Disentangling early antler diversity: is there a causal link with extrinsic (climate-related) factors?.
Antlers are the only branched, bony appendages of apophyseal nature among mammalian headgear that can be regularly shed and rebuilt in an annual cycle. Annual cycles are controlled intrinsically by fluctuating hormone levels. At higher latitudes, this cycle is seasonally synchronized depending on extrinsic photoperiodicity, suggesting the influence of climate-related factors in their evolution. A single developmental and evolutionary origin has been assumed, and is considered a synapomorphy for Cervidae. Moreover, given their complexity and the physiological effort of the regeneration process, it has been argued that deciduous antlers could have been developed from previous non-deciduous antlers. Several types of branched frontal headgear (probably of apophyseal nature) were exhibited by early Miocene pecorans in the “Old World” and in North America. Differences in ontogenetical development, histological features, surface texture, and ramification mechanisms suggest that these appendages could be non-homologous, and thereby subject to independent evolution. In fact, they emerged at the same time as other headgear in several pecoran lineages, triggered by global climatic changes. As in modern antlers, four of these appendage types underwent spontaneous autotomy (or self-amputation). However, only three show evidence of complete regeneration. In these appendage types, a true coronet (indicative of regenerated antlers) is not developed, and there are differences in mineralization processes and histology, which suggest that their growth cycle was aperiodic. Antlers with true coronet and similar seasonal cycle were first recorded later, in middle Miocene sites of Eurasia. Although the meaning and origin of such innovative features is still not fully understood, it should be born in mind that these evolutionary changes were concomitant with the Middle Miocene Climatic Transition event, suggesting that the annual cycle was an evolutionary response to a new step in increased seasonality.