Scientists describe paternal behavior unique to spiders in species that lives in Brazil

Study shows that males of the species Manogea porracea protect spiderlings and eggs against predators even if they cannot be sure of their paternity.

By André Julião  |  Agência FAPESP – Male spiders of some species cannibalize the offspring of rivals or simply abandon progeny if in doubt about their paternity. In the case of Manogea porracea, however, it is common practice for males to care for any offspring or egg sacs found in webs that are characteristic of the species.

This paternal care behavior, which is unique among solitary spider species, was first observed in M. porracea in 2016. Researchers have now shown that a male of the species may protect offspring and eggs against predators without knowing whether he is the father.

The findings are published in the journal Ethology by scientists at the State University of Minas Gerais (UEMG), the Federal University of Uberlândia (UFU) and the University of Campinas (UNICAMP) in Brazil.

“The male makes a web above the web made by the female while she’s sexually immature. When she becomes fertile, they mate. The male may remain in the web, protecting the female against competitors, or leave to look for another mate, in which case other males may mate with the female he left behind. If the female dies for some reason, her male mate will descend to her web and care for her offspring. In our experiment, males cared for offspring even when we put them in a web with the progeny of others,” said Rafael Rios Moura, a professor at UEMG’s Department of Biological Sciences in Ituiutaba.

Moura conducted the study while he was a postdoctoral intern in animal biology at UNICAMP’s Institute of Biology with a scholarship from FAPESP and supervision by Professor João Vasconcellos Neto, penultimate author of the published article.

The researchers have not been able to determine why many females of the species disappear after laying their eggs, but they believe the reason may be that they attract more predators than males because they put on fat as lipids accumulate during the reproductive period.

“On the other hand, the male of this species spins a web above the female’s and can continue to feed during adulthood. This behavior is also rare in spiders, potentially extending the lifespans of M. porracea males, so that they have opportunities to care for offspring. Few survive in the absence of their parents, so paternal care is an important evolutionary innovation for the species,” said Moura, who leads UEMG’s Center for Extension and Research in Ecology and Evolution (NEPEE) and runs a science diffusion project on social media called “Rios de Ciência”.

Paternal behavior of this kind in spiders had only been described before in an African species (Stegodyphus dumicola), which lives in large community webs where several males and females care both for their own eggs and offspring and those of others (alloparenting).

In the study that described paternal care for the first time, published in 2017, Moura and his team showed that 68% of egg sacs were cared for by males at the end of the reproductive season and that a significantly larger number of hatchlings were born from eggs protected by males than from eggs with none to care for them.

Parents are the ones who raise children

M. porracea is found throughout the Neotropics, from Panama to Argentina, in very different ecosystems. The population studied by the Brazilian research group lives in eucalyptus plantations surrounded by native areas of Cerrado (Brazilian savanna) on a property called Fazenda Nova Monte Carmelo in Estrela do Sul, Minas Gerais. They build webs near the ground, using branches and the layer of leaves and other organic material on the forest floor (litterfall) as a support.

The researchers observed 40 males, 57 females and 87 egg sacs in the laboratory. All males were found in the wild, in webs with egg sacs. In the laboratory, the females were placed in transparent boxes with eucalyptus branches and cotton balls to maintain the humidity for at least 24 hours so that they could spin webs. Each female was then removed and replaced by a male and an egg sac that was not necessarily his offspring.

To find out whether the males protected their adoptive spiderlings from predators, the researchers introduced spiders of two species (Faiditus caudatus and Argyrodes elevatus, both of which belong to the family Theridiidae) known to eat eggs and adults of M. porracea, observing them for ten days. Boxes used as controls contained predators without a protective male, and they ate the eggs. In the others, the males were seen to take a protective stance toward the egg sacs and hatchlings, warding off predators when they approached. In one case, the male killed the predator.

“The number of hatchlings that emerged from the egg sacs was two and a half times higher in the treatments where males were present, even in the absence of certainty about paternity. Both biological and adoptive fathers were equally effective in protecting them from predators,” Moura said.

Besides fending off predators, two other behaviors that characterized paternal care were observed. Males constantly sucked the egg sacs when wet, probably to remove excess moisture that could attract fungi or hinder the development of the eggs; and they rebuilt supporting threads in damaged portions of webs that without this care could fall to the ground, exposing the eggs to ants and other predators. Future studies will describe these behaviors in detail.

The authors refer to the strategy of protecting any offspring of the species by incorporating into the title of the article the famous dictum from Ode on a Distant Prospect of Eton College by Thomas Gray (1716-71).

The article “‘Where ignorance is bliss, ’tis folly to be wise’: Indiscriminate male care in a Neotropical spider” by Rafael Rios Moura, Isabella Dias Oliveira, João Vasconcellos-Neto and Marcelo Oliveira Gonzaga can be retrieved by subscribers to Ethology at: onlinelibrary.wiley.com/doi/abs/10.1111/eth.13112.