Indirect Fitness Benefits Do Not Compensate for the Loss of Direct Fitness in Yellow-bellied Marmots

The contribution of indirect fitness to inclusive fitness is expected to increase as the reproductive skew increases, with indirect fitness being the only component of inclusive fitness of sterile individuals in eusocial species. However, the relative contribution of indirect fitness to inclusive fitness has rarely been evaluated empirically. Using data from a long-term study (1962–2003), we show that female yellow-bellied marmots that have a later age of 1st successful reproduction incur a substantial loss of direct fitness with no corresponding gain in indirect fitness. Additionally, although females that survive to reproductive age but do not successfully reproduce have a greater indirect fitness than those that reproduce at least once, indirect fitness benefits of foregoing reproduction are insufficient to compensate for the loss of direct fitness resulting from later reproduction. Although indirect fitness composed 22.2% of the inclusive fitness of females that reproduced at least once, females that reproduced at least once had 2.3 times higher inclusive fitness than those that survived to reproductive age but never reproduced. These results suggest that, in yellow-bellied marmots and other species with similar life histories, the contribution of indirect fitness to inclusive fitness does not compensate for loss of direct fitness.