frequencies and concluded that indeed the pattern across North America suggested a birth pulse that occurred anywhere between June and October. They further tested if cubs born in their Idaho study area within the birth pulse documented by Logan and Sweanor ([1]; July–September) exhibited higher survival; no difference was documented. We also did not document differences in survival between pulse-born and other cubs, as well. These results might not be surprising, given the ability of mountain lions to kill prey animals throughout the year. Providing sufficient quantities of food for cub survival (e.g., maintenance) would require less prey than to provide for individual quality (e.g., body mass, growth rates). Clearly, energy requirements for maintenance are less than for growth and reproduction [10]. Although survival tended to increase after neonate ungulates switched to the ‘‘fleeing’’ strategy, the use of cub survival might not be a sensitive metric to evaluate reasons for timing of birth in mountain lion populations. Because cubs are dependent on an adept predatory mother to provide food for survival until independence, and mortality is often caused by other mountain lions [1], survival might be a poor indicator for variation in reproductive quality of birth timing. We found evidence that there were other advantages in timing of birth. Timing of birth predominantly influenced body mass of cubs and the dispersal age of juveniles. However, the benefits seemed to be best tested by using actual ungulate availability, as predicted by Logan and Sweanor [1], than basic frequency of births (i.e., birth pulse) as tested by Laundre´ and Herna´ndez [4]. We found that cubs born 1 April to 30 June were smaller and dispersed at an earlier age than cubs born 1 July to 31 January in our study area, even though June coincided with ungulate births. More cubs were born during July and August (40% of all litters) than any other 2-month period, which occurred .30 days after the births of ungulate neonates in our study area. Neonates of white-tailed deer (Odocoileus virginianus), mule deer (O. hemionus), and elk (Cervus elaphus) exhibit a scent-less hiding strategy for 2–4 weeks, presumably to avoid predators until they are mobile enough to flee [11–14]. Because of this hiding strategy, neonates might be less vulnerable to predation, by sight predators such as mountain lions, during the hiding-phase; although, the strategy might be less effective for animals with highly developed olfactory senses (i.e., Canidae [15]; Ursidae [16]). The abundance of vulnerable prey [17] during July and August, should allow mothers to more easily find food with less travel and spend more time at dens providing food for their young cubs. In fact, we found cubs born .30 days after the ungulate birth peak were 21% larger, at 1 week of age, than cubs born before this period. Cubs born prior to or during the ungulate birth peaks necessarily exhibit lower developmental growth, because prey availability is lowest at this time of the year. We suggest that there are advantages to birth timing for mountain lions. Birth following the birth peak for ungulates in North America leads to heavier body mass and older dispersal. Subadults dispersing at 17 months (born after the switch) are presumably heavier and more experienced, than those dispersing at 13 months (born before the switch). Some of the advantages might influence survival through the subadult life-stage, which translates to an evolutionary advantage for both mother and offspring. Thus, births of mountain lions should trend toward a narrower as opposed to wider birthing period. Because of the stochastic nature of most cub mortalities [1,18], timing of first litter should show less variation if it is inherent or in relation to resource availability because timing of subsequent litters are related to completion of a previous litter. We showed that timing of birth is more narrowly concentrated, in time, for 1st time mothers than for older mothers. These data are in accordance Birth Timing for Mountain Lions with the prediction that birth timing is related to prey availability [1]. However, breadth of the population birth pulse is maintained by variation in reproductive maturity, birth intervals, and the vagaries of mortality. Logan and Sweanor [1] found mean age of first litter of 29 months and mean birth intervals of 17 months, thereafter. Natural mortality of cubs is dominated by infanticide by other mountain lions, which likely occur during chance encounters. Mothers that lose cubs can rebreed as soon as 23 days following mortality event and give birth 113 days following the mortality event [1]. Despite the predictability of ungulate birth peaks and subsequent timing of anti-predator strategy switch, none of the reproductive traits for mountain lions promote the persistence of a strict annual birth peak. No study has evaluated the possibility of variation in pulse dates within populations that depend upon the age structure of the female population. We offer a hypothesis and predictions regarding the birth pulse in mountain lions and recommend testing with data from other populations. We hypothesize that mountain lion births are adaptively timed closely to the availability of primary prey in environments, where primary prey exhibit predictable birth peaks, but that reproductive biology and random mortality events after the initial litter result in a broadened birth pulse that adheres less strongly to the timing of prey availability. We predict that in environments where the primary prey for mountain lions exhibit ‘‘hider’’ predator-avoidance strategies (e.g., deer, elk) and where timing of ungulate birth peaks is predictable (e.g., north/south temperate latitudes) that first litters of mountain lions will be closely timed to follow the switch from ‘‘hider’’ to ‘‘fleer’’ by neonates. We also predict that in environments where primary prey species do not exhibit a ‘‘hiding’’ anti-predator strategy as neonates are capable of fleeing shortly after birth (e.g., bighorn sheep [Ovis canadensis], guanaco [Llama guanacoe]), yet birth peaks are predictable (e.g., non-desert systems in North America, temperate South America), that first litters of mountain lions will be closely timed to the birth peak of those primary prey species. We predict that in environments where prey are not born in predictable peaks or the sum of births in multiple primary prey species provide abundant neonates throughout the year (e.g., peccary [Pecari spp.], Sonoran Desert, tropical regions), that mountain lions will not exhibit a birth pulse or the pulse will be broader than 3–4 months, characteristic of populations studied in North America [4]. Materials and Methods Ethics Statement Our capture and handling procedures followed guidelines of the American Society of Mammalogists [19] and were approved by the Institutional Animal Care and Use Committee (Approval No. 07-A024) at South Dakota State University. Study area We studied mountain lions in the Black Hills ecoregion (N44.09375u, W103.77691u) of South Dakota, USA between 2006 and 2009. The Black Hills are a complex of forested ridges, valleys, and steep canyons [20]. Climate patterns in the Black Hills are characterized by hot summers and cold winters typical of a continental climate regime. The plant community is dominated by ponderosa pine (Pinus ponderosa) forests, but also contains spruce (Picea glauca), aspen (Populus tremuloides), birch (Betula spp.), and oak (Quercus macrocarpa) trees [21]. Potential ungulate prey included white-tailed deer (Odocoileus virginianus), mule deer (O. hemionus), elk, American bison (Bison bison), mountain goat (Oreamnos americanus), PLOS ONE | www.plosone.org 5 September 2012 | Volume 7 | Issue 9 | e44625