First tracks of newborn straight-tusked elephants (Palaeoloxodon antiquus)

The MTS proboscidean tracks and trackmakers

Rounded-to-elliptical tracks, with an axial length range from 9.6 to 54.5 cm (pes), were found mostly isolated and as manus-pes couples, or associated forming at least eight short trackways (see Table 1). They reveal good preservation in one 6-footprint trackway (see below), two converging trackways and some couples, showing anteriorly directed, wide, short and blunt toe impressions (Figs. 2, 3 and 4). Toe impressions are not commonly visible in elephant footprints^9,13, (but see²⁷), which attests to cases of exceptional preservation in Matalascañas tracks. Preservation as true tracks is identified through expulsion marginal rims (e.g., Fig. 4a, g) and possible ejecta (Fig. 3b,e). Large and flat sole surfaces sometimes show evidence of pockmarks²³ (Fig. 4f).

Irrespective of the track size, pes are elliptical to sub-rounded, with the length axis larger than the width and manus are circular or elliptical, with the width axis larger than the length (Figs. 2c and 4d, g for small and large size tracks, respectively). The safest way to differentiate between pes and manus is through the orientation of the track provided by the toe impressions, or by the orientation of the longer axis in trackways. When arranged in trackways, manus-pes couples show the typical elephantine gait, showing a short pace resulting from the fore- and hind feet on the same side swinging forward simultaneously below the body, as it is known from modern elephant gait²⁸. In some cases, the partial impression of a pes overstepping the proximal part of a manus can be seen (Fig. 2c, g). Based on similar preservational style and opposing directions of movement without overlapping at the meeting point, a converging pair of trackways was apparently produced contemporaneously by an adult and a rather small juvenile. Sharp edges of the toe impressions indicate the presence of nails. These are found mostly in well preserved, smaller-sized tracks (Fig. 2a, d, e) because nails are commonly worn down in adult elephants and not always shown in their tracks¹³. These morphological features allow us to attribute the MTS trackways to the ichnospecies Proboscipeda panfamilia used previously for describing, among other tracksites, those tracks attributed confidently to the straight-tusked elephant Palaeoloxodon antiquus in the paleogeographical context of southern Europe^11,14 (see supplementary Table S1).

Manus-pes couples, when showing overstepping, were not considered in Table 1 (Fig. 2c, g). Overstepping depends on the speed of walking; at faster speeds the overstepping is only partial or there is no overstepping; elephants maintain the footfall pattern at all speeds, shifting toward a calculated 25% phase offset between limbs as they increase speed²⁸ (Fig. 2g). The smallest tracks usually do not show overstepping possibly because of the greater activity, with longer pace and stride lengths, demonstrated by calves and juveniles when compared to adults. Manus or pes showing a large width-length ratio (below 0.80–0.96 sensu²⁵) were not considered for the estimates since they represent slippage.

Younger elephants have more pliable skin and musculature than adults. Also, the greater expansion and distribution of the weight in heavier adult animals is enough to reduce or negate toe impressions in some types of sediments, such as compacted substrates^24,29. Interpreting the sedimentological data for the paleosol where MTS was developed^15,17,30, suggests a drying clayey-sandy substrate¹⁴ that was still plastic enough to absorb the impact of the limbs during the locomotion of the elephants (presence of expulsion rims and absence of radial pressure cracks), and preserving, in many cases, the morphological details of the feet in good condition (Figs. 2a, 3, 4a; see Fig. 2h for a badly preserved example).

Ichnological inference about the height, body mass and age of Palaeoloxodon antiquus in the MTS

Several methods have been proposed for estimating the height at the shoulders for proboscideans, and the relationship between body mass and age with shoulder height ^1,31,32. A linear relationship between foot length and shoulder height was confirmed by Lee and Moss³³ from extant elephants and compared with fossil examples by Pasenko²⁴. Pes length has been especially used in studies as an indicator of shoulder height^21,34,35,36. Among Asian elephants, manus circumference has been shown to have a similar predictive relationship with shoulder height³³. These parameters were determined for each isolated track (or representative track in a trackway), including manus and pes (Table 1), using equations previously proposed^31,33 (see Methods). A similar approach has been applied to mammoth track studies in North America^21,27, where modern ontogenetic and body-mass data has been used to provide age and size estimates from fossil tracks.

From the skeletal record, sexual dimorphism of P. antiquus was observed to be more accentuated than in extant elephants, especially in terms of size differences¹. During the first 10 years of life, both male and female African bush elephant foot lengths increase rapidly, with the fastest growth shown in the first two years for calves^33,37. In P. antiquus, males would have continued to grow until their fifties according to bone data¹, while females would have been much smaller as result of energy expenditure with reproduction, flattening the growth curve just after puberty. That is why the equations of Lee and Moss³³ that discriminates the shoulder height from tracks for males and females have been applied. However, by comparison with the study of Marano and Palombo³² (based on the progress of eruption and degree of wear of teeth compared to extant elephants), and the body mass correlation of Larramendi et al.¹ for calculating the age of P. antiquus, our MTS ages obtained from the application of the regression curve of Lee and Moss³³ are underestimated and must be analysed as minimum age approximations for track lengths corresponding to adolescent and adult animals, especially for males. The obtained estimations from tracks are subject to a level of uncertainty related to biotic and abiotic factors that can distort the data (i.e., taphonomy) as it happens also with the calculations taken from skeletal proportions. Therefore, McNeil et al.²¹ even included data from frozen mammoth carcasses on the growth curve of Lee and Moss³³ for correcting size discrepancies along ontogeny. For P. antiquus, our best data for comparison comes, however, from the flesh reconstructions¹.

Ontogenetic implications

Based on the best fossil site found for this species in Europe, corresponding to 70 individual Palaeoloxodon antiquus specimens recovered in Geiseltal, Germany, Larramendi et al.¹ developed the best reconstruction, so far, of the life appearance of this species and discussed size, body mass, ontogeny and sexual dimorphism. The Neumark-Nord bone site may be contemporary or slightly older than MTS, corresponding to late Middle Pleistocene-to-Eemian interglacial period¹. The authors found that the body mass of P. antiquus males was up to three times more that of male Asian elephants and twice that of extant male African bush elephants. The large size determined for straight-tusked elephants (with an estimated > 400 cm shoulder height in the flesh and body mass of 13 tonnes) and a later complete epiphyseal-diaphyseal fusion of limb bones (not yet totally fused at an estimated age of 47 years), in comparison with extant elephants, suggests that this species had a longer lifespan of 80 years or more¹. Sexual dimorphism of P. antiquus was observed to be more accentuated than in extant elephants, with females generally not exceeding 300 cm at the shoulders with an estimated weight of not more than 5.5 tonnes, while males continued to grow until their fifties¹. Males in extant elephant species grow more rapidly than females after puberty (i.e., around 7 years in age), which are affected by a trade-off between growth and reproduction. Under normal nutritional conditions, the growth rate is generally higher in males than females leading to a marked difference in size between sexes at already around 10 years in age^33,37,38,39.

The ontogenetic variation in growth projected for the MTS, when compared to what we known from extant proboscideans, is expressed in the track size distribution plot, with the definition of five age classes (Fig. 5; see also Table 1): calves under 2 years in age (when extant elephants experience fastest growth rates in both sexes), juveniles between 2 and 7 years in age (up to when elephant females reach their sexual maturity and therefore experience a strong reduction of growth rate in comparison to males), 7–15 years in age which include pre-puberty males and young female adults, over 15 years in age and < 70 old bulls (with almost stagnation of female growth and males reaching much larger sizes). Trample grounds are important for identifying the social structure and interaction of groups of animals as they represent an extremely short period of time, and thus provide a snapshot of group behavior²¹. Thus, the MTS reflects the demographic structure of the P. antiquus present in this this habitat at the time it was formed. The age classes-frequency plot shows that a large majority of small tracks found belong to calves within one year in age, with estimated shoulder heights less than 95 cm (Figs.4, 6), and estimated body masses between 70 and 200 kg (Table 1). The post-puberty gender distribution cannot be reflected by the tracks both males and females could produce, due to the fast growth of bulls in relation to females of reproductive age except, and with a certain confidence, when trackways of large and very small animals are found interacting or show parallel direction of movement (Figs. 5, 6 and 7). Only the largest, over 50 cm long tracks already identified in the MTS can confidently be attributed to males (old bulls), but the tracks around 30 cm or less may have been produced by adolescent males or young adult females. The evidence for large bulls, up to 325 cm of estimated shoulder height and over 7 tonnes of estimated body mass, are surprisingly very rare in MTS (represented by only two tracks) and may represent solitary incursions to this habitat (see below).

In the MTS, true tracks reveal the plantar-contact surface. Depth distribution within the track reflects the applied plantar pressure on the thick pad, in all age groups. Living elephants usually touch down phase with their heels and step off of their toes²⁹. Patterns of depth-deformation for P. antiquus from MTS are consistent with the plantar pressure data for modern elephants where, for similar substrate conditions, there are no significant depth differences between animals with estimated body masses of 260 kg and over 1 ton (Figs. 6a–d, 7a). The deepest zone of the track corresponds to its anterior part following the typical rotation during the event foot on-toe off observed in extant elephants (see Fig. 3b, e for a MTS example). The pressure on the feet of elephants is greater on the distal ends of the lateral toes and increases outward from the centre of the foot^24,26,40,41. This may leave prominent anterior nail impressions in the plastic substrate of Matalascañas for contemporary small and large tracks.

Several of the small- and large-sized proboscidean tracks have been produced contemporaneously in the MTS (Fig. 6a–d). However, foot length for age in Fig. 5 reveals the preferential presence of young P. antiquus in the MTS, especially calves and juveniles. Smaller elephants are relatively more agile than larger ones, but still move with the same gait as adults^28,36. For that, they just need to adopt greater relative stride frequencies and relative stride lengths compared to larger elephants²¹. That is the reason why overstepping is not evident (but see Fig. 2c for an exception found) in the juvenile-sized trackway but is remarkably evident for the adult-size trackway which intersects it (Fig. 7a–c). Smaller animals took quicker steps with shorter contact durations with relatively longer pace lengths. For this reason, the good preservation of morphological features, without being blurred by overstepping or confounded with similar-sized tracks of other mammals especially in less preserved examples, enabled to discriminate many small proboscidean-type tracks. Their attribution to several individuals based on track size comparisons reflect the prevailing and contemporary presence of youngsters during the short-term environmental conditions that allowed the development and preservation of the MTS.

Matalascañas as a possible straight-tusked elephant reproductive habitat

Extant elephants possess a sex-segregated social structure, centred around a matriarchal family, consisting of one or more related females and their offspring. Males are raised until sexual maturity, around 14–15 years in age, when they approach the shoulder height of adult females⁴², then becoming loners and only reuniting with the female-led groups for mating⁴³. Nuclear family structure in Loxodonta cyclotis corresponds to a mother-calf pair, or 2–4 animals, and herds in Loxodonta africana are usually extended families between 4 and 14 animals^42,44. African forest elephants’ males are solitary, while making transient associations in Loxodonta africana. The remarkable rarity of very large proboscidean tracks in the MTS, with only two tracks over 50 cm in length and clearly made by old bulls, suggests the possibility that the area was visited mainly by matriarchal groups. The characteristics of the MTS as event surface and the comparison of track preservation conditions, such as depth and deformation structures in a relatively firm substrate⁴⁵ (Fig. 6) provides data to define the impression window for different trackways and tracks which support herding behavior for P. antiquus.

In the original diagnosis of Proboscipeda, Panin and Avram⁴⁶ suggest likely herding behavior for a large number of deinothere tracks preserved on a small surface, from the upper Miocene of Romania. As in MTS, Mleisa 1 in Abu Dhabi preserves evidence for a herd of Stegotetrabelodon Petrocchi of varying size crossed by single large animals indicating the presence of both herding and solitary social males⁴³. The presence of a small individual around 1 ton in estimated body mass was inferred from these tracks. A herd of probable Palaeoloxodon recki, consisting of both adults and juveniles trackways measuring 19-to-63 cm was described in MIS 5 paleolake deposits from the Arabian interior⁴⁷. Gregarious behavior and matriarchal herding were previously known for straight-tusked elephants from their track record in similar-aged, MIS 5 eolianites from SW Portugal^11,48. There, a trackway attributed to a juvenile was found parallel to other same-directed trackways considered to be two adult females⁴⁸. A trample ground in the same coastal eolianite reveals a concentration of different sized tracks from where very large asymmetric undertracks organized in parallel trackways, and interpreted as produced by large males, seem to diverge¹¹. This interpretation of social herding structure for P. antiquus seems to be different than the one in MTS, most likely because of a distinctive habitat use.

Matriarchal herd behavior among proboscideans is not surprising and likely evolved early, with its earlier records dated from Late Miocene^43,46,49, being also deduced from tracksites in the latest Late Pleistocene of both Mammuthus primigenius^20,21 and Mammuthus columbi^12,22,27. The presence of calves in these herds was demonstrated only in very rare and remarkable cases. Retallack et al.¹² described a trample ground of Mammuthus columbi, from the shores of Fossil Lake in Oregon, and dated from 43.26 ± 0.33 cal ka, showing a herd of 4 adults, a baby, and at least one subadult. The smallest trackway estimates the presence of a calf less than a year old, and intermediate sizes between 1 and 3 years old. In New Mexico, Lucas et al.²² described a likely (Columbian) mammoth tracksite of Late Pleistocene age (22.8–19.43 ka) with hundreds of proboscidean tracks from 15 to 62 cm in length, estimate shoulder heights ranged between 180 cm (juveniles) to 300 cm. Matsukawa and Shibata⁴⁹ described a tracksite produced by a herd of Stegodon aurorae in the Late Pliocene of Tamagawa. Two parallel trackways of large and small proboscidean tracks suggest a female adult and juvenile. In the MIS 5e-to-5b eolianites from Still Bay, South Africa, coeval with the MTS, Loxodonta africana tracks were described in heavily trampled surfaces, suggesting the passage of a sizeable herd of elephants⁵⁰. Tracks with the smallest length of 17–19 cm were made by calves about 1 year old, found in two parallel trackways made by two very young animals, walking side-by-side, and influenced by mutual behaviour.

In conclusion, herd structure and social behaviour have been previously inferred for the extinct gomphothere Stegotetrabelodon, deinotheres, mastodon^51,52, Stegomastodon and Stegodon aurorae from Japan⁴⁹, Columbian and woolly mammoths, Loxodonta africana, P. recki and P. antiquus, from upper Miocene and Pleistocene trackways and also from mass-death assemblages⁵³. The MTS provides detail to the existing knowledge on the social structure of P. antiquus, with the evidence of matriarchal groups with newborns, perhaps with just few days to months of existence (Fig. 6), and the inferred rare presence of adult males.

African bush elephants have a hind footprint length at birth of 12.5 cm and a weight of 90 kg, with a minimum estimated shoulder height of 69 cm from a young calf⁴². For the Amboseli Loxodonta africana, the minimum shoulder height measured was 79 cm for a newborn female³³. A comparison of the size categories of Loxodonta cyclotis with those of Loxodonta africana suggested a similar distribution of size, despite a marked difference in stature⁴². However, growth may have occurred more quickly in P. antiquus, as recorded in the MTS, than for Loxodonta africana, similar to what is known for mammoths²⁰. The Yamal baby mammoth male had a hind foot of 10 cm in length and 8 cm in width, with a measured shoulder height of 67 cm and an estimated age of 3–4 months⁵⁴ and therefore newborn P. antiquus tracks of the same size, or slightly shorter, from the very plastic sediments of the MTS may have been made by animals with equivalent shoulder height and body mass, i.e., up to 66 cm and around 70 kg.

Female-led family groups are required to remain closer to water as the young need to drink more frequently and cannot cover large distances as quickly as the adults. This tends to restrict the nurseries to areas close to available water²⁰. Therefore, the MTS seems to have been a prime area for matriarchal herds as the bias towards the inferred presence of newborn and juveniles is noteworthy. We interpret the trackways of Figs. 5 and 7a as two examples of straight-tusked elephants walking together, most likely a female moving slowly in response to a close juvenile (reconstitution in Fig. 7e).

The dietary behaviour of P. antiquus populations in different climatic conditions, resulting from microwear and isotopic analysis⁵⁵, has documented different feeding behaviours, particularly browser versus grazer-prevalent diets. African forest elephants living in moist semi-deciduous forests and rainforests are browser-frugivorous⁴⁴. However, high numbers of elephants use the coastal area of Gabon, with strongly seasonal rainfall, throughout the year⁵⁶. Elephants were observed foraging along the coastal shrub areas adjacent to the beach, particularly during the rainy season. Seasonally they spend more time in grassland than in forest during the short-wet season, when grasses grow faster⁵⁷. Reolid et al.⁶ hypothesized that in southern Spain, the P. antiquus would have seasonally migrated beween the highlands of the Guadix-Basa Basin and the lowlands of the western Guadalquivir Basin where the MTS is located. As in the coastal area of Gabon, the MTS coastal area may have provided a mosaic of open grasslands in the dune complex and evergreen forest with high tree and shrub density near the banks of the Guadalquivir river, that afforded cover and browsing to the matriarchal groups during the Last Interglacial. Straight-tusked elephants may have included fresh grasses from the humid grassland dependent on the interdune ponds, in their diet. In the African elephants, mating usually occurs during the dry season⁵⁸. If the gestation in P. antiquus was expectadly like for any modern elephant, i.e., around 20–22 months⁵⁹, this would imply giving birth during the spring after the rainy season, when the freshwater ponds in Doñana would be in their full capacity, as in present days⁶⁰. The retreating waters, starting in May, would have left behind important grasslands that may have provided available water and high-nutritious food resources for the new mothers and breast-feeding calves. The paleosol development with abundant rhizoliths clearly shows the growth of dense vegetation. The high-plasticity of the trampled clayey-sandy substrate and the presence of shrinkage cracks intersected by some tracks allow us to conclude that the water-level was retreating in the MTS interdune pond. The rare occurrence of large tracks shows that MTS was not a habitat for the male adults of P. antiquus. Nevertheless, the Doñana interdune freshwater pond system is an important source of available water in the coastal area and the pond-dependent seasonal grasslands would have been available, especially for old males requiring vegetation that was easier to chew.

Neanderthals in the MTS: food procurement and elephants

During the study of the MTS, short trackways and isolated tracks of both adult and young humans were found together with large herbivore ones (Fig. 8). Meanwhile, Mayoral et al.¹⁸ claimed to have found 87 hominin tracks, most of them constituting what they called the Hominin Trampled Surface (HTS). However, it is evident from their Fig. 3, that HTS is the continuation of the MTS in the most exposed area of the present shore to tidal and wave erosion. This important finding rises questions regarding the ecological reasons for the presence of Neanderthals in coastal environments and especially in the MTS. In fact, reconstructing hominin paleoecology is critical for understanding diets, social organizations and interactions with other animals⁶.

During MIS 5, Neanderthals would find in the coastal areas of SW Spain, visited by large herbivores according to both the track and body fossil (including P. antiquus⁶¹) record, important food resources for hunting and scavenging⁶², and the opportunity of including seafood in their diets⁶³. In fact, besides Neanderthal tracks in the MTS we recorded the presence in concentrations of Mousterian-type lithic tools which are still under study. Neanderthals may have actively selected the best body parts for meat and fat from mega-herbivores, including proboscideans. Elephants are relatively easy to locate due to their dependency on water resources, reported use of familiar paths, and the clear tracks they leave behind⁶⁴. There is an increasing amount of evidence that proboscideans, and especially young individuals, played a major role in Neanderthal’s diet and adaptation^64,65,66,67. Anzidel et al.⁶⁷ interpreted a late Middle Pleistocene occurrence in Italy of P. antiquus with Neanderthal lithic tools as resulting from scavenging activity. Contemporary from Matalascañas, in Manzanares valley (Madrid), Panera Gallego et al.⁶⁸ have found typical Neanderthal lithic tools associated to skeletal remains of a possible young straight-tusked elephant, including also a couple of tracks 17–22 cm in length that could be related to the same animal. Considering the ichnological record of elephant-human interactions, McNeil et al.²⁰ described a woolly mammoth tracksite in southwestern Canada dated between 11,3–11 ka with ichnological evidence of modern humans hunting the megafauna. Fladerer⁶⁹ in skeletal record and Haynes⁵³ in the track record found selective mortality of juvenile mammoths as result of modern humans selecting the smaller animals because they were easier to kill or to process. Mammoths-human interactions were also revealed through tracks in New Mexico²⁷.

The repeated pattern of young proboscidean procurement suggests that age played a significant role in their selection, likely related to a better nutritional value and the relative ease to hunt and butchering, as well as transporting the best nutritious parts⁶⁴. Therefore, and in parallel with the examples provided, the coetaneous presence of Neanderthal tracks and lithic tools together with megafauna trackways, including P. antiquus, rises the hypothesis that the MTS coastal pond may have been an elephant resource habitat for Neanderthals. They would be here seasonally intentionally hunting more accessible targets such as calves, juveniles or weakened females in delivery, or opportunistically scavenging stillbirths and females dead from birth.

Source: Ecology - nature.com