Who’s Your Daddy – Paternity in Ground Squirrels

Reproductive strategies are shaped by sexual selection, but selection pressures often differ between the sexes. When both sexes mate with multiple partners during the breeding season (polyandry or polygynandry) a conflict arises between the sexes. Females can be 100% sure the offspring she gives birth to are hers, but there is no such paternity assurance for her male mate. Thus, selection favors females who seek extra-pair matings, if the new male has “better” genes (or provides enhanced parental care) than her current partner. Males, on the other hand, attempt to ensure maximum paternity of the entire litter by mate guarding, producing copulatory plugs, or other behavioral tactics.

To ensure paternity, males often develop post-copulatory strategies that maximize their chances of siring all of the young in a litter. One such strategy is called first-male sperm precedence, where the first male to mate with a female has a greater chance of paternity than subsequent males (i.e. mating order is important). Other strategies include increasing ejaculate volume, attempting to remove or block sperm deposited by a previous male, and post-copulatory mate guarding (to prevent additional copulations by other males).

To test these hypotheses, Raveh and colleagues (
2011) studied free-living Columbian ground squirrels (Urocitellus columbianus)* in Alberta, Canada (Figure 1). The squirrels were trapped and marked soon after they emerged from hibernation. The researchers observed the marked population throughout the breeding season and noted which males each estrous female consorted with. They also used microsatellite loci from tissue samples to assign maternity and paternity to each of 147 litters (434 offspring total).

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Figure 1. A Columbian ground squirrel in the Canadian Rockies (Photo by Rob Pearce/Flickr)

Males that were first to consort and presumably mate with a female (mating takes place in underground burrows) sired approximately 60% of the offspring in the litter (Figure 2). However, as the number of additional males a female mated with increased, the likelihood of paternity for the first male declined. In addition, females were less likely to consort with other males if the first male spent more time guarding her from other males. Finally, males spent less time consorting with females near the end of their estrus cycle and were less likely to sire offspring from mating with late estrus females.

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Figure 2. The siring success of male Columbian ground squirrels declines sharply with mating order. (From Raveh et al., 2011)

In Columbian ground squirrels, mate guarding did not eliminate multiple matings. Every successful female mated with at least two males. Nevertheless, mate-guarding by the first male did reduce the female’s access to additional males. Thus mate-guarding likely plays an important role in sperm competition in polygynandrous mating systems.

*Raveh et al., 2011 use the genus Urocitellus, however the genus Spermophilus is used by Wilson and Reeder(2005).

References

Shirley Raveh & Dik Heg & Vincent A. Viblanc & David W. Coltman & Jamieson C. Gorrell & F. Stephen Dobson & Adele Balmer & Peter Neuhaus (2011). Male reproductive tactics to increase paternity in the polygynandrous Columbian ground squirrel (Urocitellus columbianus) Behavioral Ecology and Sociobiology, 65, 695-706

Wilson, D.E. and D.M. Reeder (2005)
Mammal Species of the World, A Taxonomic and Geographic Reference, The Johns Hopkins University Press, Baltimore.