A Mother's Love? New UD theory explains why good insect moms risk death to save their only children

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MARCH 29, 1999--Good insect moms ferociously protect their young by fanning their wings and charging predators--but only when they must pin all their hopes on a single batch of eggs, a University of Delaware scientist reports in the new issue of the journal, Animal Behaviour.

Bug moms who lay multiple batches are far more likely to hide their young, thereby avoiding guard duty, and often "turn tail and run" from egg-munching predators, says Douglas W. Tallamy, whose latest study is expected to reach journal subscribers this week.

His theory, in print for the first time, may better explain why the vast majority of insects shun parental responsibilities, while a few fight to the death to safeguard their young. Because "semelparous" insects lay a single clutch, they staunchly defend their only shot at a genetic legacy, he says.

"Today's human moms are carrying on a long and noble tradition!" says Tallamy, a professor of entomology and applied ecology. "We tend to mistakenly think we're the pinnacle of evolution, and lowly insects couldn't possibly be capable of parenting. In fact, mothering is an ancient trait, which originated in early invertebrates, then persisted in insects when they evolved."

Tallamy cautions against explaining different human mothering styles by pointing to insect behaviors. Unlike insects, he says, people are influenced by cultural and societal factors, and their behaviors are far more complex. "It would be silly to say, `Oh, this explains why So-and-So is so protective of her only daughter.' Insects and people are completely different in that regard."

But, learning why some insects show strong maternal instincts, while most immediately abandon their eggs, "should help us appreciate this amazing process of biological evolution, and the importance of parental care," says Tallamy, whose coauthor was UD graduate student William P. Brown.

Hero moms of the bug world

As early as 1764, Swedish naturalist Adolph Modeer reported that the female European shield bug, Elasmucha grisea, seemed to challenge clutch-raiding enemies by standing fast and even tilting her body toward marauders.

Over the years, many researchers have documented similar cases of seemingly heroic efforts by insect moms, says Tallamy. The delicate, female lace bug (Gargaphia) fearlessly tackles deadly damsel bugs, thereby allowing her nymphs to flee, Tallamy reported in the January 1999 issue of Scientific American. Similarly, a praying mantis mom (Oxyophthalmellus somalicus) camouflages her nymphs and then stands sentry over them, Tallamy says. The Brazilian tortoise beetle (Acromis sparsa) perches atop her symmetrically arranged larvae, which wave feces to repel ravenous intruders.

While such parental care is relatively rare among insects, Tallamy speculates that some 5,000 species-among an estimated 25 to 30 million species on Earth-routinely guard their young.

Yet, until 1971, Tallamy says, some naysayers said reports of maternal care among insects could be explained by other behaviors, such as group feeding activities. He dismisses these arguments. "If you look at the insects I've seen," he says, "there's just no other way to interpret what they're doing."

The high cost of child-care

Bug moms have been the subject of several scientific theories. In 1975, famed Harvard University researcher Edward O. Wilson said parental care occurs when insects live in "an especially rich or rough world," Tallamy says. When food is scarce or overly abundant, or when the environment is harsh or crawling with predators, Wilson predicted, increased competition for resources may trigger the evolution of parental care.

Tallamy contends, however, that Wilson's theory is too broad. "According to Wilson, if you have very rich food resources, like dung, then everything is frantically competing for it, and parents have to guard their offspring, as well as the food they eat," he says. "But, there are 8,000 species of dung beetles and only 41 have parental care. Why don't all dung beetles care for their young?"

Another scientist, David Zeh of the University of Houston, proposed in 1989 that maternal care becomes essential whenever land-based arthropods have thin-skinned eggs or lack an ovipositor, a needle-like probe for burying eggs in hiding spots. The rule usually holds true, Tallamy says, but it doesn't explain why many insects with ovipositors and protected eggs still guard.

Still another theory, set forth by various researchers, suggests that insect moms guard their young only when the benefits exceed the costs of child-care. Building on this concept, Tallamy argues that semelparous, or one-time breeders, risk death to save their young because they have little to lose and everything to gain. "Since these bugs have put all their eggs into one basket, it pays for them to guard that basket well," he says. "Their entire genetic future is invested in a single clutch."

By comparison, an "iteroparous" insect, which breeds repeatedly, can't afford to guard eggs. Females who protect their young "can't leave to find the they need to make more eggs," Tallamy notes, so the cost of maternal care is too high for iteroparous females. A few actually dump their eggs with other females to continue eating and mating, thereby avoiding any child-care costs.

Bug dads

Ever wonder how many insect fathers pitch in with child-care? Not many, Tallamy says: "Paternal care occurs only among three families of true bugs" (Hemiptera). After all, male insects have no way of knowing whether they fathered a particular clutch, and they can reproduce indefinitely.

But, a few bug dads go to great lengths to guard eggs. Waterbug dads repeatedly drip water on their eggs, keeping them moist and healthy. And, because female Rhinocoris assassin bugs usually refuse to mate with males who aren't guarding eggs, the guys get busy caring for a clutch.

TALLAMY - http://bluehen.ags.udel.edu/ento/staff/tallamy.htm

March 29, 1999