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male guppies have survival advantage in the wild
Life Sciences Editor
photo to enlarge
courtesy Kimberly Hughes
are small tropical fish native to Trinidad and other
areas of the Caribbean. They
are characterized by sexual dimorphism in size and
color patterns. The color pattern variation of the
males (left and right columns) is mostly genetic variation,
not environmental. New data indicates that rare-colored
males not only attract females (center column), they
are also more likely to survive predation.
— Any owner of a freshwater aquarium likely has had guppies (Poecilia
reticulata), those small brightly colored fish with a propensity for
breeding. Now guppy populations manipulated in natural habitats in Trinidad
have taught researchers an evolutionary lesson on the survival of a
rare genetic trait.
Reporting in the June 1 issue of the journal Nature, scientists from
six institutions detail how male guppies with the most colorful –
and most rare – patterns are more likely than their more commonly
colored counterparts to survive in the wild.
“This study provides very solid support for frequency-dependent
survival,” said principal investigator Kimberly A. Hughes, an animal biologist at the University of Illinois at Urbana-Champaign. “We found that
rare color patterns of these guppies had a highly significant survival
In evolutionary terms, frequency-dependent survival means that individuals
with rare gene variants have a survival advantage relative to common
variants, simply as a function of being rare. This process is important
because it leads to the maintenance of many different variants (polymorphism)
in the same population.
The same process could be important in the maintenance of genetic variants
in humans, said Hughes, who also is a member of the Institute
for Genomic Biology at Illinois.
For example, she said, it has been hypothesized that genes involved
in pathogen resistance (the Human Leukocyte Antigen or HLA genes) are
highly polymorphic because pathogens are most successful at attacking
individuals with common variants, and individuals with rare variants
have higher survival.
However, the theory is difficult to test, and, in general, frequency-dependent
survival has been difficult to document as an important process in nature,
The guppy system provided a way to test whether this kind of selection
could really promote polymorphism in a natural setting, because guppies
are highly polymorphic for a visible trait and they are easy to work
with in field experiments.
Researchers conducted 34 separate manipulations across 19 replicate
pools in three streams over four years. They collected guppies from
two tributaries of the Quare River and the main branch of the Mausica
River, sorted the males and females, and then returned them to the streams.
“We had two different color patterns at a particular site,”
Hughes said. “In half the replicates we made pattern 1 rare and
pattern 2 common, and in the other replicates we made pattern 2 rare
and pattern 1 common. This allowed us to determine that it was rarity
itself, and not any specific aspect of the color pattern that had the
biggest effect on survival. No matter which pattern was rare or common,
the rare type had higher survival.”
After 15 or 17 days, depending on location, the researchers again sought
and captured all adult-sized guppies from the streams. The rare males
had higher survival at all three sites. Overall, 84 percent of the rare-type
males survived to the end of the experiment, while only 69 percent of
the common-type males survived.
The most important source of mortality in wild guppies is predation
by larger fish species that live in the same streams. “It’s
possible that guppy predators, which are known to hunt visually, may
be more focused on common color patterns,” Hughes said.
“Predators can form ‘search images’ of the most common
prey types, and can be less efficient at locating and capturing prey
that look different from the norm,” she said. “These predators
have limited attention. Perhaps this generates a frequency-dependent
predation pattern that by its very operation acts to maintain polymorphism.”
An alternative theory is that male guppies altered their own behavior
in response to the manipulated changes in their common vs. rare numbers,
and that the changed behaviors affected predation, the authors wrote.
They suggest new experiments to study behaviors in both predators and
prey to determine which theory is at work.
In earlier studies, Hughes and colleagues had shown that female guppies
prefer to mate with males with color patterns novel to the females.
It could be, the authors surmise in the Nature paper, that females might
prefer the rare males because mating with them lowers their own risk
The six co-authors with Hughes were: Robert Olendorf, a postdoctoral
researcher in the School of
Integrative Biology at Illinois; F. Helen Rodd and David Punzalan,
department of zoology at the University of Toronto; Anne E. Houde, department
of biology at Lake Forest College in Illinois; Carla Hurt of the Smithsonian
Tropical Research Institute, Naos Marine Laboratory, Panama City, Panama;
and David N. Reznick, department of biology, University of California
The work was supported by grants from the National Science Foundation
to Hughes, Houde and Reznick, and by a grant from the Natural Sciences
and Engineering Research Council of Canada to Rodd. During 1996, Hughes
also was supported by a National Research Service Fellowship from the
National Institutes of Health, and Rodd by a grant from the Center for
Population Biology at the University of California at Davis.