Who's Watching the Kids?
by Paul C. Sikkel
Most people associate caring for offspring with mothers. This is undoubtedly because in mammals, the class to which we belong, it is usually the female that provides all of the care--the male helps in about ten percent of all families but there are no mammals in which only the male cares for the offspring. In birds, there is generally more gender equity--males usually help females with care but rarely provide care by themselves. In amphibians and reptiles, care is often absent, but when it occurs, it is usually provided by the female. But in the largest group of vertebrates, the bony fishes, female involvement in parental care is quite rare. In most bony fishes (about 79 percent of 245 families), neither parent cares for the offspring. However, in families that do exhibit some form of parental care, care is provided only by the male in about 50 percent, by both parents in about 18 percent, and only by the female in about 32 percent.
Why are fishes different? Part of the explanation can be related to the mode of fertilization. In birds, mammals, and reptiles, fertilization is internal. Thus, females are automatically predisposed to care for offspring, especially since males have plenty of opportunity to abandon females with the offspring inside. In fact, for the fish families that provide care and also have internal fertilization, the female alone cares for the offspring in 86 percent. By contrast, in the externally fertilizing families, females alone care in only about 18 percent and males care by themselves in 67 percent. A similar pattern occurs in amphibians where male care is more common in externally fertilizing forms while female care is more common in internally fertilizing forms.
But, mode of fertilization is only part of the picture. Some elegant work by Craig Sargent at the University of Kentucky and Mart Gross at the University of Toronto has shed light on some other key differences between fishes and their tetrapod counterparts that account nicely for the differences in their patterns of parental care. In species that provide parental care, reproduction is closely associated with the substratum--the eggs usually being glued to algae, rocks, or shells. (For an example of the importance of substrate, see Surf, Turf, and Eggs.)While both parents reap equal benefits by guarding and caring for offspring (increased survivorship), they incur different costs. For males, reproductive success is a function of the number of eggs fertilized. For females, it is a function of the number of eggs produced.
Compared to bird eggs or newborn mammals, fish eggs are extremely small. In addition, even the smallest males can usually produce enough sperm to fertilize the eggs of many females. Thus, a male fish guarding a territory can pack the eggs of many females (thousands to millions of eggs) into a very small area. In fact, having eggs often seems to attract even more females. Thus, caring for offspring will not decrease and may even increase the number of eggs a male fertilizes. In contrast, caring for eggs can have a large negative effect on the number of eggs a female produces. This is because fish can continue to grow even after reaching maturity and the number of eggs a female produces (her "fertility") is directly dependent on how big she is and how much energy she can devote to egg production. This, in turn, depends on the amount of energy she takes in relative to the amount she has to devote to other tasks. Parenting not only requires expending additional energy, but also reduces the parent's opportunity to feed. This leaves little energy available for growing and making more eggs.
Thus, females are much better off if they can leave the care to males and spend their efforts on making more eggs. In some cases, many cichlids for example, it takes two parents to fight for a nesting site and defend offspring against enemies. This probably accounts for most cases in which females join males in care of offspring. In others, certain environmental factors may actually enable a male to fertilize more eggs if he abandons care, accounting for the rare incidence of female-only care in external fertilizers. But most of the time, one parent will do just fine, and the male fish tends to be better suited for the job.
(ZooGoer 26(1) 1997. Copyright 1997 Paul C. Sikkel. All rights reserved.)
