To Twin or Not to Twin? Trade-Offs in Litter Size and Fawn Survival in Mule Deer

Parents often face a trade-off between allocating resources to many young with lower survival, or fewer young with higher survival. To examine trade-offs in litter size and juvenile survival in mule deer, and how survival was influenced by the nutritional condition of the female, we compared the survival of 30 twin and singleton mule deer (Odocoileus hemionus) fawns during their 1st summer in eastern Washington. Overall fawn mortality was high (51.6%), especially during the 1st half of the summer (61.3%). Coyote predation caused 58.3% of all identified fawn mortalities. Twin fawns had a risk of dying 2.6 times higher than single fawns, and the difference between twin and singleton survival was most drastic during the first 1.5 months of life. Body fat of females during their last trimester of pregnancy predicted the number of fetuses they were carrying and whether they had at least 1 fawn surviving until the fall, but not the number of fawns surviving. Under these conditions, a litter size of 2 would be considered optimal because mothers giving birth to twins produced an average of 0.92 fawns by fall, whereas mothers producing singles ended up with only 0.75 fawns. However, our model suggested that a population producing only twins would be expected to increase only 4% faster than one producing only singletons.