Arachnid care of eggs is a tactic for seduction and survival

A study performed at the USP Biosciences Institute shows that single males of the species Iporangaia pustulosa live longer and have more chances to copulate with young females.

By Karina Toledo
 
Agência FAPESP – Although paternal presence in the care of young is common among humans, it continues to be a rare phenomenon in nature and only occurs when the situation is advantageous to both the males and females.
 
Among arachnids of the species Iporangaia pustulosa, care of the eggs both increases the survival rate of males and makes them irresistible to females, as shown in the doctoral research of Gustavo Requena, advisee of Glauco Machado at the Universidade de São Paulo Biosciences Institute (IB-USP). The resultas were published at PLoS One.
 
“Approximately half of the males do not manage to copulate during their lifetime. However, those who do become more attractive to the females after the first eggs are laid,” Requena told Agência FAPESP.
 
For this Opiliones species, a relative of the spider commonly known as daddy longlegs, care of the eggs requires at least a month without eating.
 
“Many males cannot handle the task. If they leave the eggs for more than one or two days to find food, the chance of the nest being attacked by predators is great,” said Requena.
 
Somehow, certain males manage to convince a first female of their capability and project a positive father image when they are observed caring for the eggs of other females. In some cases, four or five females will line up for such males.
 
Copulation lasts for only two minutes and occurs before the male abandons the first spawn. The female then lays her eggs together with those of the previous female and continues with her life—without regrets or jealousy.
 
“The behavior is advantageous to the females because they can return to feeding soon after laying their eggs. Within a few days, they are ready to lay more,” explained Requena.
 
In contrast, some males spend four months without eating until all the young have hatched. Although they grow weak in the process, the study showed that their mortality rate is lower than that of those males that spend their entire lives looking for food.
 
“The main predators of these Opiliones are spider species that ambush their prey. The more time an opilione remains protecting his eggs, the fewer chances he has of encountering a predatory spider during his lifetime. Although paternal caretaking implies the cost of going without food for these males, there is a great benefit in terms of survival,” affirmed Requena.
 
Paternity testing 

To arrive at these conclusions, the scientists observed hundreds of Opiliones inside the Intervales State Park, a region of Atlantic Rainforest in southern São Paulo State. There is a large concentration of I. pustulosa in the region, one of the few Opiliones species—there are over 6,000 in all—that adopt paternal egg caretaking.
 
“They resemble spiders at first sight but do not have any venom, nor can they make webs. Furthermore, their bodies are not divided like spider bodies are,” said Requena.
 
Small in comparison to its legs, the body of I. pustulosa is approximately 1.5 cm in length. These arachnids are mostly scavengers and themselves can become a meal for amphibians, ants, crickets, wasps and even other Opiliones species. To defend themselves, these arachnids secrete a yellow, bad-smelling substance, explaining its popular name: the smelly spider.
 
“We observed the arachnids for a year to evaluate the male survival rates. We painted their feet, as it is difficult to distinguish individuals,” told Requena.
 
Opiliones require approximately a year to reach adulthood and can live for an additional two years. “The mortality rate is very high when they are young but is quite low after they become adults. As those who care for the eggs live longer, evolution must also have favored their paternal behavior,” he said.
 
The researchers still have not been able to explain why approximately half of the males never manage to copulate. However, the fact is that the males are also selective with regard to their partners. During the study, the USP group observed many situations in which a female came to the nest wanting to copulate and was refused by the male.
 
“We have some hypotheses to explain this behavior. One would be the fact that, after many couplings, the male needs some time to restore his stock of sperm,” told Requena.
 
Another possibility, he continued, would be the existence of some mechanism by which the male identifies females that are already carrying sperm from another male.
 
According to Requena, one of the study’s objectives was to apply DNA paternity testing to discover whether male I. pustulosa may also at times care for eggs fertilized by other males.
 
“Unfortunately the methodology available today, which is the same as that used in human DNA testing, does not work for Opiliones. We will have to develop another method,” he said.