Marine Animal Sex-Dispersed Migration Behavior
Biology 342 Fall 2015
Margalit Leiser and Jaelin Cola
How the Behavior increases Lifetime Reproductive Fitness
Sharks follow three distinct migration patterns. In Carcharodon carcharias, the great white shark, a decadal timescale coincides with ENSO - El Niño effect - indices. In warm-water years, more female sharks overall are observed at coastal sites, whereas coastal shark densities are skewed towards male sharks in cold-water years (Towner, et. al). On an annual scale, both great white and broadnose sevengill (Notorynchus cepedianus) sharks spend August - February in the relatively warm temperate waters off continental shelves (Pardini, et al; Barnett et al). In cooler months, sharks migrate to tropical and subtropical environments; their coastal loci are entirely bereft from mid-April until mid-July (Jorgensen, et al).
In addition to these seasonal fluctuations, one overriding behavior is present: females are philopatric, while males are roving (Pardini et al). Because populations of many sharks are relatively small, and because sharks are by nature elusive, simple animal tagging and tracking was not sufficient to establish this. A comparison of mitochondrial control regions in New Zealand, South Africa, and Australia was utilized; the estimated differentiation of the mitochondrial DNA approached maximum unity. This indicated long-term isolation of the female populations. Analysis of nuclear loci across the three regions, however, yielded almost no difference. This indicated that male-mediated gene flow occurred often enough to homogenize allele frequencies. This disparity between the interaction rates of males and females indicated that females stay in one region for the duration of their lives, while males may undertake transoceanic migrations.
There is no single explanation for this migratory behavior, rather, there are many probably factors contributing to it. Main contributors include breeding, mating, and hunting impulses.
The Darwinian paradigm of evolution indicates that behaviors that enable an organism to raise more young to adulthood will be selected for. One manifestation of this phenomenon is the thermal niche hypothesis, which suggests that pregnant female sharks seek out areas of warm water in order to enhance fetal development and optimize parturition efficiency (Economakis & Lobel).
Preliminary data has indicated that sharks may swim where the food is. The July-August peak in C. carcharias sightings for both sexes coincides with a time that fur seal pups first begin solitary swimming, and may therefore be a prime hunting opportunity for the sharks. Sharks’ tendency to congregate near shorelines in warm-water months may also be explained by food-seeking behaviors. In warm waters, surf-zone diatoms form blooms. These blooms lead to an increase in food sources for C. carcharias. Because C. carcharias is capable of body-temperature regulation, it is likely that an increase of white sharks near shore is due to a favorable increase in prey availability, rather than a physiological preference for warmer waters (Weltz et al). Explanations for the sex-segregated nature of shark migration, wherein males are more abundant in cooler water years, include the lack of larger females. This may reduce competition for prey resources (Towner et al). Similarly, it is possible that females time their inshore movements in order to avoid males, and with them, mating harassment (Towner et al).