Enrique Font

The proliferative ventricular zone in adult vertebrates: a comparative study using reptiles, birds, and mammals.

Although evidence accumulated during the last decades has advanced our understanding of adult neurogenesis in the vertebrate brain, many aspects of this intriguing phenomenon remain controversial. Here we review the organization and cellular composition of the ventricular wall of reptiles, birds, and mammals in an effort to identify differences and commonalities among these vertebrate classes. Three major cell types have been identified in the ventricular zone of reptiles and birds: migrating (Type A) cells, radial glial (Type B) cells, and ependymal (Type E) cells. Cells similar anatomically and functionally to Types A, B, and E have also been described in the ventricular wall of mammals, which contains an additional cell type (Type C) not found in reptiles or birds. The bulk of the evidence points to a role of Type B cells as primary neural precursors (stem cells) in the three classes of living amniotic vertebrates. This finding may have implications for the development of strategies for the possible treatment of human neurological disorders.

Neurogenesis and Neuronal Regeneration in the Adult Reptilian Brain

Evidence accumulated over the last few decades demonstrates that all reptiles examined thus far continue to add neurons at a high rate and in many regions of the adult brain. This so-called adult neurogenesis has been described in the olfactory bulbs, rostral forebrain, all cortical areas, anterior dorsal ventricular ridge, septum, striatum, nucleus sphericus, and cerebellum. The rate of neuronal production varies greatly among these brain areas. Moreover, striking differences in the rate and distribution of adult neurogenesis have been noted among species. In addition to producing new neurons in the adult brain, lizards, and possibly other reptiles as well, are capable of regenerating large portions of their telencephalon damaged as a result of experimentally-induced injuries, thus exhibiting an enormous potential for neuronal regeneration. Adult neurogenesis and neuronal regeneration take advantage of the same mechanisms that are present during embryonic neurogenesis. New neurons are born in the ependyma lining the ventricles and migrate radially through the brain parenchyma along processes of radial glial cells. Several lines of evidence suggest that radial glial cells also act as stem cells for adult neurogenesis. Once they reach their final destination, the young neurons extend axons that reach appropriate target areas. Tangential migration of neurons alongside the ventricular ependyma has also been reported. Most of these tangentially migrating neurons seem to be destined for the olfactory bulbs and are, thus, part of a system similar to the mammalian rostral migratory stream. The proliferation and recruitment of new neurons appear to result in continuous growth of most areas showing adult neurogenesis. The functional consequences of this continuous generation and integration of new neurons into existing circuits is largely conjectural, but involvement of these phenomena in learning and memory is one likely possibility.

Beyond 'nasty neighbours' and 'dear enemies'? Individual recognition by scent marks in a lizard (Podarcis hispanica)

True individual recognition (TIR), the ability to recognize conspecific individuals on the basis of identity cues, is required for the evolution of several social traits (e.g. the maintenance of dominance hierarchies). However, knowledge about the distribution and functional significance of TIR is scant in some vertebrate groups, such as reptiles. In this study we used a functional modification of a habituationedishabituation paradigm to investigate the existence and adaptive significance of TIR in a territorial lizard (Podarcis hispanica, Lacertidae). Males discriminated between individual rivals of similar characteristics (e.g. size, weight, familiarity) solely on the basis of their scent marks. Males also remembered the spatial location of scent marks and subsequently behaved more aggressively towards rival males that consistently marked in the core than on the periphery of their experimental terrarium. Together, these results suggest that, in this species, scent marks function to identify the potential threat posed by each individual neighbour, allowing resident males to allocate their aggressive behaviour accordingly. Our findings challenge the simplistic and commonly held view that ‘dear enemy’ phenomena in lizards are exclusively based on familiarity asymmetries, and support an alternative threat level hypothesis in which TIR may be more important than previously acknowledged.

Chemosensory assessment of rival competitive ability and scent-mark function in a lizard, Podarcis hispanica

Recent studies have stressed the role of scent marks as signals potentially mediating competitor assessment. According to this view, receivers may use scent marks to derive information about the costs of exploiting a given area, but few studies have directly addressed this hypothesis. One of its main predictions is that scent marks should reflect a signaller’s competitive ability. We simulated the situation faced by an intruding male when entering the scent-marked territories of rival males of varying competitive ability to test predictions about the role of scent marks in a lizard, Podarcis hispanica. We report that males were attracted to areas marked by males of similar or higher competitive ability (i.e. larger size), but not to areas scent marked by males of lower competitive ability, and that this preference disappeared towards the end of the breeding season. Our results show that (1) male lizards can assess rival competitive ability (i.e. rival size) on the basis of scent marks alone, (2) scent marks do not function as chemical barriers to deter intruders, and (3) male response to marked areas varies throughout the breeding season, suggesting a shift in the cost-to-benefit balance of entering a scent-marked area. We propose that male assessment of rival competitive ability may function as an indirect assessment mechanism of territory resource quality in this species, and thus that scent marks may convey information not only about costs but also about the benefits of exploiting a scent-marked area.

Chemosensory cues allow male Tenebrio molitor beetles to assess the reproductive status of potential mates

Males of many insect species, including beetles, choose their mates according to their reproductive status. However, the ways in which male beetles evaluate female reproductive status have received little attention. We tested the existence of male mate choice in the mealworm beetle, Tenebrio molitor, by observing mating and courtship behaviour of males given simultaneous access to pairs of females differing in their reproductive status: (1) mature versus immature; (2) virgin versus previously mated; (3) familiar (mated with the experimental male) versus unfamiliar (mated with a different male). Males courted and mated preferentially with mature and virgin females. To determine whether chemical cues played a role in these discriminations, we exposed males to filter paper squares bearing chemical cues from different types of females: (1) virgin versus mated; (2) mature versus immature. Males were significantly more attracted to those squares bearing chemical cues from virgin and mature females, suggesting that males can assess female reproductive status on the basis of chemical cues alone.

Quantity discrimination in Tenebrio molitor: evidence of numerosity discrimination in an invertebrate?

Numerosity discrimination, the ability to distinguish between sets with more and less items, is recognised as the foundation for higher numerical abilities. Understanding numerosity discrimination from a comparative perspective is hence pivotal in tracing the evolution of numerical representation systems. However, numerosity discrimination has been well studied only in vertebrates, where two innate systems of number representation have been described: an ‘analog magnitude system’ used to discriminate among numerosities by representing them as cardinal magnitudes and a ‘parallel individualisation system’ that allows precise discrimination among small arrays of items (≤4) by representing objects individually. We investigated the existence of quantity discrimination in an insect species (Tenebrio molitor) by using a spontaneous two-choice procedure in which males were exposed to substrates bearing odours from different numbers of females (≤4) in increasing numerosity ratios (1:4, 1:3 and 1:2). We show that males can discriminate sources of odours reflecting 1 versus 4 and 1 versus 3 females, but not 2 versus 4 or 1 versus 2, indicating that T. molitor males exhibit a marked preference for sources reflecting more female donors only when numerosity ratios are below 1:2. We discuss the functional significance of this finding and whether our pattern of results could be best explained by summation of a non-numerical continuous variable or by the existence of a numerosity discrimination mechanism with an operational signature ratio of 1:2.

Chemically Mediated Species Recognition in Closely Related Podarcis Wall Lizards

In many animals, chemical signals play an important role in species recognition and may contribute to reproductive isolation and speciation. The Iberian lizards of the genus Podarcis, with up to nine currently recognized lineages that are often sympatric, are highly chemosensory and provide an excellent model for the study of chemically mediated species recognition in closely related taxa. In this study, we tested the ability of male and female lizards of two sister species with widely overlapping distribution ranges (Podarcis bocagei and P. hispanica type 1) to discriminate between conspecific and heterospecific mates by using only substrate-borne chemical cues. We scored the number of tongue flicks directed at the paper substrate by each individual in a terrarium previously occupied by a conspecific or a heterospecific lizard of the opposite sex. Results show that males of P. bocagei and P. hispanica type 1 are capable of discriminating chemically between conspecifics and heterospecifics of the opposite sex, but females are not. These results suggest that differences in female, but not male, chemical cues may underlie species recognition and contribute to reproductive isolation in these species. The apparent inability of females to discriminate conspecific from heterospecific males, which is not because of reduced baseline exploration rates, is discussed in the context of sexual selection theory and species discrimination.

Putting information back into biological communication

At the heart of many debates on communication is the concept of information. There is an intuitive sense in which communication implies the transfer of some kind of information, probably the reason why information is an essential ingredient in most definitions of communication. However, information has also been an endless source of misunderstandings, and recent accounts have proposed that information should be dropped from a formal definition of communication. In this article, we re‐evaluate the merits and the internal logic of information‐based vs. information‐free approaches and conclude that information‐free approaches are conceptually incomplete and operationally hindered. Instead, we propose a functional notion of information that follows logically from previous adaptationist accounts. The ensuing definition of communication provides a wider, more inclusive theoretical scope that reflects more accurately the evolutionary scenario shaping animal signals. Additionally, it is a definition better equipped to deal with the extraordinary diversity of animal signals, facilitates the distinction of honest and deceptive signals at a proximate level and accommodates a number of conceptual and practical issues (e.g. redundancy, alerting components) that are lost when we fail to acknowledge the informative content of animal signals.

Neuron regeneration reverses 3-acetylpyridine-induced cell loss in the cerebral cortex of adult lizards

Systemic administration of the neurotoxin 3-acetylpyridine to adult lizards results in extensive loss of neurons in the medial cerebral cortex, other brain areas remaining largely unaffected. After the neurotoxic trauma, new cells are produced by mitotic division of cells in the ventricular wall. The new cells migrate along radial glial fibers and replace lost neurons in the medial cortex. Electron microscopic examination of cells labeled with [3H]thymidine confirms that the newly generated cells are neurons. Thus, neuron regeneration can occur in the cerebral cortex of adult lizards.

Spacing and social organization: Urban stray dogs revisited

Abstract Different aspects of the etho-ecology of the stray dog population in Valencia (Spain) were investigated. Densities of between 127 and 1304 stray dogs km −2 overlap with density estimates reported for other populations. A male:female ratio of 2:1 is also in agreement with earlier studies. Behavioral observations revealed that these dogs will occasionally form groups with dominance hierarchies and communal defense of a territory. From the stability of these groups, long-term affiliative bonds apparently exist among group members. This finding conflicts with the accepted notion that urban stray dogs are asocial and do not form stable social groups. Methodological problems may invalidate earlier claims that urban stray dogs are asocial animals. This is due to improper use of a Poisson model for assessing social organization. It is suggested that stray dogs possess, like most canids for which an adequate data base exists, remarkable behavioral plasticity allowing them to adjust their social system to prevailing ecological constraints.