Lemon Shark Mating Systems:
We found that adult female sharks consistently returned to Bimini on a biennial cycle to give birth, and over 86% of the litters had multiple fathers. Similar patterns were observed at the Marquesas Keys with polyandrous, philopatric females returning every two years to pup. In contrast, adult male sharks rarely sired more than one litter at Bimini (or Marquesas) and may mate over a broader geographic area, thus introducing gene flow between nurseries. No single adult female was ever detected as having switched between nurseries for parturition, although the mating habits of adult males we know with less certainty.
One argument for the adaptive significance of female sharks mating with multiple males despite the risk of injury associated with mating events is that it provides indirect genetic benefits to the offspring like greater genetic diversity and survival. In Bimini we found that this was not the case and it may instead be the result of convenience polyandry, whereby females make the best of a bad situation and mate multiply to avoid excessive harassment from males.
The juvenile lemon sharks of North Bimini are unique in that they are smaller at age and grow up to five times slower than individuals sampled at all other lemon shark nursery areas we have surveyed to date. By estimating the strength of natural selection acting on these traits in the North Sound we found that small size and slow growth were indeed consistently favored and heritable, which contradicts the conventional “bigger is better” theory for life history traits in fishes. Our results instead support those of some other recent studies in suggesting that bigger / fatter / faster is not always better, and may often be worse. In this case, we suggest that bigger, faster growing juvenile lemon sharks may take more risks when foraging and thus expose themselves more often to predators in the North Sound (i.e., larger, subadult sharks). This hypothesis is currently under investigation using stable isotope data and personality studies (see current research).
Coincident with coastal development on North Bimini, we found significant changes in the level of neutral genetic variation in the lemon shark, additive genetic variance (i.e., a key component of heritability) for body length and mass, and strength of selection on these same traits. This finding suggests that anthropogenic forces can induce dramatic shifts in the selective forces that a population must adapt to over a short time period.