Timing is Everything

In social mammals, females often give birth synchronously, often within a few hours or days of one another. Among the most popular explanation for this phenomenon is the “prey swamping” hypothesis, which proposes that the survival of each offspring is improved when females in a group all give birth within a short period. In effect, the risk of predation is reduced by dilution.

A second hypothesis suggests that females give birth synchronously in order to avoid competition or conflict with other females within the group. For example, offspring born to a female who gives birth later in the season may be smaller and have less access to food. Females who give birth earlier than other females in the society may lose offspring to infanticide from other females. If either competition or conflict occurs, then selection should favor birth synchrony.

Banded mongoose (
Mungos mungo) are small, diurnal carnivores that form bands of up to 40 individuals. In these highly social mammals females exhibit a high degree of birth synchrony. Although females may mate and conceive on different days (up to a week apart), most give birth on the same night (Hodge et al., 2011). Thus, banded mongoose provide an opportunity to study how competition and conflict influence birth synchrony.

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Figure 1. An adult and juvenile banded mongoose (Mungos mungo) from Africa. (From Flickr/thy)

The survival of offspring born in synchrony was compared with offspring from asynchronous births. Pup survival from birth to the time they emerged from the den (pre-emergence survival) was significantly higher in synchronous litters (Figure 2). As predicted, litters born before other litters suffered significantly higher pre-emergence mortality. Litters are well-protected in their dens and the most likely cause of mortality was infanticide from other females who had not yet given birth to their own litters.


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Figure 2. (a) the proportion of pups lost prior to emergence in synchronous and asynchronous litters. (c) the post-emergence mortality of pups from synchronous and asynchronous litters. (From Hodges et al., 2011)

In asynchronous litters, pups born later were smaller at emergence than older littermates. As predicted, the younger pups from asynchronous litters also suffered increased mortality between first emergence and weaning. In this case, smaller pups were less likely to be fed and protected by adult “escorts.”

In short, offspring born too early suffer increase risk of infanticide at the hands of other females. Offspring born too late suffer from increased competition for food and escorts. Therefore, in this highly social species, natural selection favors a high degree of reproductive synchrony.

References

Hodge, S.J., Bell, M.B.V., and M.A. Cant. (2011) Reproductive competition and the evolution of extreme birth synchrony in a cooperative mammal. Biology Letters, 7:54-56.