In some species, offspring are born (or hatch) at roughly the same time. This is called synchronous reproduction: an evolutionary strategy to give offspring the best possible chance of survival.
One of the main drivers of synchronised reproduction is predator satiation. Predators tend to target young animals because they are easy prey. If young are born asynchronously, predators can eat a large number because they can have a break between meals.
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But if a huge number are born at once, there are simply too many to eat. Therefore, mass birth decreases the chance of any single baby being eaten.
Sea turtles, particularly olive ridley and Kemp’s ridley turtles, are thought to hatch synchronously for exactly this reason. As hundreds of thousands of turtles hatch at once and overwhelm predators, each hatchling’s odds of being eaten is lower than if it had hatched alone or in a small group.
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Many antelope species, such as wildebeest, give birth synchronously for the same reason, with hundreds of thousands of calves born in the space of just a few weeks.
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But there are other reasons that a species might synchronously reproduce. In one social group of banded mongoose, more than 60 per cent of females gave birth on the same night.
As a cooperatively breeding species, pups are raised by the community rather than their mothers. But if mothers can identify their own young, they might give them preferential treatment, leading to a breakdown in the cooperative system.
Giving birth at the same time avoids this as mothers cannot tell which pups are theirs and therefore treat each pup as their own.
Further to this, younger banded mongoose pups are less able to compete with older pups for resources and therefore have a poorer chance of survival. This puts pressure on females to give birth first.
But having the first-born pups is risky, too, because first-born pups are likely to be killed by females who want their own pups to be the oldest.
These two opposing pressures among banded mongooses, where older pups are more likely to survive than younger ones, but the oldest are more likely to be killed – along with the need to conceal the parentage of pups – has resulted in this unusually extreme example of synchronous reproduction.
Top image: olive ridley sea turtles in Ostional Wildlife Refuge, Costa Rica. Credit: Kryssia Campos/Getty Images
