Researchers have unearthed the earliest evidence of parental care in an unsurprising place: Cretaceous carrion beetles, the ancient relatives of today’s burying beetles, famous for providing nutrients for their young in the form of carcasses. The findings were published in Proceedings of the National Academy of Sciences this week.
Ephemeral behaviors are really difficult preserve over millennia. After all, what does parenting look like fossilized? The answer, for beetles at least, is tiny ridges. There are over 200 beetle species in the Silphidae family today, including the American burying beetle (Nicrophorus americanus), one of the most protected beetle species in North America. They’re also famous for their biparental care, and as it turns out, their early relatives have been doing the same for 125 million years.
After a Nicrophorus male finds a small vertebrate carcass in the night, the male and female pair will transport it while lying on their backs and using their legs to move the carcass like a conveyor belt. These small mammal and bird carcasses can weigh up to 200 times more than the beetle itself. After the pair buries the carcass in soil, the female lays 20 to 40 eggs in an escape tunnel leading to a brood chamber next to the carcass. When the larvae hatch a few days later, the parents guide them to the carcass by rubbing the the edge of their wing covers over raised ridges (called files) on the dorsal side of their abdomen, Science explains. This form of parent-offspring communication -- called stridulating -- creates a raspy sound, and once the newly hatched offspring find the nest, the parents regurgitate partially digested carcass for them. The parents also stridulate to warn their offspring about predators.
To see when this behavior started, a team led by Di-Ying Huang from Nanjing Institute of Geology and Palaeontology examined 44 specimens from the Mesozoic belonging to three distinct groups: Middle Jurassic Daohugou beds (around 165 million years old) in Inner Mongolia in China, Lower Cretaceous Yixian Formation (125 million years, pictured above) in Liaoning Province and Inner Mongolia, and mid-Cretaceous ambers (99 million years) from northern Myanmar.
The Jurassic silphids, the oldest ones studied, primitively lack those “stridulatory files” that are important for parental care communications. But the team did find the requisite files to indicate that early forms of parental care and social behavior among silphids had developed by the Early Cretaceous. They also found evidence of parents guarding small vertebrate carcasses for their larvae in the mid-Cretaceous amber from Myanmar. The researchers think the behavior originated from competition between early silphids and their predators.