A new study has found that lizard mothers dress their children in different colour patterns, which guarantees success under the conditions that the babies will face as adults.
Researchers at the University of California, Santa Cruz, have found that female side-blotched lizards are able to induce different colour patterns in their broods in response to social cues, 'dressing' their progeny in patterns they will wear for the rest of their lives.
The mother's influence gives her young ones the patterns, which are most likely to ensure victory under the circumstances they will come across as adults.
The study, published in the online early edition of the journal Ecology Letters, shows that female side-blotched lizards give an additional dose of the hormone estradiol to their eggs in certain social circumstances. The extra hormone affects the back patterns of lizards that hatch from those eggs, creating either lengthwise stripes down their backs or bars stretching from side to side. Whether they get stripes or bars depends on the genes for other traits.
'This is the first example in which exposure to the mother's hormones changes such a fundamental aspect of appearance. Even more exciting is that the mother has different patterns at her disposal, so she can ensure a good match between back patterns and other traits that her offspring possess,' said Lesley Lancaster, a UCSC graduate student and first author of the paper.
'The females are dressing their progeny for success, because they need a different back pattern in different conditions,' Sinervo said. 'It's like fashion--she wants to make the rare, fashionable progeny that won't be caught by predators,' Coauthor Barry Sinervo, professor of ecology and evolutionary biology, said.
Lancaster used a combination of laboratory and field experiments to tease apart a complex set of interactions involving hormones, genetics, social interactions, behavioral strategies, and predators.
According to Lancaster, the maternal effect on back pattern is significant because continued existence in these lizards depends on different combinations of traits in different situation. Females use social cues to envisage the circumstances their progeny will encounter. Maternal influences like this probably occur in many species, but are very difficult to perceive, Sinervo said.
'Maternal effects are a nebulous thing to study, because you know there are genes for these traits, and it's really hard to tell the maternal effects apart from the effects of the genes,' he said.
Lancaster began by treating side-blotched lizard eggs with a collection of different hormones. That revealed an outstanding influence of estradiol on back patterns. She also tested eggs from lizards captured in the wild and found a wide range of naturally occurring estradiol concentrations in the egg yolks.
The researchers then performed experiments in breeding enclosures, each holding one male lizard and three females. The numbers of orange- and yellow-throated females in each enclosure were experimentally varied, and throat colors of the males varied randomly. Lancaster tested eggs from each female's clutch for hormone levels and recorded the color patterns of the progeny before releasing them into the wild to see how well they survived.
'This is a classic example of an interaction between genes and environmental influences on traits,' Lancaster said. 'Females provide an estradiol-rich prenatal environment to their entire clutch, but different progeny respond to it in different ways depending on which genes they inherit for throat color. So a female could make some progeny barred and others from the same clutch striped, depending on what's best for each individual.'
It is not yet clear why an 'orange' social environment triggers this hormonal tweaking of the eggs.
'The females are responding to cues that predict something about the future environment, but we're not sure yet exactly what that is,' Lancaster said. 'Orange-throated neighbors may indicate a trend in the frequency of orange-throated lizards within the population or in the overall population density.'
According to Sinervo, the density of the lizard population is probably a critical factor. At high densities, the aggressive orange-throated males are so busy fighting with other lizards that they are especially vulnerable to predators. As a result, the predators are likely to focus on them, giving a survival advantage to lizards with different patterns.
After Lancaster released the lizards from her breeding experiments into the wild, she found that the ones whose mothers had tweaked their back patterns had an obvious survival advantage. The highest survival rates were seen in the induced progeny types--yellow-throated lizards of both sexes with barred backs, orange-throated males with striped backs, and blue-throated females with striped backs.
'The frequencies of yellow and orange were both increasing in the wild that year, so the females exposed to orange neighbors and yellow sires in the experiments had the most fit progeny,' Lancaster said. 'But under different environments, maybe the progeny that weren't tweaked would have the highest fitness. If these same estradiol-induced types had the highest survival rates in all years, there would be no need for the females to respond to their social environment--they would just always give their eggs estradiol.'
To understand these complex interactions more fully, the researchers plan to study the hormones and their effects in the field over several generations of lizards.