The isotope half of this study contains its own control. Carbon and nitrogen signatures in muscle integrate feeding over a long period rather than reporting the last meal [10], and the assay plainly had the resolution to separate groups: sharks from Biscayne Bay and from the Tampa Bay region, included to test whether the same developmental pattern showed up on the opposite side of Florida [14], came out in distinct food webs, and diet shifted as animals grew [11]. Within a single bay, males and females drew on many of the same food sources [11]. A flat result on sex, from a measurement that was not flat on geography, carries more weight than a flat result alone.
What the isotopes cannot see is handling. They record what was assimilated, not how it was caught or crushed, so two sexes that process the same prey differently would still return the same signature. That is where a functional explanation could still sit, and this work does not reach it.
The arithmetic of the sample: 105 sharks in Biscayne Bay plus 39 in the Tampa Bay region gives 144 measured animals [15], of which 137 provided muscle for isotope analysis, about 95 percent [16]. Tampa contributed 27 percent of the total [17]. Split 39 sharks by sex and by size class and the per-cell counts get thin, and the announcement gives neither the per-sex counts nor an effect size for the curvature trend [20].
Age is the other soft spot. Sampling ran from May 2022 to May 2023, twelve months [18], and each shark was photographed against the grid board once and released quickly [8], with ImageJ used afterward to compare curvature [9]. The developmental trajectory is therefore read across individuals of different sizes rather than by following one animal through maturity. That is usually the only feasible design with free-swimming sharks, and it cannot fully separate growth from differences in which animals the longlines and the gillnets brought in [7].
The consequence for the standing story is specific. The old account came with a mechanism attached, because it placed the divergence at sexual maturity and let whatever happens at maturity do the explaining [3]. Liu reports the opposite direction of change, with young males and females both more pointed and both rounding as they matured, much more so in females [5]. If females change more, the adult difference is built mainly by female rounding rather than by males acquiring a point [19], which moves the question to a different sex and a different stretch of the life history. Bonnetheads are also the only shark known to show this kind of clear sex difference in head shape [4], so there is no sister species in which to check the pattern. Macdonald calls the result a narrowing of the drivers of the trait [13]. That is the honest verb for it.
