Two hypotheses have been proposed to explain the evolution of cathemerality in lemurs. The Evolutionary Disequilibrium Hypothesis (EDH) states that cathe-merality is of recent origin, representing a transitional stage between nocturnality and diurnality and the result of evolutionary disequilibrium caused by the relatively recent extinction of large-bodied lemurs and large aerial predators (Martin, 1972; Tattersall, 1982; van Schaik and Kappeler, 1996). The second hypothesis proposes cathemerality is more ancient and a stable strategy that may be ancestral for the genus Eulemur, or the entire lemurid clade and may have contributed to the radiation of the numerous lemurid taxa in Madagascar (Tattersall, 1982; Tattersall and Sussman, 1998; Curtis and Rasmussen, 2002, 2006).
Evolutionary Disequilibrium Hypothesis (EDH)
EDH argues that ecological release has led to the evolution of cathemerality and diurnality in lemurs (van Schaik and Kappeler, 1996). In the Neotropics, Wright (1989) suggests that A. azarai is cathemeral as a result of either ecological release from competition or predation pressure, which is allied to the proposals of the EDH. This is supported by studies on other cathemeral mammals, such as sloths, tree hyraxes, and fruit bats that document diurnality in the absence of aerial predators (Chiarello, 1998; Milner and Harris, 1999; Brooke, 2001), as well as observations on cathemeral lemurs which shift to nocturnality when canopy cover is low and provides little protection from raptors (Overdorff, 1988; Curtis et al., 1999; Donati et al., 1999; Rasmussen, 2005). EDH is not so well supported when we evaluate current predation pressure and competition in the light of the possible situation in the past in Madagascar.
Raptors are present today and exert substantial pressure on day-active lemurs (Goodman et al., 1993). Predator pressure from raptors in the past was probably no higher than today and large aerial predators may have become extinct due to the extinction of their main prey item, the large subfossil lemurs (Goodman, 1994; Wright, 1999). Equally, predation pressure from the cathemeral Cryptoprocta ferox is unlikely to be any higher than in the past, when a substantially larger extinct species (Cryptoprocta spelea) probably preyed on lemurs (Goodman et al., 2004; Colquhoun, 2006). Recent data on the activity and ecology of Cryptoprocta ferox (Hawkins, 2003) indicate that the predation risk posed by this carnivore is possibly far higher for cathemeral lemurs than predation risk from aerial predators. Colquhoun (2006) suggests that cathemeral lemurs were present on Madagascar prior to the arrival of the first carnivores, 24-18 MYA (Yoder et al., 2003), and that cathemerality may have evolved in Cryptoprocta as a direct response to the activity rhythm of these prey species. Were this the case, then predation could not have been pivotal in the evolution of cathemerality, but might play a role in the different modes of cathemerality in lemurs, which may aid in "confusing" the cathemeral viverrid.
Ecological release from competition seems equally unlikely to have played a role in the evolution of cathemerality, as there is little evidence the extinction of large-bodied lemurs led to vacation of niches now occupied by cathemeral lemurs: Subfossil lemurs were generally much larger than extant lemurs and mainly foli-vores or seed predators (Godfrey et al., 1997; Rafferty et al., 2002) that would not have competed with smaller-bodied cathemeral frugivore-folivores or bamboo specialists. Archaeolemur was probably adapted to a varied and eclectic diet and might therefore have competed with extant cathemeral frugivore-folivores. However, spatial separation would have sufficed to reduce any potential competition as the large-bodied archaeolemurids are described as more terrestrial than any living lemur (Rafferty et al., 2002). Charles-Dominique (1975) proposed that in the tropics in Africa and the New World every forest econiche can accommodate one nocturnal and one diurnal species and Curtis and Rasmussen (2006) suggest that this might be extended in Madagascar to a third sympatric species for each econiche, namely, a cathemeral species.
Recent genetic and visual morphological data support an ancient origin of cathe-merality in lemurs, as does the greater picture when the adaptive origins of primates and cathemerality in nonprimate mammals are taken into consideration. Genetic data suggest a common ancestry of diurnality and cathemerality in the indriids and lemurids dated between 32 and 52 MYA (Roos et al., 2004). Morphological evidence corroborates this, as the eye morphology of the Eulemur clade is intermediate between diurnal and nocturnal strepsirhines and based on this Kirk (2006) proposes that cathemerality characterized their common ancestor, 8-12 MYA (Yoder and Yang, 2004). Visually intermediate characteristics between nocturnal and diurnal morphology are found across the mammals in cathemeral species, leading Kirk (2006) to suggest the convergent evolution of cathemerality in many mammalian clades.
The ubiquity of day-night activity across mammals provides further evidence that this type of flexibility is probably ancient in origin. Day-night activity is present in many types of environment, at different trophic levels in both generalists and specialists, as well as in prototherian, metatherian, and eutherian mammals (Curtis and Rasmussen, 2006). There also appears to be a trend in mammals from nocturnality to mixed day-night activity with increasing body size and at different trophic levels, as shown, for example, for both herbivores and carnivores (Belovsky and Slade, 1986; Zielinski, unpublished data cited in Zielinski, 2000). This is of relevance given recent estimates of body mass in ancestral primates by Soligo and Martin (2006), suggesting that the last common ancestor of extant primates weighed around 1 kg and hence subsisted on fruit in combination with either insects or leaves. Soligo and Martin (2006) suggest that the cheirogaleids represent a dwarf lineage within the lemurs. By extension, this implies a larger body size in the common ancestor to the lemurids and indriids, if not the common ancestor to all lemurs, and hence the possibility for day-night activity.
Wright (1999) and Ganzhorn et al., (1999) have suggested that the unpredictability of the environment on Madagascar may have played a crucial role in lemur evolution and explain many of the traits we see in lemurs which are absent from primates in other tropical habitats. Such unpredictability may be implicated in the evolution of cathemerality in lemurs, but also in A. azarai, which inhabits a marginal environment for primates—outside the tropics, highly seasonal and characterized by pronounced changes in temperature. Fernandez-Duque (2003) interprets cathemerality as a thermoregulatory response in A. azarai to inhabiting the higher, cooler latitudes of the Argentinian Chaco.
Halle (2000a) suggests that in a highly predictable environment, a genetically fixed temporal program probably constitutes an evolutionary stable strategy (ESS), as it guarantees that activities will be performed at the optimal time of the day. This is not possible in an unpredictable environment where direct responses to environmental variation are required for survival and hence fixed programs are useless. The island environment of Madagascar is unstable, with frequent catastrophic climatic events such as cyclones and droughts in combination with low soil fertility and variability in peaks in abundance and scarcity of foods consumed by lemurs (Ganzhorn et al., 1999; Wright, 1999). While cathemerality is not genetically fixed, it may have evolved as a response to this unpredictability, forcing the ancestors of the extant lemurids away from their biologically inherent nocturnal cycle of activity (Bearder et al., 2006). Alternatively, should this flexibility have already been present in the ancestral lemurs, they would have been well equipped to deal with the unpredictable environment, having the capacity to shift between temporal niches in response to environmental variation.
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