Caring for offspring

Vertebrates have a broad spectrum of ways to care for their offspring. At one end of the spectrum are such species as herringand many other pelagic fishes that simply release their gametes into the water, where sperm meets egg, and the parents go off to do other things, providing no parental care at all. Avian brood parasites, which dump their eggs in other birds’ nests so the hosts rear the young, likewise provide no parental care (unless you count the effort of selecting the right nest to parasitize).

At the other end of the spectrum are species in which both male and female parents invest a lot of effort in caring for their young. For example, male and female of most species of penguin (except the emperor penguin male who does the incubating) and many other birds share the duties of both incubating the eggs and feeding the chicks. When the male does not incubate eggs, his care may be indirect: he usually feeds the female so she can stay on the egg-warming job longer. Among mammals, females of course nurse the offspring for a while, but in wolves and foxes, for instance, the male partners often bring food to the nursing mothers, and then both parents feed the young ones.

In between these extremes is an array of interesting arrangements, ranging from solo female care to solo male care:

–Solo female care: Many mammals leave all parental care to the females; males do no more than inject sperm to fertilize eggs. Porcupines, bears, and deer are local examples. A female salmon prepares a gravel bed for the eggs and she may guard her nest for a few days while the male she spawned with is off looking for other potential mates. Mallards and many other ducks also leave parental chores to the females.

Two species of very unusual frogs in Australian rainforest were not formally discovered by scientists until the 1970s but they were extinct before 1990. The females of these frogs brooded their eggs and tadpoles in their stomachs! Both eggs and tadpoles had ways of shutting down the digestive acids of the mother’s stomach while they were in residence. Geneticists and cell biologists are trying to resurrect a viable specimen of gastric-brooding frog from preserved tissue, but it would take more than one specimen before we could see this phenomenon in real time.

–Solo male care: This arrangement is known to occur regularly in certain birds, amphibians, and fishes. For example, jacanas are long-toed marsh birds that are customarily polyandrous; females are larger than males and defend territories that include the sub-territories of several males. The males do all the work of incubating and rearing chicks.

Spotted sandpipers have a variable mating system, sometimes monogamous but in some places they are polyandrous. A polyandrous female generally leaves her first male partner to do parental duties at his nest while she then pairs with another male, with whom she shares parental care.

The two species of Darwin’s frogs in Chile are called mouth-brooders, but actually the young are reared in the vocal sacs of the males, which ‘ingest’ the eggs and harbor them until they hatch. One of these frogs incubates the tadpoles until they can eat and then takes them to pools where he releases them to feed and grow up. The other species incubates both eggs and tadpoles; when the tadpoles become froglets, they just hop out of his mouth.

Fishes have been very inventive of ways for males to take over parental care. Stickleback males build nests and entice females to lay their eggs there, guarded by the males. Seahorse males incubate eggs in an abdominal pouch where the females inserted those eggs. The young ones emerge fully developed but extremely small. Intertidal sculpins often lay their eggs on the undersides of rocks, where they are guarded by males. Ravens know another place to look, cuing two local naturalists to finding sculpin eggs under horse-clam shells worn like helmets on the heads of males buried in the sediments.

Photo by Bob Armstrong

See Bob Armstrong’s blog for a video about this fascinating behavior.

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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.

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!

Willow ptarmigan

One nice day in September, I walked with a few friends in a subalpine area with small shrub thickets scattered throughout a meadow of no-longer-blooming wildflowers. A subdued clucking sound in the brush caught our ears and we stopped to look.

A little family of about six well-grown willow ptarmigan chicks wandered out of the brush and into the meadow, heads down, busily searching for bugs, seeds, berries, buds, and catkins. The brown female was nearby, clucking gently and keeping watch. As they all searched and sampled whatever looked good, we noticed another bird, standing on a small rock not far away.

Photo by David Bergeson

This bird was resplendent in a plumage of rich brownish red, with white legs and belly. He was overseeing the foraging efforts of his mate and chicks, alert for predators. He stood there like a proud papa wearing a cut-away coat with tails and white breeches. Very handsome! And the family was still intact, chicks shepherded by both parents.

Willow ptarmigan are unusual members of the taxonomic group of grouse: males participate in parental care until the chicks become independent in the fall. Males of all the other kinds of grouse are intent upon courting females and showing off to each other and have no role in chick-rearing.

In early spring, male willow ptarmigan begin to establish territories, which can be over ten acres in extent. Territory borders are defended vigorously. Neighboring males may march, rather peacefully, side by side along a shared border; and territories are advertised by vocalizations (the ‘rattling’ call) and flight displays. More intense competition involves charging at each other, knockdown-dragout fights, and long-distance chases, often well outside the area of contention. Females arrive a bit later than males, and they may be aggressive against other females.

Females are choosy when selecting mates. They like males with large red ‘combs’ over the eyes, large territories, and vigorous displays. Although sexually mature at age one, the yearlings are less likely than more experienced males to attract a female. A male courts a female by fanning tail and wings, strutting, stomping and bowing, and ‘waltzing’ around her with his fans facing her. If she likes what she sees, they form a pair. Most pairings are truly monogamous, with little extracurricular activity (unlike many other birds). If both members survive, the pair may stay together several years (annual survival in northern B. C. was reported to be roughly thirty to sixty percent (slightly higher for males than females).

Although the female builds the nest and does the incubation, the male guards her and the nest, sounding an alarm if a predator approaches. He tries to protect the family by distracting the predator: feigning injury, leading the predator away. An intensive defense includes loud vocalizations.

Females generally lay seven to nine eggs, sometimes as many as fourteen. During the egg-laying period, before incubation begins, females often leave the nest and go foraging. They commonly cover the eggs with grass or leaves while they are away. During three weeks of incubation, she covers the nest and eggs herself at least ninety percent of the time, seldom leaving to find food. Both male and female defend the eggs and, later, the chicks. A study in northern B. C. found that, on average, about fifty-four percent of females successfully raise at least some chicks; predation on eggs can be more serious than predation on chicks, but even so, about half the chicks that hatch don’t survive more than a few months.

Chicks can feed themselves soon after hatching and can regulate their own body temperatures quite well after a week or so. But the female broods even two-and three-week old chicks if the weather is cold and wet. Males sometimes brood, too, and if the female dies, he can rear chicks by himself. Chicks can fly at age ten to twelve days, and they move around together with the parents, often ranging beyond the territory borders. The family stays together until fall. Occasionally, parents may adopt chicks from another family.

Photo by Bob Armstrong

As winter approaches, ptarmigan molt into the white winter plumage that camouflages them on snowy backgrounds. Not quite all-white—there are some black tail feathers. Come spring and the breeding season, male plumage turns reddish brown on head and neck, leaving the body white, while female plumage gradually turns brown (with white in the wings). As summer progresses, the reddish brown of the male spreads over the chest and back, leaving the belly (and most of the wings) white. That is why these birds are called red grouse in Scotland, and this was the plumage sported by the elegant fellow we saw.

All the birds will be white when winter comes. Winter plumage is denser than summer plumage and that, together with a seasonally greater metabolic tolerance of low temperatures and the habit of burrowing into the snow blanket, helps keep them warm.