War of the sexes, 5

The parental dilemma

There is a fascinating aspect of avian parenting. Birds perhaps more than any other group of animals show how the environment plays a key role in driving the separate interests of males and females.

Jacanas inhabit tropical pools and lakes and can pick their way across floating vegetation, spreading their weight on their very long toes—hence the alternative name of ‘lily-trotters’. Bearing a vermilion shield of their foreheads, American jacanas have reddish-brown plumage with brilliant golden-green pinions which are conspicuous in flight. But it is their breeding arrangements that make these birds especially interesting: the females practice a particularly extreme form of polyandry, with the males undertaking all the duties normally performed by their partners.

A female jacana enjoys the services of several males, which do all the work of building the floating nests, incubating the eggs for nearly a month until they hatch and then caring for the chicks for a further two months. They make devoted fathers and when danger threatens any of the brood, the chicks either shelter beneath his wings or, on a call from him, sink under the water with only the tip of their bill showing so that they can breath.

The females are 75 per cent larger than their mates, do all the courting and scrap among themselves for territories. The most successful fighters are the heaviest with the biggest, reddest wattles. The shields display a record of their owner’s fighting history, as the scars of old injuries are yellow. Such fierce females may manage to defend a territory with as many as six males. Within her area, each male has his own nest located in his own patch of vegetation, but as he is relatively puny, he is unable to drive off the trespassing females. When there is a female intruder, he screams for his own mate to defend his share of her freehold. In the event of a new hen taking over, the males make a feeble attempt to expel her, but within a few hours they have accepted the inevitable and mate with her. Such takeovers are bad news for the vanquished females, because the victor will set about destroying the eggs and methodically hunting down the chicks of her predecessor so that she can immediately employ the males to look after her own eggs.

In effect, a female jacana acts like a fierce egg factory with no constraint on her production line, completing a clutch of four every ten days or so. By contrast, the reproductive potential of each of her partners is severely limited because, once he has received a clutch of eggs, the male is tied up with parental responsibilities for the best part of three months. The female’s sexual potential is limited only by the number of males she can exploit and retain in the face of serious competition from other hens.

Apart from laying eggs, hen jacanas behave just like the strutting cocks of other species—they are big, aggressive, passionate and less choosy than most females about their sexual partners. On the other hand, their mates act like traditional hens—the caring, gentler sex. This is such a reversal of the normal situation that it raises the question, what are the special circumstances which favour the evolution of polyandry on such a scale?

The answer may be found in the rich environment which jacanas inhabit. With no shortage of moisture and heated by the tropical sun, the swamps are among the most productive places on the planet. Such is their immense fertility that it has been estimated that the calorific value of the food available on 1 square metre (10 square feet) of ground is equivalent to two dozen chocolate bars. In fact, for the jacanas, these places are like open bird tables groaning with goodies. So easy are the pickings that, unlike most female birds, hen jacanas have evolved into ‘battery hens’, churning out egg after egg with little physiological stress. They have therefore seized the reproductive initiative, pursuing a strategy of continuous egg production while coercing a coterie of males into incubating the eggs and guarding the chicks…

Jacanas are not the only birds to indulge in polyandry. Several kinds of shore birds practice it on the Arctic or sub-Arctic breeding grounds.
 
Single mothers

Examples of polyandry are few and far between for the simple reason that the environment rarely gives females such an easy ride as it does the jacanas and Arctic wading birds. For most birds, finding enough extra food to manufacture eggs packed with nutrients is an arduous business. World wide, hen birds are constrained in the number of eggs they can lay in a season and so they, as the limited resources, are fought over as the males—which are free to copulate with as many partners as they can secure. In most wading birds, wildfowl and members of the pheasant and goose family, all parental duties are sifted firmly on the females. Their mates play no part in incubation or protecting their vulnerable chicks after they have emerged from the eggs.

In all of these cases, the young are hatched in a relatively advanced state and can run around and forage for themselves. The parent which defects—whether it is the cock or the hen in the polygamous species—is therefore not needed as provider of food, which makes his or her desertion that much easier.

But there can be intense rivalry between single mothers and lone fathers. Barrow’s golden eye, for example, is a tough little diving duck and one population breeds on Lake Myvatn in Iceland. The females nest alongside fast flowing rivers leading out of the lake, and when the ducklings hatch the mother leads them on a perilous journey upstream to the best feeding areas. The journey is dangerous because they literally risk their lives getting there.

If they pass a male whose female is late hatching and still sitting on eggs, he mercilessly beats them to death, because he doesn’t want any ducklings competing with his own offspring. If they survive the hurdle and reach the feeding area, other females already there will also attack and kill newcomers to protect the best sources of food for their own broods. In July each year, the upper reaches of Lake Myvatn can be a scene of carnage, with hundreds of dead ducklings—the result of mothers furiously fighting for the interests of their own broods at the expense of others.
 
Mammals: natural-born mothers

In just over 90 per cent of birds, monogamy prevails. This reflects the near impossibility of females producing an unlimited supply of eggs in most habitats, and the fact that male birds are able to make a significant contribution to the survival of the chicks. But there is one major group of creatures in which this is not so—the mammals. Among these equally hot-blooded, very active animals, monogamy is confined to a mere 5 per cent; in the rest the males have completely opted out of parenting…

Dwarf antelopes—such as the klipspringers and dik-diks of southern and eastern Africa—are unusual among hoofed animals in that they go around in pairs. They frequent clustered bush and thickly vegetated forest where nourishing herbage of the kind that they like is widely scattered. It therefore pays these animals to be territorial so that they can acquire an intimate knowledge of the places where their food occurs.

The buck, which is often slightly smaller than his mate, ensures success in the paternity stakes by commandeering an area of desirable bush and then behaving as a constant consort to his female, never moving more than a few paces from her aside for fear of losing sight of her in the dense vegetation—and possibly losing his sexual monopoly of her as well. It has been recorded that a pair of klipspringers spend their entire adult lives literally within 5 metres (16 feet) of each other. When the fawns arrive, the female cares for them, though the father is always nearby, preoccupied with guarding the mother. Such long bonds lessen the competition between males and so preclude the need for large, aggressive bucks of the kind found in deer and some larger antelopes.

A similar situation prevails in gibbons. These singing apes from South-East Asia appear to live like happily married couples together with their immature children. However, on close inspection, it can be seen that a male gibbon is not so much a caring father as the guardian of the adult female with whom he has chosen to breed. He is also a valiant defender of the swathe of jungle through which she and their joint offspring need to forage for tender leaves and ripe fruit. For a male gibbon, monogamy pays reproductive dividends; by keeping a close track of his ‘wife’ in the complex, cluttered canopy of the rain-forest, he can be sure of fathering her offspring. Unlikely among apes, male gibbons are virtually indistinguishable from their mates—a characteristic that reflects the low level of competition for females.

Only the male siamang—the largest of the gibbons, from the Malay Peninsula of Sumatra—shows a high level of paternal interest, taking over the daily care of his infant when it is about a year old and continuing to look after it closely for the next two years.