helps feed the young ones, but male beaver do: they regularly help build winter caches of branches on which the whole family, but especially the still-growing young ones, feed; they also help maintain dams that make the pools that protect the lodge and facilitate transport of branches. They stay with the rest of the family in the lodge over the winter, interacting and providing body warmth. Among the smaller rodents, males of the California deer mouse reportedly brood the young, keeping them warm until they can regulate their own body temperature. Prairie vole males cache food, brood and groom the babies, and even retrieve them if they wander out of the nest.

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Parental care by males

Some months ago I was captivated by a grunt sculpin at the NOAA lab, and I learned that males of this species commonly guard their mate’s eggs. When it is time for the eggs to hatch, the parent takes up the eggs in his mouth and spits them out into the water column, where the egg membrane breaks and the hatchlings are freed. Then I saw a video of a male three-spined stickleback guarding his nest from all comers (see naturebob.com); the accompanying photo is taken from that video. And all that prompted me to think more about paternal care, particularly among vertebrates. In some species either parent or both parents may be involved with parental care, but here I am concerned chiefly with vertebrate species in which males are the sole caregivers (it happens among the invertebrates too, but that’s another story). Strictly paternal care is generally less common than strictly maternal care, except in fishes, among which fatherly care is more common.

The more I thought about it, and the more I read on the subject, the more it seemed necessary to divide the intended essay into two parts, so as to cover some of the fascinating variation that is found concerning how males care for their young. In general, solo-male parental care among fishes is considered to be best developed in freshwater and small-bodied species; among both fishes and amphibians, it is most common in species with external fertilization of the eggs. Although the evolution of fatherly care has been much discussed and is still debated, here are some of the exotic parental things that male fishes and amphibians do.

The fishes offer the most fantastic array of different ways to take care of eggs and babies. The males of several very different, unrelated species customarily build nests; here are just a few examples. A male stickleback (the three-spined Gasterosteus, the nine-spined Spinachia) builds a nest and invites females to lay their eggs there. He then guards the nest against other males and potential predators, also fanning the eggs with his fins to provide a good flow of oxygen. Males of freshwater sunfish (Lepomis) scoop out shallow nests in the bottoms of lakes and ponds. They invite females to lay eggs and defend the eggs until they hatch. Again, one male may have eggs of several females in his nest (and females may mate with more than one male, too). Stream-dwelling johnny darters deposit eggs on the undersides of flat rocks. Males defend those clutches of eggs, and they also maintain sanitary conditions by removing any eggs that get infected by fungi. A male Siamese fighting fish (Betta splendens) makes a bubble nest, retrieving any eggs or hatchlings that fall out and repairing the nest as needed.

Males of sea horses (Hippocampus) take things one step farther: females deposit their eggs in a pouch on the male’s belly, where fertilization occurs. The males incubate the eggs and brood the hatchlings in that pouch—a sort of movable nest. The male’s pouch provides not only a controlled environment, but also oxygen, hormones, calcium, and lipids full of energy (in addition to the yolk of the egg), and waste management. Although apparently each brood of eggs is provided by one female, when one brood matures and leaves the pouch, the male can then mate with another female. In the related pipefishes and sea dragons, males carry the eggs either in a pouch, like the sea horses, or under his long tail.

The males of at least one species of Kurtus, a fish of slow-moving fresh and brackish waters, carry clumps of eggs on a vascularized hook on the forehead. Apparently it is still unknown if the blood supply serves to deliver nutrients to the eggs. Still other fishes brood eggs and hatchlings in the mouth of the male parents (some tilapias, a sea catfish Ariopsis, and a particular species of Betta). How offspring are distinguished from prey—an important distinction!—is unclear.

Possibly less complicated is the behavior of a tropical fish sometimes called the splash tetra (Copeina arnoldi). Male and female leap together out of the water and spawn, very quickly, on an overhanging leaf, before dropping back into the water. The male then spends a few days splashing the eggs to keep them wet and oxygenated until they hatch, and the hatchlings fall into the water.

Among the amphibians, both biparental and uniparental care occur; solo-male parental care is known from several species. For example, in the giant salamander known as the hellbender (Cryptobranchus), the male excavates a shallow scoop in the mud, where he fertilizes the eggs of each female that chooses to use his nest. He then tends the accumulated eggs (those that survive cannibalism), moving about the nest to circulate the water and keep up the oxygen supplies for the eggs. His incubation time lasts for several weeks, sometimes months.

Some male frogs build small mating pools in which eggs are laid. Males of other species carry eggs on their bodies. In one of the tropical poison-dart frogs (Phyllobates bicolor), the males tote their tadpoles on their backs, carrying them from puddle to puddle (related frogs apparently have biparental care). Male European midwife toads (Alytes obstetricans) carry a bunch of eggs on their rear ends.

Males of Darwin’s frog (Rhinoderma darwinii) in southern South America are dedicated parents. Each male guards a clutch of eggs for many days. When they are nearly ready to hatch, he takes them up with his tongue and stuffs them into his vocal sacs, which extend down his back and belly. There they hatch and grow, living off their yolk and secretions from the vocal sacs, until they metamorphose into froglets and hop out of their father’s mouth. That would be a sight to behold!

Sticklebacks

This male stickleback is fanning his nest. Photo by Bob Armstrong

Last summer, I did a little fish trapping (with a permit) in some of the ponds in the Dredge Lake area. The most common fish in my traps were three-spined sticklebacks. These tiny fish-lets get no attention from most people, because they have no ‘value’ for sport or commercial fishing, but they are exceedingly interesting biologically and very important scientifically. The research literature on this species is vast, so this short essay provides only a sampling.

The three-spined stickleback is found in both marine and fresh waters; the marine form is anadromous, breeding in fresh water but returning to the sea. These fish are very small, seldom as much as three inches long, at least in fresh water. The males acquire bright colors in the breeding season, blue eyes and usually a red belly and sides. Intensity of the red color is an indication of health, and intensely red males are commonly preferred by females. In some populations, however, males have black breeding colors, or part black and part red; black males may not be preferred by females, but they are good fathers. Males build tubular nests of plant debris and invite females to lay their eggs there. The males then care for the several hundred eggs, fanning them to increase oxygen availability, guarding them, and so on, for about a week. It seems that males with the most intense breeding colors and males who court too long or too vigorously don’t have enough energy left to be good fathers—they invest more in getting eggs than in taking care of them once they have them.

They feed on all sorts of small aquatic organisms and, in doing so, can affect the composition and productivity of the entire lake ecosystem. In some lakes, there are benthic populations that exploit food that’s found near the bottom and limnetic populations that exploit food such as zooplankton that’s found in open water. These populations are genetically distinct from each other and differ morphologically in the position of the mouth and eyes, for instance.

Sticklebacks are an important source of food for many animals. Their many predators include fish (including their own species), birds, otters, and dragonfly larvae. To some degree, sticklebacks can protect themselves physically. The marine form has well-developed lateral plates and erectable back and pelvic spines. The spines help protect the fish, partly because the erected spines make the fish larger, in effect, so small-mouthed predators are less likely to attack, and partly because predators are more likely to release them after being stabbed by the spines. Otters are reported to eat the fish but leave a little heap of spines on the ground. Bird predators may be able to manipulate their prey so that the spines are made ineffective (But very young sticklebacks don’t have these spines; they develop as the fish grows—starting to appear when the young fish reaches about one centimeter in length.) The bony lateral plates provide another form of predator protection, rather like armor. These protective devices seem to work best against predatory fishes and apparently do not help much against dragonflies.

Marine sticklebacks have repeatedly invaded fresh water systems all up and down the coasts. When they do so, these protective devices are reduced or lost, in nearly every freshwater system these fish inhabit. Without the lateral plates, the fish make quicker starts to escape from an approaching predator, such as a loon or merganser. So there is a tradeoff between armored protection and ability to escape by fleeing. Pelvic spines are often much reduced in lakes that lack predatory fishes, particularly when the lake water is low in calcium (it takes calcium to build bones and spines). If predatory rainbow trout are introduced to these lakes, populations of these poorly protected sticklebacks decline.

Most predators are not capable of wiping out whole populations of sticklebacks, but just winnow out the slow or poorly protected ones or those that live in a particular habitat. However, one predator seems to be responsible for driving several stickleback populations to extinction. This rapacious predator is the northern pike, which was foolishly introduced to Alaskan lakes in the 1990s. In some lakes, no sticklebacks are left. A few other lakes, with deeper water, still have sticklebacks, mostly the limnetic forms, largely because the deep water gives them some space to escape from the pike.

These little ‘no-account’ fish are scientifically important for at least two reasons. One is the speed at which freshwater adaptations are acquired. The classical view of evolution is that it is very slow, requiring thousands of years. Yes, some evolutionary changes have taken a long time. But the sticklebacks provide one example (of an increasing number) of very rapid evolution, with morphological changes occurring in just a few years. For example, Middleton Island was uplifted by the 1964 earthquake, creating new beaches and ponds. Sticklebacks from the sea have colonized those freshwater ponds and lost their pelvic spines and lateral plates. Sticklebacks recolonized one Alaskan lake in the 1980s. In 1990, most of the fish still had lots of lateral plates. But by 2001 (less than twenty years after arriving and only eleven years after the first sampling), most of the fish had few lateral plates.

Interestingly, the genes involved in these changes are not always the same in every freshwater system, indicating that there are multiple ways of achieving the same adaptation. In fact, different populations of sticklebacks in different freshwater systems are often quite different genetically, and probably really represent different, very closely related, species. The dynamic evolutionary changes in these populations offer scientists wonderful opportunities to study both pattern and process of evolution and species formation.

Another reason these fish are scientifically important is that the entire genome has been sequenced, so we know the precise composition of their DNA. It turns out that about 70% of their genes are just like ours. This is likely to have medical implications: If we learn which genes control what features in the fish, there is then a possibility that some of those genes also control similar features in humans. For example, if certain genes control the loss of bony plates and spines in the fish, could those genes perhaps be involved in human bone deterioration (as in osteoporosis)?