Introduction

Synchrony or birthing peaks are seen as a response to either seasonal environments or as an anti-predatory adaptation1,2. While tropical and equatorial mammals exhibit a lack of seasonality in breeding due to uniform resource availability, tropical India is highly seasonal in resource availability due to the rainfall being primarily restricted to the monsoon season. However, reproductive seasonality in ungulates may also be influenced by factors beyond forage abundance, including predation risk, social structure, mate availability, and habitat configuration3, as synchronized birthing can reduce individual predation risk via predator swamping, while habitat structure and group sociality can influence access to mates and protection. Study on different African ungulates found that breeding synchronizes with seasons of abundant forage resources, an adaptation that enhances offspring survival by ensuring that young are born during favourable conditions4. Rainfall patterns also significantly influence ungulate reproductive behaviour as it affects food availability5. Increased rainfall can lead to lush vegetation, providing better nutrition for pregnant or lactating females and improving offspring survival rates. The reproductive patterns of ungulates can vary significantly based on environmental conditions. Ungulates are timed to coincide with their breeding season with periods of highest plant quality to support offspring nutrition6. This adaptation ensures that lactating females and weaned offspring obtain the required energy and protein from the forage5,7,8,9. These relationships underscore the importance of understanding ecological dynamics to better manage ungulate populations and their habitats.

As environmental conditions continue to change due to human impact, ongoing research will be required to understand how ungulates adapt to these challenges. Changes in forage quantity and quality significantly impact the body condition of ungulates, especially females during gestation and lactation, when their energy and protein requirements are heightened. Many studies emphasize the critical role that adequate nutrition plays in ensuring the health and reproductive success of these animals during these demanding periods4,10,11,12,13,14. The onset of oestrus in females is significantly affected by conditions required for a successful pregnancy and subsequent lactation, such as food availability or favourable weather conditions15,16, as increased time and energy expenditures for food searching can lead to higher mortality in reproducing adults11. We hypothesize that though the ultimate cue for timing reproduction is likely to be the monsoon rains and the associated growing season for all vegetation, we believe that it may be possible for ungulates to adjust seasonality of reproduction to local conditions. In the case of Kanha, it would be the management practice of cool burning of grasslands and the associated new flush of sprouts that are highly digestible with high protein and energy content.

In most of the cervids, antler cycles serve as external indicators of internal reproductive changes17. In most deer species, males undergo a yearly cycle characterized by the growth and subsequent shedding of antlers18,19,20. Antlers play a crucial role in male-male confrontations to establish dominance and secure mates, serving as an indication to females of male fitness and condition during the rutting season, which is a period of intense mating activity21,22,23,24. Testosterone levels peak seasonally during the rut season25, and when testosterone levels fall below threshold after the rutting season, antlers are cast off in most cervids25,26,27. Consequently, peak rutting activity often coincides with the period when the largest proportion of males exhibit burnished antlers, indicative of their reproductive readiness28,29,30. In deer, observing actual fawning is a rare event and is often studied by implanting radio transmitters in the vagina of pregnant females, which are expelled out at the time of parturition31,32. This allows for the study of timing and specific location of parturition. We indexed the timing of fawning by estimating the proportion of lactating females in the population5,33.

Many studies on breeding seasonality have been conducted on North American and African ungulates, but in the Indian subcontinent only a few ungulate species have been studied for their reproductive patterns28,34. However, these studies have primarily focused on observing the seasonality of the antler cycle. Management decisions like prey augmentation35, where animals from high-density areas were captured and moved to low-density areas to increase the carrying capacity of large carnivores, also need information about rutting and fawning. Translocation during prey augmentation in the fawning season can cause stress due to possible mother-fawn separation, and aggressive behaviour of males that bear hard antlers during the rutting season can increase mortality, making it essential to understand the breeding season of the source chital population beforehand. In this study, we record the reproductive condition of chital throughout the year from representative samples to decipher seasonality and its likely drivers in the dry deciduous forest system of Kanha National Park.

Results

Fawning occurred throughout the year as lactation was recorded in all months. However, the proportion of lactating females in the population peaked in March-April. April has the highest proportion of lactating females (78 ± 0.8%), followed by March (73.5 ± 1.1%), while November recorded the lowest proportion (6.93 ± 0.37%) (Fig. 1, Table S1). Seasonally, summer had the highest proportion of lactating females (72.26 ± 0.24%), followed by monsoon (48.9 ± 0.46%), and winter (30.79 ± 0.34%) (Fig. 1, Table S1). None of the 200 sampled sub-adult females were observed to be lactating.

Fig. 1
figure 1

Seasonality of breeding and fawning in chital (Axis axis) as indexed by proportion of different antler stages of males and lactating females in relation to resource availability indexed by NDVI in Kanha Tiger Reserve. The bars show the proportion of males in different antler stages in the population. The blue line indicates the NDVI value for each month, and the red line represents the proportion of lactating females in the population. The dotted lines indicate the standard errors for the NDVI and the proportion of lactating females.

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In July, nearly all adult males have hard antlers (97.05 ± 0.15%). Between May and November, over 60% of the male population displayed hard antlers, while more than 50% of the adult male population sprouted velvet-covered antlers between December and April (Fig. 1, Table S2). The proportion of shed antler male observation is low due to the short interval between antler shedding and the emergence of new antlers.

In Kanha Tiger Reserve, grassland growth peaks during the monsoon and post-monsoon periods but declines during the dry winter season with minimal rainfall. The NDVI value during the post-fire season (April) shows an upward trend indicating the emergence of freshly grown grass shoots. Lactating peaked at the time of the new grass flush when the NDVI values start increasing (Fig. 1, Table S4) after a lowest in March. Births of tiger litters were observed throughout the year with a peak in December. Their weaning started after 3–4 months which coincided with the time of fawning (Fig. 2).

Fig. 2
figure 2

Percent tiger litter weaned (n = 31) depending on their birth month data obtained from Kumar, 2019 vs. proportion of lactating chital females in chital population in Kanha tiger reserve every month. Blue bars show the number of tiger litter and red line show the percentage of lactating chital females in population.

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Discussion

There seems to be a debate regarding the presence of distinct breeding seasonality in chital, especially in tropical regions where food and water are relatively stable throughout the year (Table S4). Though Krishnan (1972)36 reported chital lack any birth peak throughout peninsular India, many studies have found distinct seasonal peaks in chital rutting behaviour in India (Table S4). Studies were done in Hawaii37, Texas38, Australia39, Nepal30,40 also found a distinct peak in rutting behaviour of chital depending on resource availability. We also found similar result where fawning is influenced by resource availability. The Central Indian landscape has distinct seasonal variation, which affects the availability and quality of forage during different times of the year. Mixed foragers like Chital shift their diet towards browsing during summer when grass nutrient quality declines41,42, emphasizing the importance of dietary quality. The dry grasses become non-nutritious and are filled with silica and lignin content43. Due to the burning of grasslands during February to reduce the fuel load of grasses, preventing uncontrolled fire during the summer season, fresh biomass appears post-fire season, which is high in crude protein44,45,46,47 and other minerals like calcium and magnesium48. NDVI is a measure of green biomass, though this peaks after the monsoon along with the vegetation bulk, it is the sprouting of young grasses that provides the much needed digestible energy and proteins to support lactation and weaning of fawns that is important. Moreover, chital feed on most new flush of vegetation, irrespective of species and only become selective once the vegetation matures and produces secondary metabolites as plant defences against herbivory49. Therefore, the upward change in trend of NDVI is also important factor for chital.

We found that the parturition peak of chital is synchronous with the appearance of fresh grass forage biomass, i.e., the months of March and April. Moreover, fawns born at this time were weaned off during the monsoon period, when there is ample forage available for them to feed on. The birth peak of ungulates has been observed to coincide with seasonal rainfall and the appearance of green foliage in tropical and subtropical regions50. The influence of food availability on ungulate breeding has also been reported in Red deer51 and chital deer of Bandipur and Mudumalai52.

In Kanha, most rutting occurs in late summer and monsoon season, although some hard antler males are present in the population throughout the year. Deer females conceive only when they have good body condition53,54 and Kelly et al. (2022) found that this is true for chital also. In Kanha, chital has better body condition when ample food is available41. Similarly, fawning starts in January. Since fawning is represented by lactating females and lactation occurs up to 4 months in chital, it is difficult to determine the exact fawning peak.

Peak in tiger cub birth occurs when more than half of the female chital were lactating. When weaning of tiger cubs starts after a few months, it is the same time when the number of lactating females is also highest. When weaning starts, the need for prey also increases, as the mother needs to provide food for weaned cubs. During this period, chital are most vulnerable to predation, with males being in hard antlers and preoccupied with the rut, females caring for fawns and weak due to lactation, and ample availability of fawns. Tigers in the Kanha landscape showed a functional response by selective predation on the abundant chital fawns and rutting males55, resulting in numerical response by timing their birth peak at this time. Similar pattern has also been reported in mountain lions (Puma concolor)56. The mountain lion birth peak is synchronized with the birth of North American ungulates in May-June.

There are certain cues like resource abundance that track ecological rhythms that influence reproductive behaviour. The intricate relationship between seasonal forage dynamics, reproductive patterns, nutritional ecology, and predation underscores the importance of understanding these dynamics for effective wildlife management and conservation planning. Birthing and rutting peaks of chital in Kanha, which is a deciduous forest-grassland system, were determined by resource availability and were unlikely to be in response to predation. From Schaller’s (1967)57 study it can be inferred that chital fawning occurred during December-February and the peak in tiger cubs reaching weaning stage was during June. These timings differ significantly from those observed by us. We believe that chital and subsequently tigers have adapted to the cool burning of grasses a management practice that was initiated since the creation of the Protected Area. This is an interesting finding since it suggests that nutritional factors and not photoperiod play an important role in determining reproduction timing in tropical ungulates58,59,60. This result is open to further verification by more robust experimental studies on tropical ungulates especially chital.

This study underscores the complexity of chital reproductive strategies in tropical systems. Controlled burning enhances forage quality and providing essential nutrition for lactating females. The alignment of chital reproductive cycles with forage availability, and of tiger births with vulnerable chital demographics, emphasizes the importance of ecological synchrony in predator-prey systems.

The interplay between seasonal forage dynamics, reproductive patterns, nutritional ecology, and predation is vital for effective wildlife management and conservation. Understanding these dynamics in the deciduous forest-grassland system of Kanha reveals that chital birthing and rutting peaks are driven more by resource availability than predation pressures. Effective conservation strategies must consider these intricate ecological relationships to support the sustainable management of chital populations and their habitats.

Although this research provides important information regarding chital reproductive seasonality with respect to forage availability and its impact on predator-prey dynamics, it is limited by the lack of direct observation of mating and fawning events, but instead used indirect measures like antler condition and lactation. Future studies might aim for more precise and direct observations of mating behaviour and parturition, perhaps with the aid of radio telemetry based tracking techniques to validate the timing and determinants of seasonality in chital breeding. Further investigation into the impact of climate change on resource availability and timing would be a key follow-up to determine the potential effects these changes have on reproductive cycles in the future.

Materials and methods

Study area

Kanha Tiger Reserve is located in the Satpura Maikal hill ranges of the Central Indian highlands61. It was among the first nine Tiger Reserves gazetted in 1973. It is located between 80°26’10’’ E to 81°04’40.00’’E and 22°01’5.0” N to 27°27’48.00” N in Balghat and Mandla districts of Madhya Pradesh. The area of the Core Zone is 917.43 km2, and that of the Buffer Zone is 1134.39 km2, adding up to 2051.82 km2 as the total area of Kanha Tiger Reserve. The Tiger Reserve has flat hilltops, varying degrees of slopes, and meadows in the valleys, which offer unique, diverse types of wildlife habitat, forming ideal niches for various species of plants and animals.

There are three distinct seasons in Kanha Tiger Reserve. These are monsoon, summer, and winter. Summer extends from mid-February to June end, monsoon is from June end week to mid-October, and winter is from mid-October to mid-February. Due to a great variation in temperature, humidity, wind velocity, and precipitation in different seasons, these factors serve as regulators of vegetation and the habits of wild animals in the reserve. Kanha Tiger Reserve lies in a dry deciduous zone, and the availability of water is a very important factor for the survival of animals and plants. Major forest types of the area can be divided into four categories: sal forest, miscellaneous forest, bamboo mixed forest, and grasslands. Controlled burning practices during February are carried out in the grasslands of Kanha each year for habitat improvement and management purposes.

Kanha hosts high densities of both prey and predators. Chital, which is a major prey species of all the carnivores present in Kanha, has one of the highest densities in India62.

Field method

Estimating proportion of population in breeding condition

Observation of actual mating events and fawning in wild populations is rare, especially in tropical forest systems. We therefore indexed breeding condition in males by assessing their antler condition17,29 and fawning in females by assessing their lactation state5,63,64,65.

For obtaining an unbiased representation of the population in samples, we conducted intensive surveys along fixed vehicle routes to observe the breeding status of male and female chital groups. Each route was visited once a week for a whole year during 2022. An average of 21 Km road covered per day by vehicle for chital group observation which cover different habitat types of the study area. Initially, the population was categorized into adults, sub-adults, and juveniles where juvenile were (≤ 1 year), sub-adult were (< 2 years but > 1 year) and adult (> 2 years)57 (Figure S3). For assessing breeding signs, only adult individuals were considered. To assess the involvement of sub-adult females in breeding, a total of 200 sub-adult females were observed for lactation signs during the peak lactation period. We subdivided adult males into hard antler, velvet, and shed antler male groups as per their antler stages at different times of the year (Figure S1). We expected that the peak breeding season will coincide with the time when the largest proportion of bucks exhibit hard antlers.

To determine the fawning season, lactation signs were recorded whenever an adult female was observed, as the presence of lactating mothers indicated the birth of a fawn5,63,64,65. In chital udders and teats become visible only a few days before parturition and remain prominent up to four months after parturition66,67 (Figure S2).

Analytical method

Mating and fawning season

Data on various stages of male antlers and lactating females were pooled month-wise for exploratory analysis. Then the data were categorized into three distinct seasons: winter, summer, and monsoon, and reanalysed. Since the same route was followed weekly for observations, the computation was done using multiple sampling with replacement approach68 which accounts for the possible repeated sampling of the same individual.

NDVI change

Chital are the primary grazers at Kanha Tiger Reserve as more than 50% of their diet consists of grasses41. So change in the condition of grass affects the body condition of chital. To see the monthly productivity in the grasslands of Kanha, we analysed the Normalized Difference Vegetation Index (NDVI) of the grasslands of Kanha for the past 10 years from satellite imagery of LANDSAT 8 (USGS) in Google Earth Engine69. A harmonic model for monthly change in NDVI was fitted to the data to see monthly changes in the grasslands of Kanha (Figure S4). NDVI does not provide information about palatability of grass species or identification of specific grass species. We were interested in detecting the early flush of green biomass which is indicated by the increasing trend in NDVI values. When sprouting begins in grasslands, the NDVI value starts increasing from a low as the grasses turn green replacing the dry grass or bare land left after burning. Then monthly average values of NDVI were plotted to evaluate the synchrony between grass sprouting season and chital breeding in Kanha.

Birth of tiger Cubs

We used information from Kumar (2019)70 on the number of litters born each month for tigers in Kanha where the birth month of tiger litters were determined by aging the cubs. The birth records and the month of parturition were obtained through continuous sightings by experienced forest staff, researchers, camera trap pictures, and handheld photographs taken by tourists who shared the date and location information. Information on 31 litter births was recorded with reasonable certainty. Once the month of birth was ascertained, the relationship between chital fawning and tiger cub was explored by looking at the time of birth of offspring of both prey and predator.